View source for Human mortality from H5N1

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{{H5N1 Human Mortality}}

{{H5N1}}
{{update|date=March 2020}}
[[Influenza A virus subtype H5N1|H5N1]] influenza virus is a type of influenza A virus which mostly infects birds. H5N1 flu is a concern because [[global spread of H5N1|its global spread]] may constitute a [[pandemic]] threat. The yardstick for '''human mortality from [[H5N1]]''' is the '''[[Case fatality rate|case-fatality rate]] (CFR);''' the ratio of the number of confirmed [[human]] deaths resulting from [[transmission and infection of H5N1|infection of H5N1]] to the number of those confirmed cases of infection with the virus. For example, if there are 100 confirmed cases of a disease and 50 die as a consequence, then the CFR is 50%. The case fatality rate  does not take into account cases of a disease which are unconfirmed or undiagnosed, perhaps because symptoms were mild and unremarkable or because of a lack of diagnostic facilities. The Infection Fatality Rate (IFR) is adjusted to allow for undiagnosed cases.{{Citation needed|date=May 2024}}

The IFR is central to [[pandemic]] planning. In previous [[influenza pandemic]]s (not caused by H5N1), IFR estimates have ranged from  to 2-3% for the [[1918 pandemic]], to about 0.6% for the [[1957 pandemic|1957]] pandemic,<ref>{{cite web |last1=Lovelace Jr. |first1=Berkeley |title=The coronavirus may be deadlier than the 1918 flu: Here's how it stacks up to other pandemics |url=https://www.cnbc.com/2020/03/26/coronavirus-may-be-deadlier-than-1918-flu-heres-how-it-stacks-up-to-other-pandemics.html |website=CNBC |date=26 March 2020 |publisher=f NBCUniversal |access-date=25 April 2020}}</ref> to  0.2% for the [[1968 pandemic]].

<!-- The following paragraph is transcluded into several other pages, please edit with care --> 
<section begin="H5N1_WHO" />Between 2003 and February 2025, the [[World Health Organization]] has recorded 972 cases of confirmed H5N1 influenza, leading to 468 deaths.<ref>{{Cite web |date=15 February 2025 |title=Global Influenza Programme : Avian influenza A(H5N1) virus |url=https://www.who.int/teams/global-influenza-programme/avian-influenza/avian-a-h5n1-virus |access-date=2025-03-28 |website=World Health Organization |language=en}}</ref> The true fatality rate may be lower because some cases with mild symptoms may not have been identified as H5N1.<ref name="case-fatality rate">{{cite journal |vauthors=Li FC, Choi BC, Sly T, Pak AW |title=Finding the real case-fatality rate of H5N1 avian influenza |journal=J Epidemiol Community Health |volume=62 |issue=6 |pages=555–9 |date=June 2008 |pmid=18477756 |doi=10.1136/jech.2007.064030 |s2cid=34200426 |url=http://jech.bmj.com/cgi/content/abstract/62/6/555|url-access=subscription }}</ref><section end="H5N1_WHO" />

H5N1 infections in humans are generally caused by bird to human transmission of the virus.  Until May 2006, the WHO estimate of the number of human to human transmissions had been "two or three cases". On May 24, 2006, Dr. Julie L. Gerberding, director of the United States [[Centers for Disease Control and Prevention]] in [[Atlanta]], estimated that there had been "at least three." On May 30, Maria Cheng, a WHO spokeswoman, said there were "probably about half a dozen," but that no one "has got a solid number."<ref>{{cite news| url=https://www.nytimes.com/2006/06/04/world/asia/04flu.html | title=Human Flu Transfers May Exceed Reports | work=New York Times | date=June 4, 2006 | author=Donald G. McNeil Jr.}}</ref> The cases of suspected human to human transmission that continue to be found have been isolated and contained,<ref>{{cite news
 |url         = https://www.bloomberg.com/apps/news?pid=10000080&sid=aWESsJvt6CFE
 |title       = Seven Indonesian Bird Flu Cases Linked to Patients
 |publisher   = Bloomberg
 |date        = May 23, 2006
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20071007194340/http://www.bloomberg.com/apps/news?pid=10000080&sid=aWESsJvt6CFE&refer=asia
 |archive-date = October 7, 2007
}}</ref> and include transmission among members of a family in Sumatra, Indonesia in June 2006<ref>{{cite web
 | url=http://www.foodconsumer.org/777/8/WHO_Confirms_Human_Transmission_In_Indonesian_Bird_Flu_Cluster.shtml
 | title=WHO confirms human transmission< in Indonesian bird flu cluster
}}</ref> as well as earlier and later instances arising in other countries. However, no [[pandemic]] strain of H5N1 has yet been found. The key point is that, at present, "the virus is not spreading efficiently or sustainably among humans."<ref>{{cite news|url=https://www.who.int/csr/don/2006_06_06/en/index.html|archive-url=https://web.archive.org/web/20060615010219/http://www.who.int/csr/don/2006_06_06/en/index.html|url-status=dead|archive-date=June 15, 2006|title=Avian influenza – situation in Indonesia – update 17|publisher=[[WHO]]|date=June 6, 2006}}</ref>

H5N1 vaccines for chickens exist and are sometimes used, although there are many difficulties that make it especially difficult to decide whether vaccination will do more harm than good. In the U.S. H5N1 pre-pandemic vaccines exist in quantities sufficient to inoculate a few million people<ref>{{cite web
 | url=http://www.cidrap.umn.edu/cidrap/content/influenza/panflu/news/jul0506hhsreport.html
 | publisher=CIDRAP
 | title=HHS has enough H5N1 vaccine for 4 million people
 | date=July 5, 2006
}}</ref> and might be useful for priming to "boost the immune response to a different H5N1 vaccine tailor-made years later to thwart an emerging pandemic".<ref>{{cite web
 | url=http://www.cidrap.umn.edu/cidrap/content/influenza/panflu/news/oct1306vaccines.html
 | publisher=CIDRAP
 | title=Study supports concept of 2-stage H5N1 vaccination
 | date= October 13, 2006
}}</ref> [[Japan]] has inoculated 6,000 health care workers with a pre-pandemic vaccine, and is planning how to proceed with widespread vaccinations, particularly workers who would provide utilities during an outbreak.<ref>Pre-pandemic bird flu shots eyed / Health ministry to urge study of potential early vaccination recipients
|
(Daily Yomiuri Online + AP -- Apr. 25, 2009)
http://www.yomiuri.co.jp/dy/national/20090425TDY03103.htm</ref><ref>
{{Cite news |date=2008-04-15 |title=Japan to vaccinate medical workers for bird flu |language=en |work=Reuters |url=https://www.reuters.com/article/us-birdflu-japan-idUST22101620080415 |access-date=2023-05-11}}</ref><ref>Measures against flu needed / Govt urged to set up framework to fight new influenza outbreak
Apr. 24, 2008
http://www.yomiuri.co.jp/dy/features/science/20080424TDY04302.htm</ref> [[Switzerland]] is also considering preemptive vaccination to protect the general public.<ref>Vaccinations for new flu strains eyed for public (Apr. 17, 2008) http://www.yomiuri.co.jp/dy/national/20080417TDY02301.htm</ref> H5N1 pandemic vaccines and the technologies to rapidly create them are in the [[H5N1 clinical trials]] stage but cannot be verified as useful until after a pandemic strain emerges. Efforts to identify the changes that might result in a human-communicable strain have resulted in laboratory-generated H5N1 with substantially greater affinity for human cellular receptors after a change of just two of the H5 surface proteins.<ref name=SciAug07>http://www.cidrap.umn.edu/cidrap/content/influenza/avianflu/news/aug1007mutant.html Researchers create H5N1 mutations to pave way for new vaccines and treatments
Aug 10, 2007 (CIDRAP News)
"Focusing on genetic changes to one portion of the H5 protein, called the receptor binding domain, [the researchers] found that as few as two mutations could enhance the ability of H5N1 to recognize human cells, according to the press release."<br />
{{cite journal  |vauthors=Yang ZY, Wei CJ, Kong WP, etal |title=Immunization by avian H5 influenza hemagglutinin mutants with altered receptor binding specificity |journal=Science |volume=317 |issue=5839 |pages=825–8 |date=August 2007 |pmid=17690300 |pmc=2367145 |doi=10.1126/science.1135165 |bibcode=2007Sci...317..825Y }}</ref> Significantly, mouse antibodies were 10 times less potent against the mutants than against the pre-mutated viruses.<ref name=SciAug07/>

==H5N1 cases in humans==
A graphic exhibiting total cases and mortality incidence is kept current by the WHO at https://www.who.int/teams/global-influenza-programme/avian-influenza/avian-a-h5n1-virus and complements the country-specific information shown below.

Country-specific totals of cases and deaths kept current by the WHO may be viewed by clicking through the links provided at [https://www.who.int/data/gho/data/themes/global-influenza-virological-surveillance Global influenza virological surveillance] and the map links provided here [https://www.who.int/data/gho/map-gallery-search-results?maptopics=82c0d61f-ccfd-41df-b9bd-3fc98bc84093 Map Gallery Search Results]
Global influenza virological surveillance (in the Global Health Observatory)
{{H5N1 cases}}

==History==
A strain of [[H5N1]] killed [[chicken]]s in 1959 in [[Scotland]] and [[turkey (bird)|turkey]]s in 1991 in [[England]].<ref>{{cite web
 |url=http://www.lib.cau.edu.cn/qlga/86a.pdf
 |title=A review of avian influenza in different bird species
 |author=Dennis J. Alexander*
 |work=Avian Virology, VLA Weybridge, Addlestone, Surrey KT15 3NB, UK
 |url-status=dead
 |archive-url=https://web.archive.org/web/20070629070846/http://www.lib.cau.edu.cn/qlga/86a.pdf
 |archive-date=2007-06-29
}}</ref> This strain was "highly pathogenic" (deadly to birds) but caused neither illness nor death in humans.<ref>{{cite news
 | url=https://www.who.int/csr/don/2004_03_02/en/
 | archive-url=https://web.archive.org/web/20040307050726/http://www.who.int/csr/don/2004_03_02/en/
 | url-status=dead
 | archive-date=March 7, 2004
 | work=Disease Outbreak News: Avian influenza A(H5N1)
 | title=Situation (poultry) in Asia: need for a long-term response, comparison with previous outbreaks
 | date=March 2, 2004
 | access-date=2006-10-27
 | publisher=WHO
}}
</ref> "The precursor of the H5N1 influenza virus that spread to [[human]]s in 1997 was first detected in [[Guangdong]], [[China]], in 1996, when it caused a moderate number of deaths in [[geese]] and attracted very little attention."<ref name=Webster06/> In 1997, in [[Hong Kong]], 18 humans were infected and 6 died in the first known case of H5N1 infecting humans.<ref name="timeline">{{cite web
 |url=https://www.who.int/csr/disease/avian_influenza/Timeline_28_10a.pdf
 |title=H5N1 avian influenza: timeline
 |author=WHO
 |date=October 28, 2005
 |url-status=dead
 |archive-url=https://web.archive.org/web/20110727011806/http://www.who.int/csr/disease/avian_influenza/Timeline_28_10a.pdf
 |archive-date=2011-07-27
}}</ref> H5N1 had evolved from a zero mortality rate to a 33% mortality rate.

The first report, in the current wave of HPAI A(H5N1) outbreaks, was of an outbreak that began December 10, 2003 in the [[Republic of Korea]] and continued for fourteen weeks. This strain caused asymptomatic infections in humans and may have died out,<ref>
{{cite news
 | url=http://uk.reuters.com/article/idUKT24954320070201
 | archive-url=https://web.archive.org/web/20170213164019/http://uk.reuters.com/article/idUKT24954320070201
 | url-status=dead
 | archive-date=February 13, 2017
 | title=Don't ignore less virulent bird flu strains: experts
 | newspaper=Reuters
 | date=February 1, 2007
 | access-date=12 February 2017
 | author=Tan Ee Lyn
}}</ref><ref>South Korea raises H5N1 culling target to 5.3 million
Lisa Schnirring * Staff Writer
Apr 21, 2008 (CIDRAP News)http://www.cidrap.umn.edu/cidrap/content/influenza/avianflu/news/apr2108culling(2).html</ref> like the 1959 strain, so that its low mortality level would have little value for predicting the mortality rate of a pandemic evolving from existing HPAI A(H5N1) strains.<ref>{{cite news
 | url=http://www.cidrap.umn.edu/cidrap/content/influenza/avianflu/news/sep2106korea.html
 | publisher=CIDRAP
 | title=Five Koreans had H5N1 virus but no illness
 | date=September 21, 2006
}}</ref><ref name="Tree of Life">{{cite web
 |url=https://www.who.int/csr/disease/avian_influenza/guidelines/recommendationvaccine.pdf
 |author=WHO
 |date=August 18, 2006
 |title=Antigenic and genetic characteristics of H5N1 viruses and candidate H5N1 vaccine viruses developed for potential use as pre-pandemic vaccines
 |url-status=dead
 |archive-url=https://web.archive.org/web/20060824004040/http://www.who.int/csr/disease/avian_influenza/guidelines/recommendationvaccine.pdf
 |archive-date=August 24, 2006
}} Contains latest Evolutionary "Tree of Life" for H5N1</ref> The apparently extinct strain that caused human deaths from H5N1 in the Northern part of Vietnam in 2003, 2004 and 2005 also had a much lower case mortality rate than the currently existing strains.<ref name="Tree of Life"/> Changes are occurring in H5N1 that are increasing its pathogenicity in mammals.<ref name="Chen">{{cite journal
|vauthors=Chen H, Deng G, Li Z, Tian G, Li Y, Jiao P, Zhang L, Liu Z, Webster RG, Yu K | title=The evolution of H5N1 influenza viruses in ducks in southern China | journal=Proc. Natl. Acad. Sci. U.S.A. | year=2004 | pages=10452–7 | volume=101 | issue=28 | pmid=15235128 | doi=10.1073/pnas.0403212101 |pmc=478602| bibcode=2004PNAS..10110452C | doi-access=free }}</ref><ref>http://www.cidrap.umn.edu/cidrap/content/influenza/avianflu/news/oct0507avian.html H5N1 mutation that could help spark pandemic identified
"The change promotes better viral replication at the lower temperatures found in the upper airways of mammals..."

Additionally, discussing the same mutation, one of the researchers points out that the mutated strain is in wide circulation:{{blockquote|"The viruses that are circulating in Africa and Europe are the ones closest to becoming a human virus,"}} Kawaoka said. But he pointed out that one mutation is not sufficient to turn H5N1 into a major threat to humans.</ref>

From inception through 2007, the total number of [[WHO]]-confirmed cases was 349, with 216 of those fatalities (as reported by the U.N. on January 15, 2008, confirming earlier deaths) reflecting a 62% fatality rate among [[WHO]]-confirmed cases through 2007.<ref>
{{cite web|url=https://www.who.int/csr/disease/avian_influenza/country/cases_table_2008_01_15/en/index.html |title=WHO &#124; Cumulative Number of Confirmed Human Cases of Avian Influenza A/(H5N1) Reported to WHO |access-date=2008-01-15 |url-status=dead|archive-url=https://web.archive.org/web/20080117092338/http://www.who.int/csr/disease/avian_influenza/country/cases_table_2008_01_15/en/index.html |archive-date=2008-01-17 }}

Cumulative Number of Confirmed Human Cases of Avian Influenza A/(H5N1) Reported to WHO / 15 January 2008

For possible later updates by the WHO, see links at {{cite web|url=https://www.who.int/csr/disease/avian_influenza/country/en/ |title=WHO &#124; Confirmed Human Cases of Avian Influenza A(H5N1) |access-date=2006-04-26 |url-status=dead|archive-url=https://web.archive.org/web/20060423065725/http://www.who.int/csr/disease/avian_influenza/country/en/ |archive-date=2006-04-23 }}</ref> These overall figures fail to bring forward fluctuations that have appeared from year to year and in particular geographic areas. In 2005, when a markedly less-lethal strain in Northern Vietnam was responsible for most of the cases reported worldwide, only 42 of 97 people confirmed by the [[WHO]] to be infected with [[H5N1]] died — a 43% fatality rate. In 2006, the case fatality ratio was higher among the [[WHO]]-confirmed cases, with 79 deaths among 114 confirmed cases.<ref>{{cite web
 |url=https://www.who.int/csr/disease/avian_influenza/country/cases_table_2006_12_27/en/index.html
 |title=Cumulative Number of Confirmed Human Cases of Avian Influenza A/(H5N1) Reported to WHO
 |publisher=WHO
 |date=December 29, 2006
 |url-status=dead
 |archive-url=https://web.archive.org/web/20070416052840/http://www.who.int/csr/disease/avian_influenza/country/cases_table_2006_12_27/en/index.html
 |archive-date=April 16, 2007
}}</ref>— or 69%.  In 2007, 59 of the 86 WHO-confirmed cases ended in death, again a 69% fatality rate.<ref>(including cases reported to and confirmed by the [[WHO]] up to January 24, 2008)
{{cite web|url=https://www.who.int/csr/disease/avian_influenza/country/cases_table_2008_01_24/en/index.html |title=WHO &#124; Cumulative Number of Confirmed Human Cases of Avian Influenza A/(H5N1) Reported to WHO |access-date=2008-01-27 |url-status=dead|archive-url=https://web.archive.org/web/20080128112145/http://www.who.int/csr/disease/avian_influenza/country/cases_table_2008_01_24/en/index.html |archive-date=2008-01-28 }}

Cumulative Number of Confirmed Human Cases of Avian Influenza A/(H5N1) Reported to WHO / 24 January 2008 |

For later updates by the WHO, see {{cite web|url=https://www.who.int/csr/disease/avian_influenza/country/en/ |title=WHO &#124; Confirmed Human Cases of Avian Influenza A(H5N1) |access-date=2006-04-26 |url-status=dead|archive-url=https://web.archive.org/web/20060423065725/http://www.who.int/csr/disease/avian_influenza/country/en/ |archive-date=2006-04-23 }}</ref> And 24 of the first 31 cases of 2008 (to April 30, 2008) have been fatal,<ref>{{cite web|url=https://www.who.int/csr/disease/avian_influenza/country/cases_table_2008_04_30/en/index.html |title=WHO &#124; Cumulative Number of Confirmed Human Cases of Avian Influenza A/(H5N1) Reported to WHO |access-date=2008-06-04 |url-status=dead|archive-url=https://web.archive.org/web/20080530005232/http://www.who.int/csr/disease/avian_influenza/country/cases_table_2008_04_30/en/index.html |archive-date=2008-05-30 }} Cumulative Number of Confirmed Human Cases of Avian Influenza A/(H5N1) Reported to WHO
30 April 2008. For later totals as the WHO provides updates, click through links at {{cite web|url=https://www.who.int/csr/disease/avian_influenza/country/en/ |title=WHO &#124; Confirmed Human Cases of Avian Influenza A(H5N1) |access-date=2006-04-26 |url-status=dead|archive-url=https://web.archive.org/web/20060423065725/http://www.who.int/csr/disease/avian_influenza/country/en/ |archive-date=2006-04-23 }} Epidemic and Pandemic Alert and Response (EPR)
Confirmed Human Cases of Avian Influenza A(H5N1)</ref> or 77%.

The higher total case fatality ratio after the end of 2005 may reflect the widespread circulation in Vietnam of a less-lethal [[clade]] of H5N1 in 2005, which was subsequently brought under control. The change was nonetheless interpreted by some as indicating that the virus itself was becoming more deadly over time.<ref>{{cite web
 |url         =https://www.who.int/csr/disease/avian_influenza/country/cases_table_2007_05_24/en/index.html
 |title       = H5N1 Getting Deadlier
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20070527131033/http://www.who.int/csr/disease/avian_influenza/country/cases_table_2007_05_24/en/index.html
 |archive-date = 2007-05-27
}} based on the article {{cite news
 |url         = https://www.bloomberg.com/apps/news?pid=10000080&sid=a7CJ0uPPpg.g
 |title       = Bird Flu Fatality Rate in Humans Climbs to 64% as Virus Spreads
 |work        = Bloomberg
 |date        = May 20, 2006
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20081211175414/http://www.bloomberg.com/apps/news?pid=10000080&sid=a7CJ0uPPpg.g
 |archive-date = December 11, 2008
}}</ref> In fact, when less-virulent strains die off, the surviving strains are the more virulent. Such difficulties in interpretation underscore that the global case fatality ratio can serve as but a crude and imperfect summary of the current complex situation with its many contributing factors, and not a clear or reliable predictive tool. If and when an [[influenza pandemic]] arises from one of the currently circulating pre-pandemic strains of [[H5N1|Asian lineage HPAI A(H5N1)]], the mortality rates for the resulting human adapted [[pandemic]] strain cannot be predicted with any confidence.{{citation needed|date=August 2022}}

==Existing pre-pandemic global case fatality ratio==
The global case fatality ratio looks only to the official tally<ref>The tally may be obtained by clicking a link to the most current date shown by the [[UN]] on the [[WHO]]'s web page entitled Epidemic and Pandemic Alert and Response (EPR) {{cite web|url=https://www.who.int/csr/disease/avian_influenza/country/en/ |title=WHO &#124; Confirmed Human Cases of Avian Influenza A(H5N1) |access-date=2006-04-26 |url-status=dead|archive-url=https://web.archive.org/web/20060423065725/http://www.who.int/csr/disease/avian_influenza/country/en/ |archive-date=2006-04-23 }}</ref> of cases confirmed by the [[WHO]]. It takes no account of other cases, such as those appearing in press reports.  Nor does it reflect any estimate of the global extent of mild, asymptomatic,<ref>http://www.medpagetoday.com/InfectiousDisease/URItheFlu/tb/5964   Options For Influenza Control VI (Conference, Toronto Canada, June 18, 2007)
     Even those who were in close contact with both infected birds and infected people showed no sign of ever having been infected, Dr. Dejpichai and colleagues found.
     The study is consistent with findings in Hong Kong, China, and Cambodia, which showed viral seroprevalence of no more than 10% among poultry workers and people living in villages where H5N1 outbreaks occurred, she said.
     But it contradicts a population-based study in Vietnam, published last year, that concluded that mild cases of the virus were likely to be common. (see Mild Avian Flu Transmission May Be Common) http://www.medpagetoday.com/InfectiousDisease/URItheFlu/tb/2450

Primary source: Archives of Internal Medicine
Source reference:
{{cite journal |vauthors=Thorson A, Petzold M, Nguyen TK, Ekdahl K |title=Is exposure to sick or dead poultry associated with flulike illness?: a population-based study from a rural area in Vietnam with outbreaks of highly pathogenic avian influenza |journal=Arch. Intern. Med. |volume=166 |issue=1 |pages=119–23 |date=January 2006 |pmid=16401820 |doi=10.1001/archinte.166.1.119 |doi-access=free }}
{{blockquote|"... 45 478 randomly selected ([[cluster sampling]]) inhabitants. Household representatives were asked screening questions about exposure to poultry and flulike illness ... }}
{{blockquote|... A dose-response relationship between poultry exposure and flulike illness was noted: poultry in the household (odds ratio, 1.04; 95% confidence interval, 0.96-1.12), sick or dead poultry in the household but with no direct contact (odds ratio, 1.14; 95% confidence interval, 1.06-1.23), and direct contact with sick poultry (odds ratio, 1.73; 95% confidence interval, 1.58-1.89). The flulike illness attributed to direct contact with sick or dead poultry was estimated to be 650 to 750 cases. }}{{blockquote|CONCLUSIONS: Our epidemiological data are consistent with transmission of mild, highly pathogenic avian influenza to humans and suggest that transmission could be more common than anticipated, though close contact seems required. Further microbiological studies are needed to validate these findings."}}

But note the discussion and critique
''New Study of Bird Flu Raises Important Issues''
January 9, 2006
{{cite web|url=http://www.acsh.org/factsfears/newsID.685/news_detail.asp |title=Archived copy |access-date=2008-02-23 |url-status=dead|archive-url=https://web.archive.org/web/20090515074918/http://www.acsh.org/factsfears/newsID.685/news_detail.asp |archive-date=2009-05-15 }}
{{blockquote|"Are the conclusions of this one study enough to warrant rethinking the current bird-flu paradigm and considering this threat similar to that posed by the similar "Asian Flu," as opposed to the deadly "Spanish Flu" pandemic? (The Asian Flu pandemic occurred in 1957-8, and caused millions of cases but much lower mortality than the global "Spanish flu" of 1918-9, which killed over 20 million.) Unfortunately, no. While, on its surface, the new study seems to point in that direction, a closer analysis of the study reveals several weaknesses, the most important of which is that no blood samples were taken. As a result, no data on antibody status could be collected, nor could there be any confirmation of a specific viral cause of the reported ailments.}}
{{blockquote|Indeed, it is just as likely that the illnesses sustained by the rural Vietnamese were caused by some other virus, not a bird-type flu at all — or that if their ailments were due to bird contact, that the cause was any number of bird flu variants, rather than the lethal H5N1 strain being studied intensively now. ... " }}</ref> or other cases which are undiagnosed, unreported by national governments to the [[WHO]], or for any reason cannot be confirmed by the [[WHO]].  While the [[WHO]]'s case count is clearly the most authoritative, these unavoidable limitations result in an unknown number of cases being omitted from it. The problem of overlooked but genuine cases is emphasized by occasional reports in which later serology reveals antibodies to the H5N1 infection in the blood of persons who were never known to have bird flu, and who then are confirmed by the [[WHO]] only retroactively as "cases."  Press reports of such cases, often poultry handlers, have appeared in various countries.  The largest number of asymptomatic cases was  confirmed in 2006 among Korean workers who had assisted in massive culls of H5N1-infected poultry.<ref>
{{cite web
 | url=http://www.cidrap.umn.edu/cidrap/content/influenza/avianflu/news/sep2106korea.html
 | title=Five Koreans had H5N1 virus but no illness (21 September 2006)
 | publisher=CIDRAP
 | access-date=2006-08-23
}}</ref> This relatively benign Korean strain of H5N1 has died out, and the remaining strains of H5N1 have a higher case fatality rate in humans.

Unconfirmed cases have a potentially huge impact on the case fatality ratio. This mathematical impact is well understood by epidemiologists, and is easy to see in theory.  For example, if for each confirmed case reported by the WHO we assume that there has been another mild and unreported case, the actual global number of cases would be double the current number of [[WHO]]-confirmed cases.  The fatality ratio for H5N1 infections would then be calculated as the same number of deaths, but divided by a doubled number for total cases, resulting in a hypothetical death ratio of half the currently reported fatality ratio. Such a result would indicate to epidemiologists that the world was confronting an H5N1 virus that is less-lethal than currently assumed, although possibly one that was more contagious and difficult to track.<ref>http://www.recombinomics.com/News/10030701/H5N1_Jakarta_Cluster.html  H5N1 Cluster Raises Surveillance Concerns In Indonesia
Recombinomics Commentary
October 3, 2007
(Suggests Indonesian cases may be less lethal than feared, but more prevalent due to various undersampling errors.){{better source needed|reason=This reference needs to be replaced with a better one. Recombinomics and Henry L Niman are not credible sources according to the UN experts on bird flu.|date=July 2022}}</ref>

A case-fatality ratio based on an accurate and all-inclusive count of cases would be invaluable, but unfortunately it is impossible to attain. The ability to diagnose every case of H5N1 as it arises does not exist.  A few small reported studies have attempted to gather preliminary data on this crucial statistic, by carrying out systematic blood testing of neighbors and contacts of fatal cases in villages where there had been confirmed H5N1 fatalities. In most cases, this testing failed to turn up any overlooked mild cases, though in at least one study mild overlooked cases were identified.<ref>
{{cite news
 |
url=http://www.cidrap.umn.edu/cidrap/content/influenza/avianflu/news/jan2508bangkok-jw.html
 | title=Cambodian study hints at subclinical H5N1 cases
 | date=January 25, 2008
 | publisher=CIDRAP
}}</ref>
<ref>{{cite news
 | url=http://www.cidrap.umn.edu/cidrap/content/influenza/avianflu/news/mar2706cases.html
 | title=Mild H5N1 cases weren't found missed in Cambodian outbreak study
 | date=March 27, 2006
 | publisher=CIDRAP
}}</ref><ref>{{cite news
 | url=http://www.cidrap.umn.edu/cidrap/content/influenza/avianflu/news/sep0706cambodia.html
 | title=Cambodian study suggests mild H5N1 cases are rare
 | date=September 7, 2006
 | publisher=CIDRAP
}}</ref> These methodical studies of contacts provide significant evidence that the high death rate among confirmed cases in the villages where these studies were carried out cannot be simply attributed to a wholesale failure to detect mild cases. Unfortunately, these studies are likely to remain too few and sketchy to define the complex situation worldwide regarding the lethality of the varying H5N1 clades.  The testing and reporting necessary for mass serology studies to determine the incidence of overlooked cases for each existing clade and strain of H5N1 worldwide would be prohibitively costly.{{citation needed|date=August 2022}}

Hence the precise allocation of infections by the various H5N1 clades across the spectrum including lethal, serious, mild, and asymptomatic cases is likely to remain unknown in both humans and the hundreds of other species it can infect. Scientists are very concerned about what we do know about [[H5N1]]; but even more concerned about the vast amount of important data that we don't know about H5N1 and its future mutations.{{citation needed|date=August 2022}}

==Demographic characteristics==

Review of patient ages and outcomes reveals that H5N1 attacks are especially lethal in pre-adults and young adults, while older victims tend to have milder attacks and to survive.<ref>{{cite journal |title=Epidemiology of WHO-confirmed human cases of avian influenza A(H5N1) infection |journal=Wkly. Epidemiol. Rec. |volume=81 |issue=26 |pages=249–57 |date=June 2006 |pmid=16812929 |url=https://www.who.int/wer/2006/wer8127/en/index.html
|archive-url=https://web.archive.org/web/20060904140943/http://www.who.int/wer/2006/wer8127/en/index.html
|url-status=dead
|archive-date=September 4, 2006
|quote=The median age of confirmed cases was 20 years. The age of cases ranged from 3 months to 75 years (n = 202). Half of the cases occurred among people aged <20 years; 90% occurred among those aged <40 years (Fig. 2). Among cases aged <10 years, 21 children were aged <5 years and 32 children were aged between 5 years and 9 years.}}</ref><ref>{{cite web
 |url=http://www.wpro.who.int/NR/rdonlyres/299B9BDC-67D8-40AC-81BE-0BC8D38E296A/0/S2_0814.jpg
 |title=Human Avian Influenza A(H5N1) Cases by Age Group and Country
 |url-status=dead
 |archive-url=https://web.archive.org/web/20070629070843/http://www.wpro.who.int/NR/rdonlyres/299B9BDC-67D8-40AC-81BE-0BC8D38E296A/0/S2_0814.jpg
 |archive-date=2007-06-29
}}</ref><ref>{{cite journal
|vauthors=Smallman-Raynor M, Cliff AD |title=Avian influenza A (H5N1) age distribution in humans |journal=Emerging Infect. Dis. |volume=13 |issue=3 |pages=510–2 |date=March 2007 |pmid=17552119 |pmc=2141519 |doi=10.3201/eid1303.060849
}}</ref> This is consistent with the frequent development of a [[cytokine storm]] in the affected.<ref>{{cite news
 | url=https://www.nytimes.com/2006/09/11/world/11flu.html
 | title=Immediate Treatment Needed for Bird Flu Cases, Study Says
 | work=New York Times
 | date=September 11, 2006
| first=Donald G.
| last=McNeil Jr
| access-date=May 4, 2010
}}</ref>  Few persons over 50 years of age seem to have become infected by H5N1, and very few have died following an H5N1 attack.<ref>U.N. chart, "Human Avian Influenza (H5N1) Cases by Age Group and Outcome"  {{cite web|url=http://www.wpro.who.int/NR/rdonlyres/FD4AC2FD-B7C8-4A13-A32C-6CF328A0C036/0/Slide4.jpg |title=Archived copy |access-date=2008-04-15 |url-status=dead|archive-url=https://web.archive.org/web/20081119194526/http://www.wpro.who.int/NR/rdonlyres/FD4AC2FD-B7C8-4A13-A32C-6CF328A0C036/0/Slide4.jpg |archive-date=2008-11-19 }}</ref> Instead, the age-fatality curve of H5N1 influenza attacks in humans resembles that of the 1918 Spanish pandemic flu, and is the opposite of the mortality curve of seasonal flu strains, since seasonal influenza preferentially kills the elderly and does not kill by [[cytokine storm]]. An additional factor which may be active is that H1N1 was the predominant human flu circulating from 1918 until 1957 when the H2N2 strain emerged.<ref name=Gioia08>{{cite journal  |vauthors=Gioia C, Castilletti C, Tempestilli M, etal |title=Cross-subtype immunity against avian influenza in persons recently vaccinated for influenza |journal=Emerging Infect. Dis. |volume=14 |issue=1 |pages=121–8 |date=January 2008 |pmid=18258091 |pmc=2600140 |quote=We also observed that seasonal vaccination is able to raise neutralizing immunity against influenza (H5N1) in a large number of donors. |doi=10.3201/eid1401.061283}}</ref>  Hence those over 50 years old have had the opportunity to be exposed to H1N1, and to develop some immune response to the N1 group contained in that human form of flu. Likewise, annual flu vaccination includes inoculation against a type-A human H1N1 flu, leading to the possibility that the annual flu shot or Flumist inoculation might confer some immunity against H5N1 bird flu infection, and indeed testing the blood of volunteers to look for immune response to H5N1 found that some blood samples showed immunity, but more of the blood samples of persons who had received the flu shot showed an immune response.<ref name=Gioia08/>

Another factor complicating any attempt to predict lethality of an eventual pandemic strain is the variability of the resistance of human victims to the pathogen.  Many people with the current H5N1 influenza have been blood relatives (but rarely<ref>{{cite journal  |vauthors=Olsen SJ, Ungchusak K, Sovann L, etal |title=Family clustering of avian influenza A (H5N1) |journal=Emerging Infect. Dis. |volume=11 |issue=11 |pages=1799–1801 |date=November 2005 |pmid=16422010 |pmc=3367331 |doi=10.3201/eid1111.050646}}<br />listed 15 family clusters, in which three included a husband and wife pair. (Only two of these pairs had all four members actually confirmed as H5N1 positive.)
The "blood relative theory" is, so far, too weak to be called a theory. It is an observation, with some reasoning that could support it as a hypothesis (the genetic tendency possibility, for instance).</ref> spouses) of other victims.  Though this observation seemed to suggest that a familial genetic susceptibility might have played a role in human infection,<ref>{{cite journal
 | title=Three Indonesian Clusters of H5N1 Virus Infection in 2005
 | author=I. Nyoman Kandun| journal=NEJM
 | volume=355
 | pages=2186–94
 | date=November 23, 2006
 | issue=21
 | doi=10.1056/NEJMoa060930
 | pmid=17124016
|display-authors=etal| hdl=10722/45196| hdl-access=free
 }}</ref> a study by researchers at the Harvard School of public health noted no significant familial pattern of infection.<ref>{{cite journal |author=Pitzer VE, Olsen SJ, [[Carl Bergstrom|Bergstrom CT]], Dowell SF, Lipsitch M |title=Little evidence for genetic susceptibility to influenza A (H5N1) from family clustering data |journal=Emerging Infect. Dis. |volume=13 |issue=7 |pages=1074–6 |date=July 2007 |pmid=18214184 |pmc=2878232 |quote=Abstract The apparent clustering of human cases of influenza A (H5N1) among blood relatives has been considered as evidence of genetic variation in susceptibility. We show that, by chance alone, a high proportion of clusters are expected to be limited to blood relatives when infection is a rare event. |doi=10.3201/eid1307.061538
}}</ref>  Clearly, those whose immune systems are best able to fight off the virus are the most likely to survive a pandemic.  Those with impairment of the needed immune function, whether from familial genetics or from AIDS, have poorer chances.  Moreover, the health care system is generally expected to be overwhelmed throughout a pandemic.  Persons needing access to medical care, whether for influenza or for unrelated serious maladies, are unlikely to receive the accustomed care, and without it their survival chances will be reduced.{{citation needed|date=August 2022}}

==Predicting pandemic mortality rate==
Although the actual rate of mortality during a pandemic is unknowable in advance, it is pressing to predict the possible ranges for that lethality responsibly in advance. The pre-pandemic case fatality ratio of over 50% provides a grim backdrop for the fact that the currently circulating H5N1 strains have certain genetic similarities with the [[Spanish Influenza]] pandemic virus. In that pandemic, 50 million to 100 million people worldwide were killed during about a year in 1918 and 1919.<ref>
{{cite book
 | url=http://www.nap.edu/books/0309095042/html/7.html
 | title=The Threat of Pandemic Influenza: Are We Ready? Workshop Summary (2005)
 | publisher=NAP
 | access-date=2006-08-21
| doi=10.17226/11150
 | pmid=20669448
 | year=2005
 | isbn=978-0-309-09504-4
 | author1=Institute of Medicine (US) Forum on Microbial Threats
 | last2=Knobler
 | first2=S. L.
 | last3=Mack
 | first3=A.
 | last4=Mahmoud
 | first4=A.
 | last5=Lemon
 | first5=S. M.
 }}</ref> The highly lethal second and third waves of the 1918 Spanish flu evolved through time into a less virulent and more transmissible human form. Although the overall fatality rate for the Spanish flu is estimated to have been 10% to 20% of the population,{{citation needed|date=March 2020}} the lethal waves of the Spanish flu are not reported to have emerged with anything like the over-50% case fatality ratio observed to date in human H5N1 infection. Studies indicating that an H5N1 pandemic may be more pathogenic than was the Spanish flu include a mouse study in which the H5N1 virus elicited significantly higher levels of pro-inflammatory cytokines in the lungs.<ref>{{cite journal
 | author=Lucy A. Perrone, Julie K. Plowden, [[Adolfo García-Sastre]], Jacqueline M. Katz, Terrence M. Tumpey
 | title=H5N1 and 1918 Pandemic Influenza Virus Infection Results in Early and Excessive Infiltration of Macrophages and Neutrophils in the Lungs of Mice
 | journal=PLOS Pathogens
 | volume=4
 | issue=8
 | pages=e1000115
 | doi=10.1371/journal.ppat.1000115
 | pmid=18670648
 | pmc=2483250
 | date=August 2008
 | doi-access=free
 }}</ref>

{{update|date=March 2020}} A human H5N1 pandemic might emerge with initial lethality resembling that over-50% case fatality now observed in pre-pandemic H5N1 human cases, rather than with the still-high 1-2% seen with the Spanish flu or with the lower rates seen in the two more recent influenza pandemics.<ref name="Billions_could_die">{{Cite web |date=2006 |title=WHO PDF |url=https://www.who.int/csr/resources/publications/influenza/WHO_CDS_EPR_GIP_2006_3C.pdf}}</ref> As a [[WHO]] working group noted,

'''Determinants of virulence and transmissibility.'''
:... One especially important question is whether the H5N1 virus is likely to retain its present high lethality should it acquire an ability to spread easily from person to person, and thus start a pandemic. Should the virus improve its transmissibility by acquiring, through a reassortment event, [[H5N1 genetic structure|internal human genes]], then the lethality of the virus would most likely be reduced. However, should the virus improve its transmissibility through adaptation as a wholly avian virus, then the present high lethality could be maintained during a pandemic.<ref>Influenza research at the human and animal interface

Report of a WHO working group

Geneva, Switzerland

21–22 September 2006

Numbered page 15, (19th page including non-numbered introductory page) (emphasis original)
http://www.who.int/csr/resources/publications/influenza/WHO_CDS_EPR_GIP_2006_3C.pdf</ref>

The U.S. [[Centers for Disease Control and Prevention|CDC]] presents a similarly sobering conclusion authored by [[Robert G. Webster]] et al.:

:... We cannot afford simply to hope that human-to-human spread of H5N1 will not happen and that, if it does, the pathogenicity of the virus will attenuate. Notably, the precursor of the severe acute respiratory syndrome (SARS)–associated coronavirus (31) repeatedly crossed species barriers, probably for many years, before it finally acquired the capacity for human-to-human transmission, and its pathogenicity to humans was not attenuated. We cannot wait and allow nature to take its course. SARS was interrupted by early case detection and isolation, but influenza is transmissible early in the course of the disease and cannot be controlled by similar means.<ref name=Webster06>{{cite journal |vauthors=Webster RG, Peiris M, Chen H, Guan Y |title=H5N1 outbreaks and enzootic influenza |journal=Emerging Infect. Dis. |volume=12 |issue=1 |pages=3–8 |date=January 2006 |pmid=16494709 |pmc=3291402 |doi=10.3201/eid1201.051024}}</ref>

Although some mammalian adaptations have been noted, [[H5N1]] remains better adapted for infecting [[bird]]s than mammalian hosts,<ref>{{cite journal  |vauthors=Yen HL, Lipatov AS, Ilyushina NA, etal |title=Inefficient transmission of H5N1 influenza viruses in a ferret contact model |journal=J. Virol. |volume=81 |issue=13 |pages=6890–8 |date=July 2007 |pmid=17459930 |pmc=1933302 |doi=10.1128/JVI.00170-07 |quote=Our results suggest that despite their receptor binding affinity, circulating H5N1 viruses retain molecular determinants that restrict their spread among mammalian species.}}</ref> which is why the disease it causes is called a [[Avian influenza|bird flu]].  No [[pandemic]] strain of H5N1 has yet been found. The precise nature and extent of the genetic alterations that might change one of the currently circulating [[avian influenza]] strains into a [[human flu]] strain cannot be known in advance.

While many of the current H5N1 strains circulating in birds can generate a dangerous [[cytokine storm]] in healthy adult humans, the ultimate pandemic strain might arise from a less-lethal strain, or its current level of lethality might be lost in the adaptation to a human host.<ref name="reuters">{{Cite news|url=https://uk.reuters.com/article/idUKT24954320070201 |archive-url=https://web.archive.org/web/20170213164019/http://uk.reuters.com/article/idUKT24954320070201 |url-status=dead |archive-date=February 13, 2017 |title=Don't ignore less virulent bird flu strains-experts |newspaper=[[Reuters]] |date=February 1, 2007 |author=Tan Ee Lyn}}</ref><ref>{{cite report
 | url=https://www.who.int/csr/resources/publications/influenza/WHO_CDS_EPR_GIP_2006_3C.pdf
 | title=Influenza research at the human and animal interface
 | author=WHO working group on influenza research at the human and animal interface
 | date=21–22 September 2006
 | pages=15
 | publisher=[[World Health Organization]]
 | access-date=12 April 2007
}}
([https://archive.today/20121212231913/http://www.who.int/entity/csr/resources/publications/influenza/WHO_CDS_EPR_GIP_2006_3/en/index.html alternate version])</ref><ref>{{cite web
 | url=http://www.cidrap.umn.edu/cidrap/content/influenza/panflu/news/sep1106storm.html
 | title=Clinical study points to cytokine storm in H5N1 cases
 | date=September 11, 2006
 | publisher=CIDRAP News
}}</ref><ref>{{cite journal
 | title=Fatal outcome of human influenza A (H5N1) is associated with high viral load and hypercytokinemia
 | journal=Nature Medicine
 | author=Menno D de Jong| date=September 10, 2006
| doi= 10.1038/nm1477
 | volume=12
 | pages=1203–7
 | pmid=16964257
 | issue=10|display-authors=etal| pmc=4333202}}</ref><ref>http://www.cidrap.umn.edu/cidrap/content/influenza/avianflu/news/jul1607cytokine.html Study: Inhibiting cytokine response might not reverse H5N1 infections</ref>

If H5N1 mutates so that it can jump from human to human, while maintaining a relatively high level of mortality, how many people could die? [[Risk communication]] analysts Peter M. Sandman and Jody Lanard give a round-up of the various estimates:

{{blockquote|Worldwide mortality estimates range all the way from 2-7.4 million deaths (the "conservatively low" pandemic influenza calculation of a flu modeling expert at the U.S. Centers for Disease Control and Prevention) to 1000 million deaths (the bird flu pandemic prediction of one Russian virologist). The estimates of most H5N1 experts range less widely but still widely. In an H5N1 pandemic, the experts guess that somewhere between a quarter of us and half of us would get sick, and somewhere between one percent and five percent of those who got sick would die — the young and hale as well as the old and frail. If it's a quarter and one percent, that's 16 million dead; if it's a half and five percent, it's 160 million dead. Either way it's a big number.<ref>{{cite web
 | url=http://www.psandman.com/col/pandemic.htm
 | title=Pandemic Influenza Risk Communication: The Teachable Moment
 |author1=Peter M. Sandman |author2=Jody Lanard | date=December 4, 2004
 | access-date=2006-10-08
}}</ref>}}

The renowned virus expert [[Robert G. Webster]] provided perhaps the most extreme estimate when he acknowledged in March 2006 that H5N1 has the theoretical capacity to mutate into a form that could kill one half of the human population,<ref>{{cite journal
 | url=http://www.scs.carleton.ca/~soma/biosec/readings/influenza/influenza.html
 | title= The world is teetering on the edge of a pandemic that could kill a large fraction of the human population
 | author=[[Robert G. Webster]], Elizabeth Jane Walker
 | date=March–April 2003
 | access-date=2006-10-08
 | journal=American Scientist
 | volume=91
 | issue=2
 | pages=122
 | doi= 10.1511/2003.2.122
| url-access=subscription
 }}</ref> stating, "Society just can't accept the idea that 50 percent of the population could die.  And I think we have to face that possibility".<ref>{{cite news
 | url=https://abcnews.go.com/WNT/AvianFlu/story?id=1724801
 | publisher=ABC News
 | date=March 14, 2006
 | title=Renowned Bird Flu Expert Warns: Be Prepared
 | access-date=2006-10-08
}}</ref>

===Genetic factors===
{{main|H5N1 genetic structure}}

[[H5N1]] may cause more than one [[influenza pandemic]] as it is expected to continue mutating in birds regardless of whether humans develop [[herd immunity]] to a future pandemic strain.<ref>{{cite journal
 | title=H5N1 Influenza — Continuing Evolution and Spread
 |author1=Robert G. Webster |author2=Elena A. Govorkova, M.D.
 | journal=NEJM
 | volume=355
 | pages=2174–2177
 | date=November 23, 2006
 | issue=21
 | doi=10.1056/NEJMp068205
 | pmid=17124014
| doi-access=free
 }}</ref> Influenza pandemics from its genetic offspring may include [[Influenzavirus A|influenza A virus]] subtypes other than H5N1.<ref>{{cite journal |vauthors=Taubenberger JK, Morens DM |title=1918 Influenza: the mother of all pandemics |journal=Emerging Infect. Dis. |volume=12 |issue=1 |pages=15–22 |date=January 2006 |pmid=16494711 |doi=10.3201/eid1201.050979 |pmc=3291398 }}</ref> While genetic analysis of the H5N1 virus shows that influenza pandemics from its genetic offspring can easily be far more lethal than the [[Spanish flu]] pandemic,<ref name=preparation>{{cite journal|title=Why is the world so poorly prepared for a pandemic of hypervirulent avian influenza?|date=December 2006 |journal=Microbial Ecology in Health and Disease |volume=18 |issue=3–4 |pages=113–132|doi=10.1080/08910600600866544 |s2cid=218565955 |last1=Christophersen |first1=Olav Albert |last2=Haug |first2=Anna |doi-access=free }}</ref> planning for a future influenza pandemic is based on what can be done and there is no higher [[Pandemic Severity Index]] level than a Category 5 pandemic which, roughly speaking, is any pandemic as bad the Spanish flu or worse; and for which ''all'' intervention measures are to be used.<ref>{{cite web | url=http://www.cidrap.umn.edu/cidrap/content/influenza/panflu/news/feb0107pandemic.html|access-date=2007-02-03|date=February 1, 2007|last=Roos|first=Robert|author2=Lisa Schnirring|title=HHS ties pandemic mitigation advice to severity|publisher=[[University of Minnesota]] Center for Infectious Disease Research and Policy (CIDRAP)}}</ref>

There "is evidence of at least three independent virulence factors connected with three different [[gene]]s. It is highly unlikely that all of the high-virulence [[allele]]s will simultaneously mutate and disappear if and when the [[haemagglutinin]] gene changes so as to make the haemagglutinin molecule better adapted for the human-type (alpha-2,6-linked) receptor (which is a necessary prerequisite in order that a pandemic with H5N1 virus may start). It is more probable that evolutionary adaptation of the haemagglutinin of H5N1 viruses to the human-type receptor will happen without any simultaneous change in those other genetic properties that now are important for explaining the exceptionally high [[virulence]] of certain strains of avian-adapted H5N1 influenza virus. The change of the haemagglutinin molecule from [[Avian influenza|avian adaptation]] to [[human flu|human adaptation]] must be expected to act as an additional virulence factor because it will enhance the total number of cells that can be infected (per host organism), increase the total rate of virus replication and potentiate the effects of the other virulence factors already present."<ref name=preparation/> The [[H5N1 genes]] work together in ways we don't yet understand.<ref name=ferret>{{cite journal  |vauthors=Maines TR, Chen LM, Matsuoka Y, etal |title=Lack of transmission of H5N1 avian-human reassortant influenza viruses in a ferret model |journal=Proc Natl Acad Sci USA |volume=103 |issue=32 |pages=12121–6 |date=August 2006 |pmid=16880383 |pmc=1567706 |doi=10.1073/pnas.0605134103 |bibcode=2006PNAS..10312121M |doi-access=free }}</ref> [[Influenza research]] is continuing. The genetic factors that make H5N1 so deadly are only partly understood. Known factors involve the surface [[antigen]] encoding gene segments H ([[hemagglutinin]])<ref>{{cite journal  |vauthors=Tumpey TM, Maines TR, Van Hoeven N, etal |title=A two-amino acid change in the hemagglutinin of the 1918 influenza virus abolishes transmission |journal=Science |volume=315 |issue=5812 |pages=655–9 |date=February 2007 |pmid=17272724 |doi=10.1126/science.1136212 |bibcode=2007Sci...315..655T |s2cid=24627071 }}</ref> and N ([[neuraminidase]]) genes (causing it to be [[H5N1]] for example), as well as the matrix M2 gene, and the polymerase genes.

:"In order to cause a pandemic, H5N1 viruses will have to acquire the ability to transmit efficiently from person to person. The H5 [[hemagglutinin]] (HA) is found in influenza viruses that typically infect avian species, so efficient person-to-person spread could happen if the H5N1 virus reassorts, or exchanges genes, with circulating human influenza viruses giving rise to a virus with the H5 HA (to which the population is not immune) in a gene constellation that confers the property of transmissibility. Alternatively, efficient person-to-person spread could occur if the H5N1 virus evolves and adapts to more efficient replication and transmissibility in the human population."<ref>{{cite journal |vauthors=Subbarao K, Luke C |title=H5N1 viruses and vaccines |journal=PLOS Pathog. |volume=3 |issue=3 |pages=e40 |date=March 2007 |pmid=17335350 |pmc=1808069 |doi=10.1371/journal.ppat.0030040 |doi-access=free }}</ref>

A change of just two genes identified in laboratory testing appears to substantially increase the affinity of H5N1 for binding with human cell surface receptors.<ref name=SciAug07/>

[[Neuraminidase]] is an [[antigen]]ic [[glycoprotein]] [[enzyme]] found on the surface of the [[influenza]] [[virus]]es. It helps the release of progeny viruses from infected cells. Flu drugs Tamiflu and Relenza  work by inhibiting some strains of [[neuraminidase]]. They were developed based on N2 and N9. "In the N1 form of the protein, a small segment called the 150-loop is inverted, creating a hollow pocket that does not exist in the N2 and N9 proteins. [...] When the researchers looked at how existing drugs interacted with the N1 protein, they found that, in the presence of neuraminidase inhibitors, the loop changed its conformation to one similar to that in the N2 and N9 proteins."<ref>[http://www.scidev.net/gateways/index.cfm?fuseaction=readitem&rgwid=4&item=News&itemid=3050&language=1 Scidev.net News] {{Webarchive|url=https://web.archive.org/web/20080205161953/http://www.scidev.net/gateways/index.cfm?fuseaction=readitem&rgwid=4&item=News&itemid=3050&language=1 |date=2008-02-05 }} article ''Bird flu protein's 'pocket' could inspire better drugs'' published August 16, 2006</ref>

The amino acid substitution (Ser31Asn) in the M2 gene in some H5N1 genotypes is associated with [[amantadine]] resistance which increases lethality. However the pathogenicity of H5N1/97 was related to the nonstructural (NS) gene. NS codes for two nonstructural proteins (NS1 and NEP). The NS1 gene of the highly pathogenic avian H5N1 viruses circulating in poultry and waterfowl in Southeast Asia is believed to be responsible for an enhanced proinflammatory cytokine response (especially TNFa) induced by these viruses in human macrophages. H5N1 NS1 is characterized by a single amino acid change at position 92. By changing the amino acid from glutamic acid to aspartic acid, researchers were able to abrogate the effect of the H5N1 NS1. This single amino acid change in the NS1 gene greatly increased the pathogenicity of the H5N1 influenza virus. This is one genetic factor in why H5N1 is so deadly.{{citation needed|date=August 2022}}

Polymerase encoding gene segments are also implicated in why H5N1 is so deadly. PA genes code for the PA protein, which is a critical component of the viral polymerase. The PB1 gene codes for the PB1 protein and the PB1-F2 protein. The PB1-F2 protein probably contributes to viral pathogenicity and might have an important role in determining the severity of pandemic influenza. Until H5N1, all known avian influenza viruses had a Glu at position 627, while all human influenza viruses had a lysine.  Recently, some 75% of H5N1 human virus isolates identified in Vietnam had a mutation consisting of Lysine at residue 627 in the PB2 protein; a change believed associated with high levels of virulence.

===Areas of research===
Areas of research to identify the likelihood of rapid or slow evolution to human contagion, or for predicting the greater or lesser likelihood of a rather lethal human-adapted influenza include:{{citation needed|date=August 2022}}
* bird species susceptibility
* bird migration paths
* [[cell-based vaccine]] development
* adjuvant testing
* human vaccine clinical trials
* bird vaccine testing and use
* computer simulations of pandemic spread patterns (e.g. will grounding flights help?)
* detailed shape and gene code analysis of each of the RNA strands for as many flu virus strains as possible and making them available on a database for study
* wild bird testing for flu viruses
* testing humans for asymptomatic H5N1 infection
* training exercises in case of a pandemic

Computer simulations and direct gene manipulation have yielded inconclusive results.

===Scientific advances===
Scientific advances may attenuate probable lethality. The genetic lethality potential of the initial [[flu pandemic]] strain is only one important factor in determining the ultimate outcome in number of human lives lost. Another factor that grows potentially more important with the passage of time is human preparation. For example, no [[influenza vaccine]] specific to [[H5N1]] could be produced when it emerged in [[Hong Kong]] in 1997, because it was lethal to eggs. Reverse DNA techniques have since made a vaccine possible, and several H5N1 vaccines have been tested and are in production in at least limited quantities. Vaccine development and production facilities are being ramped up, and possible pre-pandemic vaccines are being produced and studied. If a human pandemic does not emerge in the next few years, its eventual emergence may become almost a non-event if a very-effective pre-pandemic vaccine has prepared the population with sufficient [[herd immunity]] to blunt its lethality. Indeed, if there is sufficient immunity to stop it at the source, it will not become pandemic.{{citation needed|date=August 2022}}

As long as the likelihood of protecting the population continues to rise with the passage of time, that likelihood becomes an increasingly important factor in predicting the loss of lives and the amount of economic dislocation that will ultimately occur. In light of human potential to develop herd immunity via vaccination in advance of a pandemic strain, the time that it allows us to do so before it evolves may become as crucial or more crucial to the measure of damage it causes than its own lethality and contagiousness.{{citation needed|date=May 2023}}

Among the more attractive alternatives available for reducing mortality is vaccine stockpiling and prepandemic vaccination. "Human H5N1 vaccines are currently available and can induce heterotypic immunity. WHO and governments should give urgent consideration to the use of these vaccines for the priming of individuals or communities who would be at greatest risk of infection if an H5N1 influenza pandemic were to emerge."<ref>{{cite journal |vauthors=Jennings LC, Monto AS, Chan PK, Szucs TD, Nicholson KG |title=Stockpiling prepandemic influenza vaccines: a new cornerstone of pandemic preparedness plans |journal=Lancet Infect Dis |volume=8 |issue=10 |pages=650–8 |date=October 2008 |pmid=18922487 |doi=10.1016/S1473-3099(08)70232-9 }}</ref>  Death associated with influenza A viruses "is usually mediated by superinfection with bacteria, mainly Streptococcus pneumoniae.",<ref>Stegemann, Dahlberg, Kröger, Gereke, Bruder, Henriques-Normark, '''Increased Susceptibility for Superinfection with Streptococcus pneumoniae during Influenza Virus Infection Is Not Caused by TLR7-Mediated Lymphopenia''' http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0004840</ref> suggesting that lethality may be reduced by vaccination against pneumonia.

===Preparation===

Among others, the Secretary of the [[United States Department of Health and Human Services (HHS)]] has repeatedly pointed out the key role of preparation in reducing pandemic mortality, including as examples research in cell- and DNA-based vaccines, as well as stockpiling available vaccines and antivirals and increasing vaccine manufacturing capacity.<ref>{{Cite web|url=https://www.cdc.gov/flu/pandemic-resources/index.htm|archiveurl=https://web.archive.org/web/20090115170040/http://www.pandemicflu.gov/plan/panflureport6.html|url-status=dead|title=Pandemic Influenza &#124; Pandemic Influenza (Flu) &#124; CDC|date=May 12, 2020|archivedate=January 15, 2009|website=www.cdc.gov}}</ref>

==Planning reports==
Governments and other organizations at many levels and in many places have produced "planning" reports that, among other things, have offered speculation on the mortality rate of an eventual H5N1 pandemic. That speculation has varied widely.<ref>{{usurped|1=[https://web.archive.org/web/20070301091447/http://www.upmc-cbn.org/report_archive/2006/06_June_2006/cbnreport_060206.html CBN Report: Severe Pandemic Planning Assumptions May Be Too Low]}}</ref>  One such report stated that "over half a million Americans could die and over 2.3 million could be hospitalized if a moderately severe strain of a pandemic flu virus hits the U.S.".<ref>{{cite web
 |publisher=Senior Journal
 |date=June 24, 2005
 |title=Pandemic Flu Projection Says More Than Half Million Could Die in U.S.
 |url=http://www.seniorjournal.com/Spotlights/FLU2005-06/5-06-24PandemicProjections.htm
 |url-status=dead
 |archive-url=https://web.archive.org/web/20070309203425/http://www.seniorjournal.com/Spotlights/FLU2005-06/5-06-24PandemicProjections.htm
 |archive-date=March 9, 2007
}}</ref> No one knew if "moderately severe" was an accurate guess or not. A report entitled ''A Killer Flu?''<ref>{{cite web
 | url=http://healthyamericans.org/reports/flu/Flu2005.pdf
 | title=Healthy Americans Flu 2005 report PDF
| date=26 April 2019
}}</ref> projected that, with an assumed (guessed) contraction rate of just 25%, and with a severity rate as low as that of the two lowest severity flu pandemics of the 1900s, a modern influenza A pandemic would cause 180 thousand deaths in the US, while a pandemic equaling the 1918 Spanish flu in level of lethality would cause one million deaths in the US. Again, the report offered no evidence that an emerging H5N1 flu pandemic would be between these figures.<ref>{{cite magazine
 | url=http://www.msnbc.msn.com/id/12610942/site/newsweek/
 | magazine=Newsweek
 | title=A Dramatic Disconnect
 | first=Jennifer| last=Barrett
 | date=May 3, 2006
 | access-date=2006-12-11
| url-status=dead
 |archive-url = https://web.archive.org/web/20061210121525/http://www.msnbc.msn.com/id/12610942/site/newsweek/ <!-- Bot retrieved archive --> |archive-date = 2006-12-10}} estimates two million dead in the US, for example</ref>

The current avian flu, in humans, is fatal in over 50% of confirmed cases.  Yet early projections like those above have assumed that such a lethal avian strain would surely lose genes contributing to its lethality in humans as it made the adaptations necessary for ready transmission in the human population. This optimistic assumption cannot be relied on. As the WHO reported in November 2006, initial outbreaks of an H5N1 pandemic could rival the current lethality of over 50%.<ref name=Billions_could_die/> Further information necessary to make an accurate projection of initial lethality of an H5N1 pandemic does not exist, as no data was collected that could show the pre-pandemic virulence in any potential flu strain until after the last pandemic of the 20th Century. There is no basis for assuming that an H5N1 pandemic will emerge with only the far lower 1-2% lethality rate of the Spanish flu, once assumed to be a worst-case scenario.  There exists no reliable prediction of the mortality rate of an H5N1 pandemic, and it would be irresponsible to confine planning to only optimistic assumptions out of step with the currently observed case fatality ratio.{{citation needed|date=August 2022}}

Although marred by unrealistically low ranges of assumed mortality, the earlier planning reports nevertheless show convincingly that we are not prepared ''even'' for a pandemic as severe as the milder pandemics of the past century.,<ref>Dr. Martin Meltzer of the Centers for Disease Control, an expert on the societal impact of diseases, warns that "There is no healthcare system anywhere in the world that can cope with even a mild pandemic like the one in 1968" Meltzer MI, Lancet Asia Forum, Singapore, May 2006</ref> let alone the much higher case fatality ratios seen more recently.

==Sources and notes==
{{Reflist}}
{{Influenza}}

[[Category:Epidemiology]]
[[Category:Influenza A virus subtype H5N1]]