H5N1 influenza virus is a type of influenza A virus which mostly infects birds. H5N1 flu is a concern because its global spread may constitute a pandemic threat. The yardstick for human mortality from H5N1 is the case-fatality rate (CFR); the ratio of the number of confirmed human deaths resulting from 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. The IFR is central to pandemic planning. In previous influenza pandemics (not caused by H5N1), IFR estimates have ranged from to 2-3% for the 1918 pandemic, to about 0.6% for the 1957 pandemic, to 0.2% for the 1968 pandemic. Between 2003 and December 2025, the World Health Organization has recorded 993 cases of confirmed H5N1 influenza, leading to 477 deaths. The true fatality rate is presumably lower because cases with mild symptoms may not have been identified as H5N1. 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." The cases of suspected human to human transmission that continue to be found have been isolated and contained, and include transmission among members of a family in Sumatra, Indonesia in June 2006 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." 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 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". 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. Switzerland is also considering preemptive vaccination to protect the general public. 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. Significantly, mouse antibodies were 10 times less potent against the mutants than against the pre-mutated viruses.
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 Global influenza virological surveillance and the map links provided here Map Gallery Search Results Global influenza virological surveillance (in the Global Health Observatory)
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