Human Mortality From H5N1

Human mortality from H5N1 or the human fatality ratio from H5N1 or the case-fatality rate of H5N1 refer to the ratio of the number of confirmed human deaths resulting from confirmed cases of transmission and infection of H5N1 to the number of those confirmed cases. For example, if there are 100 confirmed cases of humans infected with H5N1 and 10 die, then there is a 10% human fatality ratio (or mortality rate). H5N1 flu is a concern due to the global spread of H5N1 that constitutes a pandemic threat. The majority of H5N1 flu cases have been reported in southeast and east Asia. The case-fatality rate is central to pandemic planning. While estimates of case-fatality (CF) rates for past influenza pandemics have ranged from about 0.1% (1957 and 1968 pandemics) to 2.5% (1918 pandemic); the official World Health Organization estimate for the current outbreak of H5N1 avian influenza to date is around 60%. While the real H5N1 CF rate (what it would be if we had perfect knowledge) could be lower (one study suggests that the real H5N1 CF rate is closer to 14–33%); it is unlikely that, if it becomes a pandemic, it will go to the 0.1–0.4% level currently embraced by many pandemic plans.

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 transmission 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.