History
A strain of H5N1 killed chickens in 1959 in Scotland and turkeys in 1991 in England. This strain was "highly pathogenic" (deadly to birds) but caused neither illness nor death in humans. "The precursor of the H5N1 influenza virus that spread to humans 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." In 1997, in Hong Kong, 18 humans were infected and 6 died in the first known case of H5N1 infecting humans. 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, 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. 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. Changes are occurring in H5N1 that are increasing its pathogenicity in mammals.
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. 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.— or 69%. In 2007, 59 of the 86 WHO-confirmed cases ended in death, again a 69% fatality rate. And 24 of the first 31 cases of 2008 (to April 30, 2008) have been fatal, 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. 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 Asian lineage HPAI A(H5N1), the mortality rates for the resulting human adapted pandemic strain cannot be predicted with any confidence.
Read more about this topic: Human Mortality From H5N1
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