Increasing Lethality of H5N1 Seroprevalence and Animal Studies Agree
31 January 2006
Monotreme – at 22:34
There was much speculation in the news recently suggesting that H5N1 may be becoming less lethal. WHO case reports did not support this idea. However, cases that had not been confirmed by the WHO affiliated lab in Mill Hill were often cited as proof that H5N1 was now less lethal. However, it now appears that many of the apparent cases were in fact not infected with H5N1. The tentative case fatality rate in Turkey is now thought to be 33%, similar to what has been reported in some southeast asian countries. The issue of seroprevalence studies came up when discussing the “real” case fatality rate. DemFromCT added a table from the WHO that appeared in the New England Journal of Medicine summarizing some of the seroprevalence studies. I noticed that there was some evidence of mild cases from the samples collected from the 1997 outbreak, but almost none from the 2004 outbreak. What could this mean? One interpretation is that the virus became more lethal between 1997 and 2004. Is there any other information that would support this idea? Indeed there is. Animal studies were conducted with H5N1 strains collected from 1997 and 2004. The results? Rapid disease progression and high lethality rates in ferrets distinguished the highly virulent 2004 H5N1 viruses from the 1997 H5N1 viruses. Thus both seroprevalence and animal studies suggest the same conclusion, H5N1 became more lethal to mammals, including humans, from 1997 to 2004. There is no evidence that H5N1 has become less lethal since 2004. There may be evidence that it is becoming easier to transmit (more and larger clusters) but we must wait for proper documentation from the WHO to be sure. There is no data to suggest that H5N1 is becoming less lethal or that it must do so before becoming a pandemic strain. Fact-based planning should include the possibility that a pandemic strain of H5N1 may kill 30% or more of its victims, as terrible as that is to contemplate.