Sunday, December 12, 2004

Trying to count future flu deaths

By now many of you will have seen a variety of predictions about the death toll an influenza pandemic might exact, whether from "bird flu" or other strain to which the world's population has little immunity (discussions on this site here and here). WHO, after sounding the alarm that included death estimates from 7 million to more than 100 million, "clarified" their position in a prepared statement: "While it is impossible to accurately forecast the magnitude of the next pandemic, we do know that much of the world is unprepared for a pandemic of any size." But then WHO added that a pandemic virus could "affect between 20-50% of the total population."

The difficulty of making these predictions is well discussed in a news story from the The Center for Infectious Disease Research and Policy (CIDRAP) at the University of Minnesota. CIDRAP's Director, Michael Osterholm, has himself said a pandemic could kill 1.7 million in the US and as many as 177 million globally. The CIDRAP story notes that a Pittsburgh medical researcher and critic of WHO, Henry L. Niman, has said the global toll could even reach 1 billion, assuming the 72% mortality of recent avian influenza cases held.

Much depends, it appears, on the likely proportion of the world's population that would become infected by a highly contagious disease like influenza, the distribution of severity of disease among those infected, and the mortality rate for those with severe disease. These are three critical unknowns, making it understandable that predictions might differ. Trying to bound the estimates for these factors on the basis of past pandemics is a chancy enterprise. The world is not only very different now than in 1918 but also different than 1957 and 1968 when the last pandemics struck. But if a mutated strain of H5N1 allowed as efficient human-to-human transmission as currently circulating strains with the virulence seen in the bird-to-human cases, the higher estimates could not only be plausible, but even plausible underestimates. Consider the worst case, where half the globe's population is infected (3.2 billion people) and half of those are serious cases with 70% mortality. Niman's estimate of 1 billion would actually be 10% too small. If 20% of the US population were infected (50 million people) and half were severe cases (25 million), in order to get as low as Osterholm's 1.7 million US deaths, the mortality rate would have to be only 7%. Even with excellent medical care this would likely not be attainable with a virulent strain and epidemic conditions.

So while it is not suprising, given the uncertainties, that estimates might differ widely, I am surprised that by simply putting together the component parts, it is so easy to get numbers much higher than any of them.

In related news, Reuters is reporting that big pharma companies Aventis-Pasteur and Chiron Corporation will shortly begin human trials of an H5N1 vaccine. Having an effective vaccine would not prevent a pandemic, but it could slow its spread and lessen its impact if widespread vaccination could be accomplished. Cutting a death toll of 150 million people by a third would save 50 million lives. We do not know at this point the licensing conditions of these vaccines. Simple self-interest would dictate that these companies be required to license the vaccines gratis to allow production at multiple sites around the world and the vaccine should be provided at cost by any manufacturer. This is something that should be settled in advance rather than when a crisis is upon us.