Tuesday, October 18, 2005

"Overdue for a pandemic"

It is a commonplace for experts to say we are "overdue for a pandemic." But let's look at the evidence. There were influenza pandemics in 1889, 1918, 1957 and 1968, intervals of 29, 49, 11 and (so far) 37 years. So it is true that pandemics happen sporadically but not true they happen periodically, i.e., at regular intervals.

There is no biological or epidemiological reason they should. The emergence of a pandemic is a complicated but so far poorly understood interaction between random genetic changes in the flu virus (I include here reassortment and recombination as well as copy error), the accessibility (to the virus) of changing ecological niches and the relative genetic "distances" of differential fitness in those niches from the circulating viruses. The existence of niches depends on human and natural history, like World War I or perhaps in the present era, the special agricultural practice of industrial poultry farming, both of which crowded natural hosts together in an abnormal and catastrophic way. But those niche changes depend on completely different factors and time scales than the inherent biological processes of viral genetic variation. A pandemic comes from their interaction.

In short, we expect a flu pandemic not because we are "overdue" for one, but because once again it appears the stars are aligned in the right way: a panzootic with a novel virus, ability to infect humans with a serious disease and--although we hope not--evidence it is adapting to humans in ways that might allow it to be as efficiently transmitted as the "usual" human flu virus. Flu pandemics aren't lotteries where our number comes up. They are the result of determining factors, some of which we know or can speculate about, but most of which we don't.