Why should we plan for this if we aren’t even sure its going to happen. Is there really a potential for this to be a pandemic?
Almost all of our predictive models for pandemic flu are based on 1917/1918 Spanish flu (which actually originated in Kansas); the 1957/1958 pandemic and the 1968/1969 pandemic. The 1918 Spanish flu is known in virology circles as H1:N1. Genetic reconstruction has allowed us to isolate this virus from pathologic specimens collected in 1917 and 1918 and stored by the U.S. military and other organizations. This means that we can now study the actual virus H1:N1 aka the Spanish flu and compare it to the current pandemic risk H5:N1 aka Avian flu. What makes avian flu more likely to be a pandemic?
As we all know now from the media, influenza virus mutates over time. Small mutations are known as antigenic drift while large mutations are known and antigenic shift. These drifts and shifts slowly change the virus from something that the human immune system can recognize and therefore protect against to something that is novel or new to the human population – a pandemic. Antigenic drift occurs every seven years while antigenic shift occurs every 13 years. When antigenic drift and antigenic shift overlap (every 7 time 13 years plus or minus 3 years), a new and novel influenza virus is created. In short, approximately every 91 years there is a virus that the human immune system has never seen before.
In 1918 the H1:N1 strain was seen. Like all of pandemics before, it struck with a predictable infection rate (attack rate); approximately 1 in 3. Of these 1 in 3 on average in the population half would become seriously ill. Half of those would develop severe lung disease and half of those with the severe lung disease would ultimately die.
The picture changes significantly, however when you look at the infection by age group. H1:N1 caused virtually no more deaths in those over age 65 than the average flu. In fact in 1918 you were no more likely to die of the pandemic if you were over age 65 than you had been in 1915 or than you would be in 1920.
So what does that mean for the coming Avian flu (H5:N1)? Pandemics are very consistent and we can rely on seeing this one between 2006 and 2012. H5:N1 will act virtually the same H1:N1 did in 1917.
