The World Health Organization has declared the end of the pandemic caused by H1N1 influenza virus. According to Director-General Margaret Chan,
The world is no longer in phase 6 of influenza pandemic alert. We are now moving into the post-pandemic period. The new H1N1 virus has largely run its course.
As we enter the post-pandemic period, this does not mean that the H1N1 virus has gone away. Based on experience with past pandemics, we expect the H1N1 virus to take on the behaviour of a seasonal influenza virus and continue to circulate for some years to come.
According to the Director-General, levels and patterns of H1N1 transmission are now different from those observed during the pandemic. Out-of-season outbreaks are no longer being reported, and their intensity is similar to that seen during seasonal epidemics. In addition, multiple influenza viruses are being isolated in many countries, a pattern typical of many recent seasonal epidemics.
I take particular interest in what the Director-General believes did not happen:
I continue to wonder why the Director-General, and many others, feel that influenza virus must change to a more lethal form. Although the four previous influenza pandemics occurred in multiple waves of increasing lethality, there is no evidence that they are a consequence of viral mutation.
This time around, we have been aided by pure good luck. The virus did not mutate during the pandemic to a more lethal form. Widespread resistance to oseltamivir did not develop. The vaccine proved to be a good match with circulating viruses and showed an excellent safety profile.
I believe that a major selective force for viral evolution is the need to maintain efficient transmission among hosts. This may be achieved by any number of phenotypic changes, such as increases in stability and virion production. Changes in lethality might also lead to more effective transmission – for example, by inducing more severe coughing, the virus could be better transmitted among humans. But there is no genetic evidence that such changes have occurred during influenza virus pandemics.
How has the idea that influenza virus mutates to greater lethality permeated our popular culture? I don’t know the answer, but John Barry’s The Great Influenza is a prime suspect.