Scientists review fate of pandemic H1N1 flu virus

In a new commentary, scientists have reviewed the fates of previous pandemic influenza viruses in the years following a pandemic and speculated on possible future courses for the 2009 pandemic H1N1 (pH1N1) virus during the upcoming flu season and beyond.

Scientists from the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health, estimate that at least 183 million Americans (about 59% of the total US population) have some immunity to pH1N1 because they were exposed to related viruses or vaccines prior to 2009, and developed immunity following infection with the pandemic virus.

To stay in circulation in the face of such high levels of population immunity, the pH1N1 virus must adapt either through abrupt or gradual changes.

The authors briefly examined a number of earlier pandemics and trace paths taken by the causative viruses. Some — for reasons not well understood — died out, while others, like those of 1889 and 1918, returned in an explosive fashion.

Such an explosive return of pH1N1 virus is unlikely, noted the authors, because global levels of immunity are already high and will increase further through immunisation with 2010-2011 seasonal influenza vaccines, which contain the pH1N1 strain.

In the light of what is known about pH1N1, the NIAID authors expressed cautious optimism that unless it disappeared entirely, the virus would follow a route like that of the 1968 pandemic virus, that is, it would persist in a form that causes relatively few deaths.

Nevertheless, the authors cautioned against complacency. They acknowledged that many gaps remain in understanding of how a given pandemic influenza virus adapts to increased immunity in humans.

For that reason, influenza vaccination for everyone older than six months is a wise public health measure, to maintain high levels of population-wide immunity.

Immunisation with the 2010-11 seasonal flu vaccine is particularly urged for babies older than six months, children, teens and young adults, as this is the best way to protect individuals in those potentially more susceptible age groups.