Lessons from History: Methodological Problems arising from Comparing the Influenza A (H1N1) Pandemic 2009-10 to Seasonal Influenza 2010-2019 at the United States

Pandemics of human influenza are when influenza viruses that have little or no immunity become capable of transmitting from one person to another. A novel H1N1 influenza virus was discovered in children in the southwest United States in April 2009. Retroactively, it was shown that these cases were the result of an ongoing epidemic in Mexico. A number of national vaccination programs were established in response to the pandemic. Surprisingly early clinical trials data from humans have shown that one dose of nonadjuvanted pandemic flu A (H1N1) 2009 monovalent, inactivated vaccine (pMIV), has resulted in a significant seroprotective response. This is despite previous studies showing no cross-reactivity between pandemic and seasonal H1N1 viruses.