To the Editor: The study by Dr Belongia and colleagues1 reported that the rate of serious complications associated with the pandemic influenza A 2009 (H1N1) strain was similar to that incurred during the 2007-2008 H3N2-dominated season. Because modern H3N2 influenza A viruses are arguably more pathogenic than H1N1 variants2- 4 (possibly due to “original antigenic sin,” a phenomenon in which the immune response to influenza is conditioned by childhood exposure to influenza viruses5), the findings by Belongia et al raise the question of whether the pandemic H1N1 virus is more severe than the usual H1N1 virus.
Adalja AA. Comparing Severity and Outcomes for Seasonal and 2009 H1N1 Infections. JAMA. 2011;305(1):39-40. doi:10.1001/jama.2010.1888