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A new study shows that molecular similarities exist between the 2009 H1N1 influenza virus and other strains of seasonal H1N1 virus that have been circulating in the population since 1988. These results suggest that healthy adults may have a level of protective immune memory that can blunt the severity of infection caused by the 2009 H1N1 influenza virus.

Vaccination campaigns currently under way to protect populations from pandemic influenza are among the largest in the history of several countries, and numbers are growing daily. Given this scale of vaccine administration, at least some rare adverse reactions, not detectable during even large clinical trials, could occur, underscoring the need for rigorous monitoring of safety. Results to date are encouraging.