Does Seasonal Influenza Vaccination Affect the Incidence of pH1N1 Influenza?

Since the onset of the 2009 pandemic, several observational public health investigations in Canada have identified evidence that suggests that adults, particularly younger adults, who have previously received seasonal influenza vaccine are at increased risk of infection with the 2009 pandemic strain of H1N1 (pH1N1). Investigations in Australia, the United States and the United Kingdom have not identified this effect. While it is not possible to have an answer to whether this affect is real prior to the second wave of the 2009 pandemic, it remains vital to future influenza vaccination programs that the hypothesis that, in 2009, seasonal vaccine increases or decreases the risk of pH1N1 infection be confirmed or refuted. The objective of this study is to determine whether Ontario adults aged 18-60 years who receive the 2009 seasonal influenza vaccine will be at a 2 fold or greater increased risk of infection with influenza pH1N1 during the second or third wave of the 2009 pandemic.