Impact of seasonal influenza vaccination on pandemic influenza

Canadian studies have found that receipt of seasonal influenza vaccine increased the risk of infection with 2009 pandemic influenza A ( H1N1 ) , and Australian studies have not confirmed this finding. Ferret experiments have suggested protection against pandemic influenza A ( H1N1 ) from prior seasonal influenza infections but not from prior seasonal vaccination. Modelling studies suggest that influenza infection leads to heterosubtypic temporary immunity. These observations can explain the discrepant findings in Canada and Australia. In Australia there was no recent prior circulation of seasonal influenza and thus no temporary immunity to pandemic influenza. There was no association of seasonal influenza vaccine with pandemic influenza infection. In Canada seasonal influenza preceded circulation of pandemic influenza. An unvaccinated proportion of the population developed temporary immunity to pandemic influenza from seasonal infection but a vaccinated proportion of the population did not get seasonal infection and hence did not develop temporary immunity to pandemic influenza. It may therefore have appeared as if seasonal vaccination increased the risk of infection with 2009 pandemic influenza A( H1N1). Key words: Influenza vaccines;  Seasonal influenza;  Pandemic influenza;  Temporary immunity