The association of seasonal influenza vaccination with pandemic influenza H1N1 2009 infection

In 2010 Skowronski and colleagues reported that seasonal influenza vaccine appeared to increase the risk of pandemic influenza H1N1 2009 (pH1N1) infection during the first pandemic wave in Canada [1]. They suggested a number of possible explanations for their unexpected finding: firstly, that the results were an artefact of selection bias or confounding; secondly, that the results were due to partial mediation through a biological mechanism; and thirdly, that the results were due to a direct immune mechanism, such as antibody dependent enhancement [1]. In a recent paper in Vaccine, Rosella and colleagues have investigated in detail the first of these possibilities, confirming that it is unlikely an unidentified confounder could have explained the findings [2].