Assessing the impact of confounding (measured and unmeasured) in a case-control study to examine the increased risk of pandemic A/H1N1 associated with receipt of the 2008-9 seasonal influenza vaccine

► Confounding was examined between the seasonal influenza vaccine and pandemic H1N1. ► Confounder adjustment was done using both regression and propensity methods. ► The association remained after considering multiple confounders. ► A highly prevalent unmeasured confounder with strong associations would be needed. ► The relationship was not likely explained by an unmeasured confounder.