Selection bias explains seasonal vaccine's protection

Abstract
Vaccine efficacy is estimated to be around 80% for trivalent inactivated flu vaccine during seasons when the components closely match circulating strains.1 The surface HA/NA proteins included in the 2008-9 vaccine are antigenically very distant from those of the 2009 pandemic A/H1N1 (pH1N1) virus. Few adults given the 2008-9 vaccine developed neutralising antibody against pH1N1,2 but Garcia-Garcia and colleagues report 73% (95% confidence interval 34% to 89%) cross-protection against pH1N1 with it.3 This finding probably stems from a selection bias associated with their second paradoxical …