Partial Protection Against 2009 Pandemic Influenza A (H1N1) of Seasonal Influenza Vaccination and Related Regional Factors: Updated Systematic Review and Meta-analyses

This updated systematic review and meta-analyses aims to systematically evaluate the cross-protection of seasonal influenza vaccines against the 2009 pandemic A (H1N1) influenza infection, and investigate the potential effect of the influenza strains circulating previous to the pandemic on the association between vaccine receipt and pandemic infection. In addition, subgroup analysis was performed based on the study locations and previous circulating influenza viruses. Relevant articles in English and Chinese from 2009 to October 2013 were systematically searched, and 21 eligible studies were included. For case-control studies, an insignificant 20% reduced risk for pandemic influenza infection based on combined national data (OR = 0.80; 95%CI: 0.60, 1.05) was calculated for people receiving seasonal influenza vaccination. However, for RCTs, an insignificant increase in the risk of seasonal influenza vaccines was observed (RR = 1.27; 95% CI: 0.46, 3.53). For the subgroup analysis, a significant 35% cross-protection was observed in the subgroup where influenza A outbreaks were detected before the 2009 pandemic. Moreover, the results indicated that seasonal influenza vaccination may reduce the risk of influenza-like illnesses (ILIs) (RR = 0.91; 95% CI: 0.84, 0.99). Our findings partially support the hypothesis that seasonal vaccines may offer moderate cross-protection for adults against laboratory-confirmed pandemic influenza A (H1N1) infection and ILIs. Further immunological studies are needed to understand the mechanism underlying these findings.