Challenges encountered during the systematic review of newer and enhanced influenza vaccines and recommendations for the future.

There are a variety of challenges in the conduct of systematic reviews of influenza vaccines. We describe our experience of completing four systematic reviews of newer and enhanced inactivated seasonal influenza vaccines. The reporting of the included studies created significant challenges for study identification, data extraction and analysis. Those challenges have implications for the resources required to conduct reviews and, more significantly, for the accuracy of the estimated treatment effect. There is a substantial burden of morbidity and mortality associated with seasonal influenza, and the evidence used to support vaccination strategies requires regular review. An improved review process will facilitate robust decision-making both nationally and internationally. We recommend the development of reporting guidelines, increased engagement between researchers and decision makers, a database of identified trials, and research into search optimisation.