Vaccine efficacy and effectiveness
Measuring the efficacy and effectiveness of inactivated influenza vaccine and forming generalizable conclusions about it is challenging because of both the difficulty of identifying the burden of influenza disease and the difficulty of matching the vaccine to the circulating strains which may vary each year.
The WHO field guide for evaluation of influenza vaccine effectiveness supports the generation of reliable national data on the impact of influenza vaccine and provides advice on the design of observational influenza vaccine effectiveness studies.