Table 1.

Effects of inactivated influenza vaccines and preferred designs of primary studies to assess them

Effect	Definition	Preferred study design	Relevance for public health
Efficacy	Capacity of the vaccine to induce antibody responses (immunogenicity) to influenza viruses	Placebo controlled RCT	Important for the yearly registration of new vaccines containing the forthcoming “season's” viral antigens. Immunogenicity is the only way of testing the likely efficacy of the candidate vaccine in the absence of viral circulation
Field efficacy	Capacity of the vaccine to prevent influenza A or B virus and its complications	Placebo controlled RCT	High, if viral circulation is high (as in an epidemic or a pandemic). Studies assessing field efficacy are usually well resourced with reliable and quick virological feedback and cases of influenza are recognised as such. Estimates of efficacy cannot be generalised to seasons with low circulation of the influenza virus even if other respiratory viruses have a higher circulation
Effectiveness	Capacity of the vaccines to prevent influenza-like illness and its consequences	Placebo controlled RCT	High in conditions of good match between vaccine and viral antigen and high viral circulation. Higher if effects on major outcomes are reported
Harms	A harmful event potentially associated with exposure to influenza vaccines	Placebo controlled RCT or non-randomised, comparative study	Depends on incidence, latency, and type of harm

RCT, randomised controlled trial.