Eurosurveillance
Volume 18, Issue 7, 14 February 2013
http://www.eurosurveillance.org/Public/Articles/Archives.aspx?PublicationId=11678

Editorials
Complexities in assessing the effectiveness of inactivated influenza vaccines
by H Kelly, I Steffens

Excerpt
For many years it has been generally accepted that well-matched vaccines are the most effective single measure to protect people who are predisposed to a more severe outcome following infection with the influenza virus [1]. This group includes those aged at least 65 years, pregnant women and those who suffer from specific chronic conditions and/or are immunocompromised.
There is nonetheless widespread professional and public interest in, and some debate about, the level of protection afforded by annual influenza vaccination [2,3]. The level of such protection is assessed in two main ways: as estimates of efficacy from randomised controlled trials and as estimates of effectiveness from observational studies. Both are estimates of the proportion of vaccinated people, compared to the proportion of unvaccinated people, who are protected from a specified influenza outcome. Efficacy estimates (from trials) may be higher than effectiveness estimates (from observational studies) because trials are conducted in a controlled environment. A recent meta-analysis of trials using laboratory-confirmed influenza based on culture or PCR testing as the study endpoint estimated influenza vaccine efficacy for healthy adults at 59% (95% confidence interval (CI): 51 to 67) for vaccines licensed in the United States (US) [4]. The majority of those included in the studies were younger than 40 years. Unfortunately there were too few methodologically acceptable observational studies for a pooled analysis of vaccine effectiveness (VE)…