Abstract
Background
Different types of influenza vaccines are currently produced worldwide. Healthy adults are presently targeted mainly in North America.
Objectives
Identify, retrieve and assess all studies evaluating the effects of vaccines against influenza in healthy adults.
Search strategy
We searched the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library, 2010, issue 2), MEDLINE (January 1966 to June 2010) and EMBASE (1990 to June 2010).
Selection criteria
Randomised controlled trials (RCTs) or quasi-RCTs comparing influenza vaccineswith placebo or no intervention in naturally-occurring influenza in healthy individuals aged 16 to 65 years. We also included comparative studies assessing serious and rare harms.
Data collection and analysis
Two review authors independently assessed trial quality and extracted data.
Main results
We included 50 reports. Forty (59 sub-studies) were clinical trials of over 70,000 people. Eight were comparative non-RCTs and assessed serious harms. Two were reports of harms which could not be introduced in the data analysis. In the relatively uncommon circumstance of vaccine matching the viral circulating strain and high circulation, 4% of unvaccinated people versus 1% of vaccinated people developed influenza symptoms (risk difference (RD) 3%, 95% confidence interval (CI) 2% to 5%). The corresponding figures for poor vaccine matching were 2% and 1% (RD 1, 95% CI 0% to 3%). These differences were not likely to be due to chance. Vaccination had a modest effect on time off work and had no effect on hospital admissions or complication rates. Inactivated vaccines caused local harms and an estimated 1.6 additional cases of Guillain-Barre Syndrome per million vaccinations. The harms evidence base is limited.
Authors' conclusions
Influenza vaccines have a modest effect in reducing influenza symptoms and working days lost. There is no evidence that they affect complications, such as pneumonia, or transmission.