Different types of influenza vaccines are currently produced world-wide. Healthy adults are at present targeted only in North America. Despite the publication of a large number of clinical trials, there is still substantial uncertainty about the clinical effectiveness of influenza vaccines and this has a negative impact on their acceptance and uptake.
To identify, retrieve and assess all studies evaluating the effects (efficacy, effectiveness and harms) of vaccines against influenza in healthy adults.
We searched the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library, Issue 4, 2005) which contains the Cochrane Acute Respiratory Infections Group trials register; MEDLINE (January 1966 to January 2006); and EMBASE (1990 to January 2006). We wrote to vaccine manufacturers and first or corresponding authors of studies in the review.
Any randomised or quasi-randomised studies comparing influenza vaccines in humans with placebo, no intervention. Live, attenuated, or killed vaccines or fractions of them administered by any route, irrespective of antigenic configuration were assessed. Only studies assessing protection from exposure to naturally occurring influenza in healthy individuals aged 16 to 65 years were considered. Comparative non-randomised studies were included if they assessed evidence of the possible association between influenza vaccines and serious harms.
Data collection and analysis
Two review authors independently assessed trial quality and extracted data.
Forty-eight reports were included: 38 (57 sub-studies) were clinical trials providing data about effectiveness, efficacy and harms of influenza vaccines and involved 66,248 people; 8 were comparative non-randomised studies and tested the association of the vaccines with serious harms; 2 were reports of harms which could not be introduced in the data analysis.
Inactivated parenteral vaccines were 30% effective (95% CI 17% to 41%) against influenza-like illness, and 80% (95% CI 56% to 91%) efficacious against influenza when the vaccine matched the circulating strain and circulation was high, but decreased to 50% (95% CI 27% to 65%) when it did not. Excluding the studies of the 1968 to 1969 pandemic, effectiveness was 15% (95% CI 9% to 22%) and efficacy was 73% (95% CI 53% to 84%). Vaccination had a modest effect on time off work, but there was insufficient evidence to draw conclusions on hospital admissions or complication rates. Inactivated vaccines caused local tenderness and soreness and erythema. Spray vaccines had more modest performance. Monovalent whole-virion vaccines matching circulating viruses had high efficacy (VE 93%, 95% CI 69% to 98%) and effectiveness (VE 66%, 95% CI 51% to 77%) against the 1968 to 1969 pandemic.
Influenza vaccines are effective in reducing cases of influenza, especially when the content predicts accurately circulating types and circulation is high. However, they are less effective in reducing cases of influenza-like illness and have a modest impact on working days lost. There is insufficient evidence to assess their impact on complications. Whole-virion monovalent vaccines may perform best in a pandemic.