Published Online:
June 4, 2013
Authors of this review assessed all trials that compared vaccinated people with unvaccinated people. The combined results of these trials showed that under ideal conditions (vaccine completely matching circulating viral configuration) 33 healthy adults need to be vaccinated to avoid one set of influenza symptoms. In average conditions (partially matching vaccine) 100 people need to be vaccinated to avoid one set of influenza symptoms. Vaccine use did not affect the number of people hospitalised or working days lost but caused one case of Guillian-Barré syndrome (a major neurological condition leading to paralysis) for every one million vaccinations. Fifteen of the 36 trials were funded by vaccine companies and four had no funding declaration. Our results may be an optimistic estimate because company-sponsored influenza vaccines trials tend to produce results favorable to their products and some of the evidence comes from trials carried out in ideal viral circulation and matching conditions and because the harms evidence base is limited..
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 vaccines with 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:
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-Barré 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.
WARNING:
This review includes 15 out of 36 trials funded by industry (four had no funding declaration). An earlier systematic review of 274 influenza vaccine studies published up to 2007 found industry funded studies were published in more prestigious journals and cited more than other studies independently from methodological quality and size. Studies funded from public sources were significantly less likely to report conclusions favorable to the vaccines. The review showed that reliable evidence on influenza vaccines is thin but there is evidence of widespread manipulation of conclusions and spurious notoriety of the studies. The content and conclusions of this review should be interpreted in light of this finding.
WARNING:
This review includes 15 out of 36 trials funded by industry (four had no funding declaration). An earlier systematic review of 274 influenza vaccine studies published up to 2007 found industry funded studies were published in more prestigious journals and cited more than other studies independently from methodological quality and size. Studies funded from public sources were significantly less likely to report conclusions favorable to the vaccines. The review showed that reliable evidence on influenza vaccines is thin but there is evidence of widespread manipulation of conclusions and spurious notoriety of the studies. The content and conclusions of this review should be interpreted in light of this finding.
This record should be cited as:
Jefferson
T, Di Pietrantonj C, Rivetti A, Bawazeer GA, Al-Ansary LA, Ferroni E.
Vaccines for preventing influenza in healthy adults. Cochrane Database
of Systematic Reviews 2013, Issue 6. Art. No.: CD001269. DOI:
10.1002/14651858.CD001269.pub4
