Limited evidence for effectiveness of influenza vaccine in healthy adults

January 01, 0001

Limited evidence for effectiveness of influenza vaccine in healthy adults

Clinical Question:How effective are vaccines in preventing influenza in healthy adults (aged between 16 and 65 years)?

Bottom line: Inactivated influenza vaccines decreased the risk of symptoms of influenza and time off work, but their effects were minimal. In the relatively uncommon circumstance of the vaccine matching the viral circulating strain and high circulation, the NNT* to avoid influenza symptoms was 33. In average conditions (partially matching vaccine) the NNT was 100. There was no evidence vaccines affected hospital admissions, complication rates or transmission. Inactivated vaccines caused local harms (local erythema, tenderness and soreness), and an estimated 1.6 additional cases of Guillain-BarrŽ syndrome per million vaccinations. * NNT= number needed to treat to benefit 1 individual.

Caveat:These results may be an optimistic estimate because company-sponsored influenza vaccines trials tend to produce results favourable to their products, and some of the evidence came from trials carried out in ideal viral circulation and matching conditions; also because the harms evidence base was limited. Fifteen of the 36 trials in the review were funded by vaccine companies and 4 had no funding declaration.

Context:Over 200 viruses cause influenza and influenza-like illness (which produces the same symptoms). At best, vaccines might be effective against only influenza A and B, which represent about 10% of all circulating viruses. Healthy adults are presently targeted for influenza vaccination mainly in North America.

Cochrane Systematic Review: Jefferson T et al. Vaccines for preventing influenza in healthy adults. Cochrane Reviews, 2010, Issue 7. Article No. CD001269. DOI: 10.1002/14651858.CD001269.pub4. This review contains 50 studies involving over 80,000 participants.

Cochrane PEARLS Practical Evidence About Real Life Situations. No. 287, November 2010. .
Written by Brian R McAvoy. Published by the Cochrane Primary Care Group

Category: K. Cairclatory. Keywords: influenza, vaccination, prevention, adults
Synopsis edited by Dr Linda French, Toledo, Ohio. Posted on Global Family Doctor 22 March 2011

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Pearls are an independent product of the Cochrane primary care group and are meant for educational use and not to guide clinical care.