Vaccines for preventing influenza in healthy adults

  • Review
  • Intervention


  • V Demicheli,

  • D Rivetti,

  • JJ Deeks,

  • TO Jefferson

Dr Vittorio Demicheli, Servizo Sovrazonale di Epidemiologia, ASL 20, Via Venezia 6, Alessandria, Piemonte, 15100, ITALY.



Three different types of influenza vaccines are currently produced worldwide. None is traditionally targeted to healthy adults. 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 negative impact on the vaccines acceptance and uptake.


  • To assess the effects of vaccines on influenza in healthy adults.

  • To assess the effectiveness of vaccines in preventing cases of influenza in healthy adults.

  • To estimate the frequency of adverse effects associated with influenza vaccination in healthy adults.

Search strategy

We searched the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library, Issue 1, 2004) which contains the Cochrane Acute Respiratory Infections Group trials register; MEDLINE (January 1966 to December 2003); and EMBASE (1990 to December 2003). We wrote to vaccine manufacturers and first or corresponding authors of studies in the review.

Selection criteria

Any randomised or quasi-randomised studies comparing influenza vaccines in humans with placebo, control vaccines or no intervention, or comparing types, doses or schedules of influenza vaccine. Live, attenuated or killed vaccines or fractions thereof administered by any route, irrespective of antigenic configuration were considered. Only studies assessing protection from exposure to naturally occurring influenza in healthy individuals aged 14 to 60 (irrespective of influenza immune status) were considered.

Data collection and analysis

Two reviewers independently assessed trial quality and extracted data.

Main results

Twenty five reports of studies involving 59,566 people were included. The recommended live aerosol vaccines reduced the number of cases of serologically confirmed influenza by 48% (95% confidence interval (CI) 24% to 64%), whilst recommended inactivated parenteral vaccines had a vaccine efficacy of 70% (95% CI 56% to 80%). The yearly recommended vaccines had low effectiveness against clinical influenza cases: 15%(95% CI 8% to 21%) and 25% (95% CI 13% to 35%) respectively. Overall the percentage of participants experiencing clinical influenza decreased by 6%. Use of the vaccine significantly reduced time off work but only by 0.16 days for each influenza episode (95% CI 0.04 to 0.29 days); Analysis of vaccines matching the circulating strain gave higher estimates of efficacy, whilst inclusion of all other vaccines reduced the efficacy.

Authors' conclusions

Influenza vaccines are effective in reducing serologically confirmed cases of influenza. However, they are not as effective in reducing cases of clinical influenza and number of working days lost. Universal immunisation of healthy adults is not supported by the results of this review.

Plain language summary

Plain language summary

There is not enough evidence to recommend universal vaccination against influenza in healthy adults

Influenza is an acute, viral respiratory infection with symptoms of headache, fever, muscle pain, cough and a runny nose. It spreads easily and can cause serious illness. New strains develop regularly. Each year, the World Health Organization recommends which strains to include in vaccinations for the next 'season'. People considered 'at risk' of complications may be offered vaccination. This review found that vaccinating healthy adults not at risk of complications reduced their chances of developing flu-like illness by only a quarter, and the number of working days lost by less than half a day. Vaccination had no reported relevant adverse effects.