Vaccines for preventing influenza in healthy adults
Abstract
Background
Three different types of influenza vaccines are currently produced world wide. 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.
Objectives
To identify, retrieve and assess all studies evaluating 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 methods
MEDLINE was searched using the strategy of the Cochrane Acute Respiratory Infections Group. The bibliography of retrieved articles, the Cochrane Controlled Trials Register (CCTR), and EMBASE (1990 to 1997) were also searched. Handsearch of the journal Vaccine from its first issue to the end of 1997 (Jefferson and Jefferson, 1996; Jefferson, 1998). 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
Both clinically defined cases and serologically confirmed cases of influenza were considered as outcomes according to the authors' definitions. Time off work, complication and hospitalisation rates were considered, together with adverse effects. Vaccine schedules were analysed including one component matching the recommended vaccine (WHO or government recommendations) for the year of the study, and whether they matched the circulating viral subtypes.
Main results
The recommended live aerosol vaccines reduced the number of cases of serologically confirmed influenza A by 48% (95% confidence interval 24% to 64%), whilst recommended inactivated parenteral vaccines had a vaccine efficacy of 68% (95% confidence interval 49% to 79%). The vaccines were less effective in reducing clinical influenza cases, with efficacies of 13% and 24% respectively. Use of the vaccine significantly reduced time off work, but only by 0.4 days for each influenza episode (95% confidence interval 0.1 to 0.8 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 A. However, they are not as effective in reducing cases of clinical influenza. The use of WHO recommended vaccines appears to enhance their effectiveness in practice.
PICOs
Plain language summary
Not enough evidence that vaccination against influenza in not at risk adults is effective as a public health measure
Influenza is an acute respiratory infection caused by a virus. Symptoms of headache, coughs and runny noses can last for weeks and lead to serious illness. Influenza spreads easily and new strains develop regularly. The World Health Organisation recommends each year which strains should be included in vaccinations for the forthcoming 'season'. People considered 'at risk' are offered vaccination to prevent complications and as a public health measure in many countries. The review of trials found vaccinations against influenza in healthy adults, not at particular risk of flu did not change the number of people who got the flu. Vaccination did not relieve symptoms, but also did not result in adverse effects.
