Vaccines for preventing influenza in healthy adults

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Clinical Summary

Influenza is an acute respiratory viral infection. Its symptoms include fever, aches and pains, tiredness, headache and cough. Complications of influenza include otitis media (an ear infection), pneumonia, secondary bacterial pneumonia, exacerbations of chronic respiratory disease and bronchiolitis in children. Efforts to prevent or minimise the impact of seasonal influenza from the second part of the 20th century have centred on the use of vaccines. Due to the yearly changes in viral antigenic configuration and the lack of carry-over protection from year to year, annual vaccination campaigns require a huge scientific and logistic effort to ensure production and delivery of that year's vaccines for high population coverage.
In 2006, the US Advisory Committee on Immunization Practices (ACIP) published a document where 11 categories of people were listed as being at high risk of complications from influenza. These included healthy adults, 50 to 65 years of age, and healthcare workers.
This Cochrane review, focusing on the effects of influenza vaccines in healthy adults aged 16 to 65, was first published in 1999 and was updated in 2004 and 2007. It has now been updated again and is able to draw on 40 trials with more than 70,000 participants and 8 non-randomised studies of harms. It provides the most robust evidence base of all of the Cochrane reviews of influenza. The main findings are based on 26 clinical trials of the effectiveness of inactivated vaccines and ten studies of the harms of the same vaccines.

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To place the findings of the review in context, it is important to understand the background risk of developing influenza (which might be effected by the vaccine) and influenza-like symptoms due to other viruses (which will not be effected by the vaccine). Data to help with this comes from the control groups in 274 influenza vaccines studies published up to 2007 and 13 ad hoc studies, which include a total of more than 3 million observations. All of these studies had virological confirmation and defined denominators, which provides more reliable estimates than public surveillance data which are biased and incomplete. The data reveal that, on average, there are seven cases or episodes of influenza-like illness (ILI) in a year among 100 people. But, only one of these is caused by influenza.

This means that influenza is a relatively rare cause of ILI. In these circumstances, influenza vaccines have a low probability of hitting such a target and producing sizeable benefits. This needs to contrasted with the large costs and time involved in vaccinating whole populations.
The findings of the Cochrane review confirm the relatively small effects of vaccination on influenza. When expressed in absolute terms, the low background risk for influenza means that you would need to vaccinate 100 people to avoid one set of influenza symptoms in real world situations, or 33 people if it had been possible to create a perfect match between the vaccine and antigens and circulating viruses. The authors of the Cochrane review are also cautious about the size of these benefits, given the potential for selective reporting biases and the relationship between the source of funding and the findings of trials. These might mean that the true effects are smaller than those found by their review, and that the numbers needed to vaccinate would be larger. The review did not find any evidence that vaccines prevent viral transmission or complications. Harms were rarely studied and the comparative safety of vaccines seems to be taken for granted.
Large studies encompassing several influenza seasons are required to allow the reliable assessment of the effect of the vaccines on rare outcomes such as complications and death. However, the authors of the review conclude that it is unlikely that these will be done because influenza vaccines are centre stage in an “industry” upon which government departments, pharmaceutical giants, the media and researchers depend.

Acknowledgements

The authors of the Cochrane review would like to thank the following for their help with the review and its interpretation: Fiona Godlee and Deborah Cohen (BMJ); Tom Clarke, Jon Snow and Phil Carter (ITN C4 News); Stephane Horel (La Canard Enchane) ; Serena Tinari (Falo’ RSI, Swiss State TV); Mark Jones, Maryann and Richard Napoli (The Cochrane Collaboration); Jeanne Lenzer (Atlantic Magazine); Francesca Nava (La7 TV); Manuela Perrone (Il Sole 24Ore); Catherine Mark (Al Jazeera English); Imogen Foulkes (BBC World Service); Eben Harrel (Time Magazine); Veronika Hackenbrock (Der Spiegel); Per Hermanrud (TV4Se); Frank Konopatzki (ARD Frontal 21); Elsa LePeutrec (Canal Plus); Ronan Lawlor (The Last Word Today FM, Irish BC); Amrita Tripathi (CNN-CNBC India) and Mike Clarke (UK Cochrane Centre). The original review was funded by the UK Ministry of Defence, the 2004 update was supported by the two Italian Local Health Authorities in which two of the review authors were employed, and the 2007 update was funded by the same Local Health Authorities and the UK's Department of Health Cochrane Incentive Scheme. The 2010 update was not funded

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