Each year enormous effort goes into producing influenza vaccines and delivering them to appropriate sections of the population. But a review of the evidence in this week's BMJ suggests that they may not be as effective as we think.
So is this effort justified, asks vaccine expert Tom Jefferson?
Public policy worldwide recommends the use of inactivated influenza vaccines (vaccines that contain dead viruses) to prevent seasonal outbreaks.
But because influenza viruses mutate (change) and the number doing the rounds varies from year to year, it's difficult for scientists to study the precise effects of vaccines. The most reliable way to judge their effects is to use systematic reviews - impartial summaries of evidence from many different studies.
Evidence from systematic reviews in this field shows that inactivated influenza vaccines have little or no effect on many influenza campaign objectives, such as hospital stay, time off work, or death from influenza and its complications.
Furthermore, most studies are of poor quality (especially in the elderly) and show evidence of bias. And there is surprisingly little evidence on the safety of these vaccines.
The large gap between policy and what the data tell us is surprising, writes Jefferson. Reasons for this are not clear, but may stem from the confusion between influenza and influenza-like illness (the acute respiratory infection which looks like influenza but is not), a lack of accurate and fast surveillance systems, and the fact that vaccines are available.
The optimistic and confident tone of some predictions of viral circulation and of the impact of inactivated vaccines, which are at odds with the evidence, is striking, he says. But given the huge resources involved, a re-evaluation should be urgently undertaken.
"The problem is that the UK has no transparent process for evaluating the effectiveness or cost effectiveness of vaccines," adds BMJ Editor, Fiona Godlee. "NICE would like to take this on. The government should let it."