CONCLUSIONS: Influenza vaccines have a modest effect in reducing influenza symptoms and working days lost. There is no evidence that they affect complications, such as pneumonia, or transmission.
WARNING: This review includes 15 out of 36 trials funded by industry (four had no funding declaration). An earlier systematic review of 274 influenza vaccine studies published up to 2007 found industry funded studies were published in more prestigious journals and cited more than other studies independently from methodological quality and size. Studies funded from public sources were significantly less likely to report conclusions favorable to the vaccines. The review showed that reliable evidence on influenza vaccines is thin but there is evidence of widespread manipulation of conclusions and spurious notoriety of the studies. The content and conclusions of this review should be interpreted in light of this finding.
Also: link to the Cochrane Collaboration review (The Cochrane Library):
Background
Different types of influenza vaccines are currently produced worldwide. Healthy adults are presently targeted mainly in North America.
Objectives
Identify, retrieve and assess all studies evaluating the effects of vaccines against influenza in healthy adults
Authors' conclusions
Influenza vaccines have a modest effect in reducing influenza symptoms and working days lost. There is no evidence that they affect complications, such as pneumonia, or transmission.
WARNING:
This review includes 15 out of 36 trials funded by industry (four had no funding declaration). An earlier systematic review of 274 influenza vaccine studies published up to 2007 found industry funded studies were published in more prestigious journals and cited more than other studies independently from methodological quality and size.
Studies funded from public sources were significantly less likely to report conclusions favorable to the vaccines. The review showed that reliable evidence on influenza vaccines is thin but there is evidence of widespread manipulation of conclusions and spurious notoriety of the studies. The content and conclusions of this review should be interpreted in light of this finding.
Plain language summaryVaccines to prevent influenza in healthy adultsOver
200 viruses cause influenza and influenza-like illness which produce
the same symptoms (fever, headache, aches and pains,
cough and runny noses). Without laboratory tests, doctors
cannot tell the two illnesses apart. Both last for days and rarely
lead to death or serious illness. At best, vaccines might be
effective against only influenza A and B, which represent about
10% of all circulating viruses. Each year, the World Health
Organization recommends which viral strains should be included
in vaccinations for the forthcoming season.
Authors of
this review assessed all trials that compared vaccinated people with
unvaccinated people. The combined results
of these trials showed that under ideal conditions (vaccine
completely matching circulating viral configuration) 33 healthy
adults need to be vaccinated to avoid one set of influenza
symptoms. In average conditions (partially matching vaccine) 100
people need to be vaccinated to avoid one set of influenza
symptoms. Vaccine use did not affect the number of people hospitalised
or working days lost but caused one case of Guillian-Barré
syndrome (a major neurological condition leading to paralysis)
for every one million vaccinations.
Fifteen of the 36 trials
were funded by vaccine companies and four had no funding declaration.
Our results may be an optimistic estimate because
company-sponsored influenza vaccines trials tend to produce results
favorable
to their products and some of the evidence comes from trials
carried out in ideal viral circulation and matching conditions
and because the
harms evidence base is limited.
