Experts in the Cochrane Collaboration's Acute Respiratory Infections Group have reviewed all trial data in the medical literature, comparing vaccinated individuals with those who are unvaccinated. The researchers Jefferson T, et al., report that under ideal conditions -- especially when the vaccine matches the circulating virus -- studies have shown that 33 healthy adults need to be vaccinated to avoid one set of influenza symptoms.
The review noted, "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 hospitalized or working days lost but caused one case of Guillian-Barré syndrome for every 1 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."
According to the review, more than 200 viruses trigger influenza and influenza-like illness which produce the same symptoms, including fever, headache, aches and pains, cough and runny noses, and that without laboratory tests, physicians cannot distinguish between the two illnesses. At best, teh reviewers note, "vaccines might be effective against only influenza A and B, which represent about 10 percent of all circulating viruses." Each year, the World Health Organization recommends which viral strains should be included in vaccinations for the forthcoming season.
Reference: Jefferson T, Di Pietrantonj C, Rivetti A, Bawazeer GA, Al-Ansary LA, Ferroni E. Vaccines for preventing influenza in healthy adults. Cochrane Database of Systematic Reviews 2010, Issue 7. Art. No.: CD001269. DOI: 10.1002/14651858.CD001269.pub4.