A Cochrane review has been published (1) which was conducted with the objective to identify, retrieve and assess all studies evaluating the effects of vaccines against influenza in healthy adults.
The authors have stated the very high cost of yearly vaccination for large parts of the population and the extreme variability of influenza incidence during each ’season’ as the background for this review.
They specified that the majority of viral respiratory disease (influenza-like illness (ILI)) is caused by many different agents which are not clinically distinguishable from one another. A variable proportion of ILI (7% to 15% on average) is caused by influenza viruses and is known as influenza.
Any randomised controlled trial (RCT) or quasi-RCT comparing influenza vaccines in humans with placebo or no intervention or comparing types, doses or schedules of influenza vaccine was included. Only studies assessing protection from exposure to naturally occurring influenza were considered.
Fifty reports were included. Forty (59 sub-studies) were clinical trials of over 70,000 people. Eight were comparative non-RCTs and assessed serious harms. Two were reports of harms which could not be introduced in the data analysis. In the relatively uncommon circumstance of vaccine matching the viral circulating strain and high circulation, 4% of unvaccinated people versus 1% of vaccinated people developed influenza symptoms (risk difference (RD) 3%, 95% confidence interval (CI) 2% to 5%). The corresponding figures for poor vaccine matching were 2% and 1% (RD 1, 95% CI 0% to 3%). These differences were not likely to be due to chance. Inactivated vaccines caused local harms and an estimated 1.6 additional cases of Guillain-Barré Syndrome per million vaccinations. The harms evidence base is limited.
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.
Parenterally administered influenza vaccines appear significantly better than their comparators and can reduce the risk of developing influenza symptoms by around 4%, if the WHO recommendations are adhered to and the match is right. However, whilst the vaccines do prevent influenza symptoms, this is only one part of the spectrum of “clinical effectiveness” as they reduce the risk of total “clinical” seasonal influenza (i.e. influenza-like illness) symptoms by around 1%. When the results are expressed as RD the effect appears minimal. This is remarkable as healthy adults are the population in which inactivated vaccines perform best. No evidence was found that vaccines prevent viral transmission or complications. None of the studies included in the review presented results evaluating the ability of this vaccination to interrupt the spread of the disease.
Some studies presented data on reduction of working days lost and showed a very limited effect. Similarly a very limited effect was found on morbidity and no effect was found on hospitalization.
It was also concluded that it is not possible to give a definite indication on the practical use of live aerosol vaccines, because the assessment of their effectiveness is based on a limited number of studies presenting conflicting results. The effectiveness, according to WHO criteria, appears relatively low.
Inactivated influenza vaccines decrease the risk of symptoms of influenza and time off work, but their effects are minimal, especially if the vaccines and the circulating viruses are mismatched. There is no evidence that they affect complications or transmission. Efforts to update and enhance these vaccines should have priority.
The review conclusions are uncertain about the safety profile of inactivated vaccines which is a reflection of the size of the evidence base. The authors also conclude that the 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.
(1)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
http://www.cochranejournalclub.com/vaccines-for-preventing-influenza-clinical/pdf/CD001269_full.pdf
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