Flu vaccination was in the news last week. Apparently, some scientists at Oxford University in the UK have developed a vaccine that might protect us against all strains of the flu virus. Clinical trials are just getting underway now. This news was greeted with much excitement from the press, as expected.
The British Medical Journal last week carried a piece about the flu vaccine [1]. Though it wasn’t about the new improved flu vaccine that might do this and might do that, it was about the existing one. The piece drew our attention to a recently published study in the American Journal of Respiratory and Critical Care Medicine which sought to re-assess the claim that flu vaccination helps stop old people dying [2].
Previous studies have found that if you look at an elderly population, those who have had the flu vaccination are about half as likely to die compared to those who remain unvaccinated. It’s obvious then: vaccination saves lives.
However, studies of this nature are what are known as epidemiological or observational studies. They tell us that vaccination is associated with reduced mortality. They don’t tell us for sure that it’s the vaccination that is the protective factor.
One could argue, as the authors of this recent study did, that people who get flu vaccinations are also more health conscious (and perhaps they eat better, exercise more and smoke less) than those who eschew flu vaccination. Perhaps it is these or similar factors, not flu vaccination, that accounts for the reduced mortality in vaccinees?
To address this issue, these researchers assessed the mortality in about 350 vaccine recipients and 350 non-vaccinated individuals (controls) who had been hospitalised with pneumonia. The really clever twist to the design of this study was its timing: the study was done outside the flu season. Now, with no flu around to be protected from, any difference in mortality is unlikely to be due to flu vaccination.
What the researchers found was that those who had had the flu shot were (still) about half as likely to die compared to the non-vaccinated. The authors of this study suggest that the apparent mortality benefit in the elderly (as observed in epidemiological studies) is probably not due to the vaccine but, but a healthy user effect.
The BMJ piece tells us this paper has been by Dr Gary Greenberg, a specialist in preventive medicine at the School of Public Health at the University of North Carolina. His argument is that some people who got genuine benefit by being protected from contracting pneumonia by the vaccine were left out of the study. This would, if true, dilute the apparent mortality benefits of the vaccine.
However, one of the authors of the study, Dr Majumdar (from the University of Alberta in Canada), pointed out that a study published earlier this year did not find that flu vaccination protects against pneumonia [3]. This kinda delates Dr Greenberg’s argument somewhat.
The BMJ piece concludes by stating: Dr Majumdar concluded that policies calling for more vaccination of elderly people are based on the “flimsiest of evidence” and reiterated his call for controlled studies.
References:
1. Lenzer J. Bias alone could account for benefit attributed to flu vaccine, study finds.
BMJ 2008;337:a1550
2. Dean T., et al. Mortality Reduction with Influenza Vaccine in Patients with Pneumonia Outside “Flu” Season – Pleiotropic Benefits or Residual Confounding? American Journal of Respiratory and Critical Care Medicine 2008;178:527-533
3. Jackon ML, et al. Influenza vaccination and risk of community-acquired pneumonia in immunocompetent elderly people: a population-based, nested case-control study. Lancet 2008; 372:398-405
