Lowered heart attack risk-flu vaccination link called into question

A British study that suggested flu vaccinations reduced the risk of heart attacks in people over the age of 40 has been called into question by researchers who have studied the benefits of flu immunization.

A scientific team from the United Kingdom studied the data of 16,000 Britons who had acute myocardial infarctions over a six year period, CIDRAP News reports. The team calculated that, after adjusting for differences in risk factors for infarctions, those who were inoculated against influenza in the previous year were 19 percent less likely to suffer a heart attack. Their results were published in the Canadian Medical Association Journal.

But two other researchers who were asked to provide comments on the study, CIDRAP News reports, said that it was flawed because the study’s authors failed to conduct separate assessments of the effects of vaccination on heart attacks during the influenza season and other times of the year.

Such an assessment would be a critical test of whether the reduction of risk might be due to healthier people generally getting vaccinated more often, the so-called “healthy user effect,” according to CIDRAP News.

Lisa Jackson, a senior researcher with the Group Health Research Institute in Seattle and a doubter of the study, told CIDRAP News that if vaccination lowers the risk, it should only happen when flu is around. Jackson conducted a 2005 study that examined the effects of flu vaccination on hospitalization and death rates among 72,000 seniors and found that the largest risk reductions occurred in the pre-flu season, which pointed to a preferential receipt of the vaccine by healthier people.

Lone Simonsen, a research director in George Washington University's Department of Global Health, found similar problems and agrees with Jackson. She pointed out to CIDRAP News that adjusting for confounds skewed the results. Unadjusted data should have no effect or even a negative effect of vaccination on the risk of heart attacks.

"Given the recent elegant demonstration of how that sort of analytic adjustment can actually move unadjusted data away from the truth and create profound mismeasurement, it is clear that this paper may have been profoundly affected by the same problem," Simonsen told CIDRAP News.