For many people, getting a seasonal flu shot may help them avoid a week or more of misery. But for some individuals, especially those with heart disease, vaccination against influenza appears to help reduce the likelihood of major adverse cardiovascular events, such as heart attack and stroke.

Researchers, whose findings appear today in JAMA, found that receiving influenza vaccine was associated with a 36% lower risk of cardiovascular events compared with not being immunized against flu. For individuals with recent acute coronary syndrome, such as a heart attack or unstable angina, influenza vaccine was associated with a 55% lower risk of cardiovascular events within 12 months compared with those who had a recent acute coronary syndrome but did not receive the vaccine. The findings are based on a systematic review and meta-analysis involving 6 randomized clinical trials, 5 published and 1 unpublished. These trials collectively enrolled 6735 patients with a mean age of 67 years; 36% had a history of heart disease.

Lead author Jacob A. Udell, MD, MPH, of the University of Toronto in Canada, discusses his team’s findings.

news@JAMA: Why did you do the study?

Dr Udell: There have been reports suggesting that getting the flu shot was protective against heart attack and stroke, but most of these reports were observational. So we went back and systematically reviewed all clinical trials involving vaccine or placebo to see if this signal of cardioprotection was reproducible and consistent across the studies.

news@JAMA: What did you find?

Dr Udell: We found there was a 36% risk reduction overall for getting a cardiac event in those who were vaccinated compared with those who did not get the vaccine. We also found that those who had a heart attack had even more benefit. So in the higher-risk patients, the flu vaccine gave more benefit.

news@JAMA: Although your study cannot answer this question, can you speculate why influenza vaccination is associated with reduced heart risk?

Dr Udell: The flu may be a severe illness, causing a lot of inflammation, and that will have an effect on all your organs, including the heart and brain. This inflammation may also disrupt stable hardened arteries and free atherosclerotic plaque, causing a heart attack. Another theory is that the flu may push people over a tipping point, especially among the frail and elderly.

news@JAMA: So what would you tell others about the implications of your study findings for vaccination against influenza?

Dr Udell: For the skeptics out there, I’d note that we now have yet another reason why receiving influenza vaccine might be a beneficial thing to do. And those hospitalized with a heart attack should be vaccinated before they walk out the door so they don’t have care gaps that could be very dangerous.