LOS ANGELES — Get a flu shot to ward off a case of influenza, and as a bonus you'll reduce your risk of a heart attack, stroke or other type of unpleasant "cardiovascular event," a new study has indicated.
For some time, researchers have suspected that flu shots can protect heart health as well as respiratory health. They have tested this theory in a handful of clinical trials, and the results have been mixed.
Now, an international group of researchers has compiled data from a dozen randomized clinical trials to see whether they could get a clearer answer. What they found was "a consistent association between influenza vaccination and a lower risk of cardiovascular events," according to their report in the most recent edition of the Journal of the American Medical Association.
Five of the trials the team examined were published in peer-reviewed journals and compared a flu vaccine to a placebo vaccine or other type of control. In those trials, 3,238 patients got a real vaccine and 95 of them — 2.9 percent — went on to experience "a major adverse cardiovascular event," the JAMA report said. For the sake of comparison, 3,231 patients in those trials got a placebo or control, and 151 of them — or 4.7 percent — later had a cardiovascular event, according to the study.
That works out to a 36 percent reduced risk of a serious heart problem simply by getting a flu shot. Based on these figures, the researchers calculated that one death or serious illness due to heart trouble could be prevented by vaccinating 58 additional people.
Three of the trials the researchers examined focused on patients with coronary artery disease. Some of those patients had experienced acute coronary syndrome recently, and they benefited the most by getting a flu shot. Patients who got the vaccine were 55 percent less likely to have a serious heart problem than patients in the control groups, the researchers found. The team calculated that for these patients, only eight additional people would need to get a flu shot to prevent one case of death or serious illness.
When the researchers considered cardiovascular deaths on their own, they found no statistically significant difference between the 1.3 percent of people who died after getting flu shots and the 1.7 percent who died after being assigned to a placebo or control group.
Experts aren't sure why flu increases the risk of serious heart problems, but they have their theories. It might cause a plaque that has built up inside the arteries to rupture, or it might cause the heart muscle to become inflamed, among other possibilities. Knowing what's going on inside the body would help researchers figure out who would get an extra boost from a flu shot, and why.
Even with those uncertainties, the conclusion that a flu shot can protect patients with heart disease is convincing, according to an editorial that accompanies the study in JAMA.
"The estimate of 1.7 major cardiovascular events prevented for every 100 persons with cardiovascular disease vaccinated is plausible and would represent a significant public health benefit," wrote Dr. Kathleen Neuzil, a vaccine expert at PATH, a Seattle-based nonprofit that focuses on global health.