As the flu season grinds on, people with heart problems have an extra reason to be worried — and to get vaccinated. Scientists have long suspected that heart attacks, myocardial infarctions (MIs), can be triggered by acute respiratory infections, including infection with the influenza virus. A new Canadian study confirms the association between influenza infection and subsequent hospitalization for acute myocardial infarction.

Getting an annual flu shot, particularly for those over 65, can help protect against MI, stroke and other coronary events caused by atherosclerosis, a hardening and narrowing of the arteries, the authors write. The evidence linking influenza virus to acute MI could lead to changes in practice that may improve the influenza vaccination rate.

The inflammation and respiratory distress brought on by the flu makes acute heart problems more likely.

“A lot of people think respiratory viruses are no big deal; they may just get a little sick,” Jeff Kwong, lead author on the study, told TheDoctor. And while that is true for most people, “This study highlights the fact they can cause significant illnesses like heart attacks and other complications that can put people in the hospital.”

The researchers looked at nearly 20,000 samples from adults who tested positive for the influenza virus. They identified over 350 patients who were hospitalized for a heart attack one year before or one year after a laboratory-confirmed influenza diagnosis. Twenty of these patients were hospitalized within a week of their influenza test, during what was called the “risk interval.” The remaining 344 who were hospitalized either during the year before specimen collection or the year after it served as the “control interval.”

The median age of the people in the study was 77 years old. Many had cardiovascular risk factors such as diabetes, hypertension and high triglycerides. Fewer than a third had had a flu shot for the current flu season.

People who tested positive for influenza were six times more likely to be hospitalized for acute MI during the seven risk interval days after testing positive for the flu virus, compared to the rate during control interval. Those who were older, had influenza type B and those having their first acute MI were at higher risk. Being infected with respiratory viruses other than influenza increased the risk of hospitalization with acute MI, but the risk was less than that associated with the flu.

Infections cause inflammation in the body as it fights off the viral or bacterial attackers, and this inflammation can lead to damage to the lining of the blood vessels that serve the heart and increase the risk of clotting in those blood vessels, Kwong explained, and that can trigger a heart attack. Your heart also tends to beat faster when you have an infection because your blood pressure is lower. All these things make your heart work extra hard to compensate and add to your chances for a heart attack.

“It is important to try to do things to protect yourself against infection, particularly if you are at increased risk of heart attack or stroke to begin with,” said Kwong, a senior scientist at the Institute for Clinical Evaluative Sciences in Toronto, Canada. He recommends getting a flu shot every year, washing your hands frequently, and staying away from others who are sick. And if you are sick, stay home.

The study is published in the New England Journal of Medicine.