A bout of flu can raise people’s risk of a heart attack even days later, researchers reported Wednesday.
The Canadian study confirms what doctors have long suspected: the influenza virus kills both directly and indirectly, and it has serious effects on the heart.
People who caught flu in Canada between 2009 and 2014 had a six-fold higher risk of suffering a heart attack in the seven days after they were diagnosed, the team at the Institute for Clinical Evaluative Sciences (ICES) and Public Health Ontario reported.
"Our findings are important because an association between influenza and acute myocardial infarction reinforces the importance of vaccination," Dr. Jeff Kwong, who led the study team, said in a statement.
Other respiratory viruses also raised the heart attack risk, but none as much as flu did.
Related: Here's how flu kills some people so fast
“No increased incidence was observed after day seven,” they wrote in their report, published in the New England Journal of Medicine.
The influenza season has been especially intense across the U.S. this year. Flu has peaked at the same time across the continent and has killed at least 30 children so far.
Most of those who die from flu are over 65, and this study confirms one of the reasons.
The researchers looked at 332 patients who were hospitalized for a heart attack in Ontario after a flu diagnosis.
Related: Flu season intensifies across U.S.
“We found a significant association between respiratory infections, especially influenza, and acute myocardial infarction (heart attack),” they wrote.
"People at risk of heart disease should take precautions to prevent respiratory infections, and especially influenza, through measures including vaccinations and handwashing," Kwong said.
The risk was equally high in people who had never had a heart attack as in those who had.
Some of the people who had flu and who had heart attacks had been vaccinated, but Kwong’s team said this does not mean the vaccine doesn’t prevent severe flu or heart attacks.
What it shows is that if people get vaccinated and still get sick enough to go to the doctor, they have as much risk as having a heart attack as unvaccinated patients do. The study had no way of showing how many heart attacks may have been prevented by vaccination, because people with mild flu don’t get tested for influenza.
“Since most patients with milder symptoms do not undergo testing for respiratory viruses, these findings may not be generalizable to milder infections,” the team wrote.
Influenza is a big killer. Every year, depending on the severity of the strains, flu kills between 12,000 and 50,000 people in the U.S. alone, the Centers for Disease Control and Prevention says.
Doctors who study the body’s immune response say there are three main reasons flu becomes deadly: co-infection with another germ, usually bacteria such as strep; aggravation of existing conditions such as heart disease and asthma; and a so-called cytokine storm, marked by an overwhelming immune system response to infection.
Any of these can lead to heart attack.
