Having the flu appears to increase the risk of having a heart attack, especially among those aged 65 and older, an Ontario study suggests.

“What we found is that you’re six times more likely to have a heart attack during the week after being diagnosed with influenza, compared with a year before or a year after the infection,” said Dr. Jeff Kwong, lead author of the study published Wednesday in the New England Journal of Medicine.

“What we were also surprised about is that we found that there was an increased risk with other respiratory viruses as well,” said Kwong, a scientist at Public Health Ontario and the Institute for Clinical Evaluative Sciences in Toronto.

Getting infected with an influenza virus appears to have the most profound effect, but the risk of having a heart attack was also somewhat increased with infections such as respiratory syncytial virus (RSV) and cold-causing adenoviruses and rhinoviruses, he said.

To conduct the study, researchers looked at almost 20,000 adult cases of laboratory-confirmed influenza infection from 2009 to 2014 and identified 332 patients who were hospitalized for a heart attack within one year before and one year after their flu diagnosis.

Of these, 20 patients had a heart attack within seven days of their flu diagnosis, said Kwong, noting that about 75 per cent were aged 65 and older and about 25 per cent had experienced a previous heart attack. About one-third of the patients died.

Kwong said 31 per cent of the patients who had a heart attack had not been vaccinated against seasonal flu, although he cautioned the connection “requires a bit of careful interpretation.”

“We know that influenza vaccines aren’t 100 per cent effective,” he said. “Some people who get vaccinated are still going to get influenza.

“If you got vaccinated and you still got influenza, you were still at an increased risk of a heart attack at the same level as those who didn’t get vaccinated and got influenza.

“That doesn’t mean it’s not worth getting vaccinated,” Kwong stressed. “It just means that it only works [to reduce the risk of a heart attack] by preventing infection.”

Studies show the protective antibody response to vaccines mounted by seniors is not as robust as it is for younger people, due to the immune system waning in strength with age.

Even so, people aged 65-plus — as well as those with underlying medical conditions or compromised immune systems — are urged to get the flu shot as they are more susceptible to complications if they do come down with the infection.

That includes people with cardiovascular disease, he said.

“This is just one more piece of evidence to encourage people, to warn people so they know that influenza has been shown to cause heart attacks,” Kwong said.

“It may not cause them in everybody, obviously, but in some people it really can increase their risk substantially.”