New Canadian research shows just how dangerous the flu can be, revealing that the illness raises the chances of a heart attack by six times during the first week.

It’s long been known that the flu can lead to serious and sometimes fatal complications, but this study, published in the New England Journal of Medicine, is the first to quantify the risk between heart attacks and laboratory-confirmed influenza.

The researchers, led by Dr. Jeff Kwong at the Institute for Clinical Evaluative Sciences (ICES), used data on patients from Ontario who were diagnosed with flu within a year of having a heart attack.

They found the chance of a heart attack was increased six times during the week after flu infection compared to the year before or after.

“We were shocked by how strong the association was,” Dr. Kwong told CTV News. “Six-fold is very high… To have it at six is a strong association. It’s comparable to the association between lung cancer and smoking.”

The researchers found that the risk of heart attacks was somewhat higher for adults over the age of 65, and for patients who were infected with an influenza B strain of the flu, rather than influenza A.

The team also found that the risk of a heart attack was elevated to a lesser extent after infection with other common respiratory viruses, such as RSV, or respiratory syncytial virus.

Dr. Kwong explains that there are several ways that a respiratory infection like the flu can lead to a heart attack.

The infections can lower blood pressure and thus lower the amount of oxygen in the blood, which forces the heart to compensate by beating faster. For those with early heart disease, that kind of strain can lead to a heart attack.

As well, viral infections cause inflammation, which promotes blood clots forming in the blood vessels that serve the heart, again leading to a heart attack.

The authors note that it’s not clear whether their findings apply only to those who develop flu infections, serious enough to result in laboratory testing, since those were the only patients they studied.

“Since most patients with milder symptoms do not undergo testing for respiratory viruses, these findings may not be generalizable to milder infections,” they write.

Still, the findings are a good reminder, says Dr. Kwong, of why it is so important for everyone to get a flu shot, particularly those who have early heart disease. That’s especially true this flu season, because influenza B strains have been circulating widely.

“For those people who aren’t convinced that getting an influenza vaccines are a good idea, this is good reason why they should get one,“ Dr. Kwong said.

Dr. Kwong also stressed the importance of staying home when sick and regularly washing hands to avoid spreading the flu.

The study was funded by grants from the Canadian Institutes of Health Research, Public Health Ontario, and ICES.

With a report from CTV medical specialist Avis Favaro and producer Elizabeth St. Philip