New Discovery: scientists found the flu can cause heart attacks.

This year's flu season is undoubtedly bad. Vaccines are less effective than usual, and the strains seem particularly virulent.

Now, scientists have found evidence that the virus may be even more dangerous than we previously thought. On Jan. 24, researchers from Canada published the first paper connecting the flu to heart attacks in adults.

“We can say that this is influenza, without a question, that is associated with [heart attacks],” Jeffrey Kwong, a physician and epidemiologist at the Institute for Clinical Evaluative Sciences in Toronto, told STAT.

For their work, published (paywall) in the New England Journal of Medicine, researchers looked at data from lab and hospital data from facilities in Ontario over a period of five years between 2009 and 2014.

During that time, there were 19,729 cases of people over 35 who had confirmed lab cases of the flu. 

Subsequently, the team looked at hospital discharge records to examine who had been hospitalized for a heart attack in the area. Of those who tested positive for the flu, 332 patients were admitted to a hospital for a heart attack within the year before their bout of flu, or 51 weeks after (34 poor souls had multiple heart attacks in that time frame).

Scientists wanted to compare the rates of heart attacks within a week of getting a flu diagnosis to the control rate of heart attacks within a nearly two-year period. There were 20 instances of heart attacks in the seven-day period after a confirmed case of the flu. The other 344 heart attacks occurred at a rate of roughly 3.3 per week. This led researchers to the conclusion that a flu diagnosis increased the risk of having a heart attack by a factor of six. 

Intuitively, this conclusion makes sense. If the body is fighting off a virus, it's going to be experiencing some degree of stress. Scientists have found some associations between other illnesses that share flu symptoms and heart attacks, but it's been hard to pin down the flu specifically because so few people actually get tested for the virus itself.

However, these statistics don't likely apply to everyone. The flu tends to make people who are 65 or older sicker. Additionally, about a fifth (pdf) of men between 60 and 79 suffer from heart attacks, and after age 80 more than 30% of men do (the stats for women in the same age groups are roughly 10% lower).

It's definitely possible to have a heart attack at a younger age, but less than a percent of the population has a heart attack before 40, and only about 6% of men and 5% of women do before 60.

The median age of patients who had heart attacks in this study was 77—well within a group already at elevated risk for heart attack and severe flu symptoms.

Scientists don't know exactly what the risk is of heart attack with this year's nasty strain of influenza, but broadly, this study offers an extra reason to get your flu shot in the future—and to bring your older loved ones with you.