Heart attacks are a leading cause of death. They occur when a section of the heart dies because it is not receiving enough oxygenated blood. (Image: iStock)

A team of Ontario researchers has provided the most definitive evidence to date that having the flu increases a person's risk for a heart attack.

The study was recently published in The New England Journal of Medicine, one of the most respected medical journals.

The flu is an infectious disease caused by a virus that attacks the tissues in your nose, throat and lungs. By infecting these tissues, the flu produces a variety of symptoms including a headache, sore throat and a runny nose. In some people, the infection can also cause more serious problems.

"For many years, researchers have suspected that the flu can trigger heart attacks," explains Dr. Jeff Kwong, a family physician at Toronto Western Hospital and a Senior Scientist at the Institute for Clinical Evaluative Sciences (ICES).

"In the 1930s, researchers first noticed that more people die of heart attacks during flu season. Since then, researchers have been trying to establish whether having the flu puts a person at a greater risk of having a heart attack; however, proving this has been difficult given that most people who display flu-like symptoms aren't tested to confirm they are infected by the flu virus."

To address this, Dr. Kwong and his colleagues leveraged a powerful resource: the extensive health information for over 13 million Ontarians available through ICES. By examining ICES data combined with flu test results from Public Health Ontario and academic hospitals, the researchers found nearly 20,000 flu cases that were confirmed by laboratory tests between 2009 and 2014.

Among these cases, the researchers identified 332 patients who had a heart attack within one year of their flu diagnosis. They discovered individuals were six times more likely to have a heart attack during the first week of being diagnosed with the flu.

"Our findings provide the best evidence yet that the flu can trigger heart attacks. Importantly, they support international guidelines that advocate for immunizing adults over the age of 65 against the flu to protect them from heart attacks," says Dr. Kwong.

This work was supported by the Canadian Institutes of Health Research, Public Health Ontario and ICES, which receives funding from the Ontario Ministry of Health and Long-Term Care.

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