They may not seem related, but in the seven days after being diagnosed with the flu, the chances of having a heart attack are six times higher than usual.
All the more reason to get the flu shot.
In a study led by the University of Toronto and published in the New England Journal of Medicine, researchers looked at seven years of data, sifting through nearly 20,000 Ontario adult cases of laboratory-confirmed influenza infection from 2009 to 2014. They identified 364 patients with confirmed cases of the flu within a year before or after having a heart attack.
Of these cases, 20 heart attacks happened in the single week after a flu diagnosis, compared to 344 (just 3.3 admissions per week) during the rest of the year.
Severe cases of the flu are a shock to the entire system. The resulting inflammation can make the heart beat faster, and it can also activate platelets that then increase the odds that blood will clot in the arteries that serve the heart.
The team also looked at other confirmed acute respiratory infections, such as respiratory syncytial virus (RSV) or non-viral infections. Although heart attack risk was also elevated to triple their normal incidence, flu infections saw the most significant association.
“Our findings, combined with previous evidence that influenza vaccination reduces cardiovascular events and mortality, support international guidelines that advocate for influenza immunization in those at high risk of heart attacks,” said epidemiologist and lead author Jeff Kwong in a statement.
An unconfirmed link between the flu and heart attacks was first noted in the 1930s. However, many studies did not definitively prove flu infection (versus other similar acute respiratory infections), or were designed in ways that could have given biased results.
This confirmation is important for many reasons. It adds to the growing evidence that the annual flu shot is important for everyone who is medically able to get it – not just because the flu itself is deadly, but because of the potential complications. This is especially true for patients already at increased risk of heart disease.
It also emphasizes the importance that patients patients to go for medical evaluation without delay if they experience heart symptoms, like chest pain or shortness of breath, within the first week of an acute respiratory infection.
People should also take standard precautions, like frequent handwashing and keeping distance from people with the flu.