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People over the age of 50 who have mild flu and few risk factors still are at double the risk of heart attack and ischemic stroke in the two weeks following infection — and that risk quadruples in people with severe cases of flu who are at high risk for heart attack and stroke, according to a recent study.

Researchers from Spain pinpointed the risks in a report published Feb. 8 in The Journal of Infectious Diseases. The team evaluated the risk for heart attack and ischemic stroke in almost 2.2 million older adults who had the flu between 2011 and 2018. During that time, 7% of participants were diagnosed with the flu clinically (by symptoms). Of them, 84 had a heart attack or ischemic stroke within 90 days of the diagnosis and 1,618 had a cardiac event during a period of no flu exposure.

The incidence of cardiac events was more than two times likely during the first seven days of having flu as well as within 8 to 14 days of infection. There weren’t increases in other time periods studied, up to 90 days of having the infection.

Of participants, 1.7% had flu tests at a hospital, and 30% of those were positive. Among them, there were 48 cardiovascular events that occurred in the next 90 days, and 558 of those cardiovascular events happened without flu exposure. People were more than four times as likely to have a cardiovascular event within one to seven days of getting the flu, and more than five times likely during eight to 14 days within the infection period. The likelihood then declined (still to more than twice likely) between 15 to 29 days and 30 to 60 days after infection. There wasn’t a significant increase in cardiac events from 61 to 90 days after infection.

There are increased risks for those 65 years old and up to have a heart attack or stroke within seven days after a clinical flu diagnosis, and that risk exists until 60 days after a lab-confirmed infection. Those risks are compared to people in the 50-and-up group.

Respiratory infections, especially the flu, can make cardiovascular diseases worse, the authors noted.

“This work reinforces the official recommendations for influenza prevention in at-risk groups and should also increase the awareness of even milder influenza infection and its possible complications in the general population,” the authors wrote.