The choice had glaring implications, apparently tied to a powerful effect: reduced inflammation.
An apple a day might keep the doctor away, but new research suggests getting a shot just once a year significantly keeps heart attack and stroke risk at bay.
In their study published April 2026 in the journal Eurosurveillance, a team of six public health researchers in Denmark and Sweden noted a rarely discussed contributor to heart attack and stroke that’s preventable, separate from the oft-mentioned lifestyle factors like diet or exercise. “Influenza infection can trigger acute cardiovascular events through short-lived systemic inflammation,” they said, citing three Canadian, French, and Spanish studies that pinpointed an increase in acute heart attack risk during the first seven days after a positive flu test confirmed within a laboratory.
They added that other studies have found the highest flu-related heart risk peaks between three to 14 days after infection, while in Denmark, the association resulted in three times greater stroke risk and five times greater heart attack risk.
Their research investigated: “Influenza vaccination has been shown to reduce this risk by preventing infection, but it is unclear whether it also offers protection among people who become infected despite vaccination.” They set out to quantify how much more adults in Denmark are at risk of heart attack and stroke shortly after catching influenza, and to assess whether flu vaccination could reduce the risk.
The team examined data from nearly 203,000 patients in Denmark between 2015 and 2024 who’d been hospitalized for heart attack or stroke. Just under 5,000 of those had experienced flu infection within that flu season, while 1,221 had were age 40 or older who’d had a “temporally linked” flu infection, meaning a week before their cardiac event. The vaccinated versus unvaccinated groups were split 50-50.
“Our findings add to the evidence that influenza vaccination confers cardiovascular protection,” the researchers report. Specifically, people who got the flu despite being vaccinated saw about half the excess cardiovascular risk compared to unvaccinated people who contracted the flu. The interaction ratio was 0.51, meaning the spike in heart attack and stroke risk after flu infection was roughly cut in half for vaccinated people, compared to unvaccinated people.
Further, in the week after a confirmed flu infection, unvaccinated people had a 4.7-fold increase in heart attack and stroke risk compared to their own baseline. Vaccinated people who still got the flu had a 2.4-fold increase. In sum, vaccination was associated with close to halved post-infection cardiovascular spike.
The team noted that just in 2021, heart events cost the European Union $282 billion. (A comparable 2025 CDC statistic suggests an annual cost of nearly $420 billion in the United States.) “These results strengthen the case for prioritizing influenza vaccination,” the team says, including for high-risk groups whose economic circumstances might not allow for easy access to vaccinations—if not for reasons of humanity, then for the sake of national cost-saving.
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