Influenza Vaccine Reduces Heart Attack and Stroke Risk Even After Breakthrough Infection

Apr 3, 2026

A study by Croci et al. published in Eurosurveillance in April 2026 found that influenza vaccination reduced heart attack and stroke risk by half even among people who contracted flu after vaccination. The research analyzed 1,231 laboratory-confirmed influenza cases in Denmark from 2014 to 2025, involving 1,221 adults aged 40 and older with a median age of 75. About half of infections occurred in vaccinated patients, yet vaccination still provided significant cardiovascular protection. The study found that influenza infection increased stroke risk threefold and heart attack risk fivefold during the first week after testing positive. Researchers suggest these findings could strengthen the case for prioritizing influenza vaccination among people at cardiovascular risk and improve the economic justification for vaccination programs.

Cardiovascular risk reduction from flu vaccination after breakthrough infection

▪The study by Croci et al. included 1,221 adults aged 40 or older in Denmark to examine if the influenza vaccine could reduce cardiovascular risk even if people were infected after vaccination.
▪About half of the 1,231 laboratory-confirmed influenza virus infections occurred in patients who had been vaccinated.
▪Influenza vaccination reduced the risk of heart attack and stroke by half among people who were infected with influenza despite being vaccinated for that influenza season.
▪Existing evidence has shown that influenza vaccine reduces the risk of heart attack and stroke by preventing infection in the first place.

Study methodology and population characteristics

▪In the Croci et al. study, 65% of patients were hospitalized with a stroke while 35% had a heart attack.
▪The Croci et al. study population comprised 660 males and 561 females aged 40 years and above, with a median age of 75.
▪The Croci et al. study used conditional Poisson regression to estimate incidence rate ratios with a self-controlled case series design that compares event timing within the same person.
▪The Croci et al. study used Danish health registry data from 2014 to 2025 covering nine consecutive influenza seasons.
▪The Croci et al. study included an observation window of 365 days before and after a positive influenza test.
▪The Croci et al. study excluded a pre-exposure period of two weeks before testing to minimize reverse causality where cardiovascular symptoms may have prompted testing.

Timing and magnitude of heart attack and stroke risk following influenza infection

▪Catching influenza increases the short-term risk of cardiovascular conditions.
▪The risk for first-time hospitalization for heart attack and stroke during the first week after testing positive for influenza was significantly higher than any other period before or after.
▪The risk of stroke increased threefold during the first week after testing positive for influenza.

Public health implications for vaccination strategies

▪Croci et al. stated that highlighting the dual protection offered by influenza vaccination against both infection and its cardiovascular complications could have a substantial public health impact.
▪The Croci et al. study findings were published in Eurosurveillance in April 2026.
▪Croci et al. stated that if confirmed by additional studies in other settings, the findings would strengthen the case for prioritizing influenza vaccination among people at risk of heart disease or stroke.
▪Factoring the influenza vaccine's added protection against cardiovascular conditions into economic and burden analysis could help make a stronger economic case for influenza vaccination programs.

Perspective of Croci et al.

▪Croci et al. stated that if confirmed by additional studies in other settings, the findings would strengthen the case for prioritizing influenza vaccination among people at risk of heart disease or stroke.

Perspective of Public health economists

▪Factoring the influenza vaccine's added protection against cardiovascular conditions into economic and burden analysis could help make a stronger economic case for influenza vaccination programs.