Influenza vaccine halves risk of heart attack after epidemic infection

The flu shot could significantly reduce the risk of flu-related heart attack and stroke even among people who become infected after getting the shot, according to research just published in Eurosurveillance.

Influenza infection increases the short-term risk of cardiovascular disease, and existing evidence has shown that the vaccine reduces the risk of heart attack and stroke by preventing the infection in the first place. Croci et al.’s study, which included 1,221 adults aged 40 and over in Denmark, looked at how much the risk increased after infection and whether the vaccine could reduce this additional risk, even if people had been infected anyway.

“If confirmed by additional studies in other settings, this would strengthen the view to prioritize influenza vaccination in people at risk of heart disease or stroke and support recommendations for improvement across Europe.” say Croci et al.

Hospitalizations for heart attack and stroke spike after flu infection

The study used data from the Danish health register from 2014 to 2025 and included people aged 40 and over with a first hospital admission for a heart attack or stroke within about a year after an influenza virus infection. It included all laboratory-confirmed influenza virus infections that occurred during nine consecutive influenza seasons with a 365-day observation window before and after a positive influenza test. Testing, hospitalization, vaccination and mortality records were matched using unique Danish personal identifiers.

The study population included 660 men and 561 women aged 40 years or older, with a median age of 75. Most patients were hospitalized with a stroke (65%), while 35% had a heart attack. Of the 1,231 influenza virus infections confirmed by PCR testing, about half occurred after patients were vaccinated. The researchers used conditional Poisson regression to calculate incidence rate ratios (IRRS) and 95% confidence intervals using the self-controlled case series design, a study design that compares the timing of events in the same person, thereby controlling for other individual factors such as comorbidities, genetic predisposition, and socioeconomic status. To minimize reverse causality, i.e. where cardiovascular symptoms may have driven control, a pre-exposure period of two weeks prior to testing was excluded.

The risk of first hospitalization for heart attack and stroke during the first week after testing positive for the flu was found to be significantly higher than for any time before or after. tripled for a stroke and quintupled for a heart attack.

This increased risk was halved for people who had been infected but had been vaccinated against the flu for that flu season.

The study did not take into account differences in effectiveness between flu vaccines, which can vary depending on how well the vaccine formulation matches the strains of virus circulating at the time. It also could not assess whether timing of vaccination or gender affected the results. The results may also not apply directly to populations or settings with different influenza epidemiology, health care systems, or vaccination strategies.

The protective effect could have implications for vaccination strategies

According to Croci et al., “Demonstrating the dual protection afforded by vaccination, both against infections and its cardiovascular complications, could have a significant impact on public health.“

Consideration of the vaccine’s additional protection against these conditions in an economic and burden analysis could also help strengthen financial support for influenza vaccination programs.