Flu vaccine halves heart attack risk after breakthrough infection

Influenza vaccination could significantly reduce the risk of influenza-associated heart attack and stroke even among people who get infected after getting the vaccine, according to research just published in Eurosurveillance.

Catching influenza increases the short-term risk of cardiovascular conditions, and existing evidence has shown that the vaccine reduces the risk of heart attack and stroke by preventing infection in the first place. The study by Croci et al., which included 1,221 adults aged 40 or older in Denmark, looked into how much the risk increased after infection and if the vaccine could reduce this added risk even if people were infected anyway.

"If confirmed by additional studies in other settings, this would strengthen the case for prioritising influenza vaccination among people at risk of heart disease or stroke and would support refining recommendations across Europe," say Croci et al.

Sharp rise in hospitalizations for heart attack and stroke after influenza infection

The study used Danish health registry data from 2014 to 2025 and included individuals aged 40 and above with a first-ever 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 an observation window of 365 days before and after a positive test for influenza. Testing, hospitalisation, vaccination and mortality records were matched through Denmark's unique personal identifiers.

The study population comprised 660 males and 561 females aged 40 years and above, with a median age of 75. Most patients were hospitalised with a stroke (65%), while 35% had a heart attack. Of 1,231 influenza virus infections confirmed by PCR testing, about half were happening after patients had been vaccinated. Researchers used conditional Poisson regression to estimate incidence rate ratios (IRRS) and 95% confidence intervals using the self-controlled case series design, a study design that compares event timing within the same person, thereby controlling for other individual factors such as comorbidities, genetic predisposition, and socioeconomic status. To minimise reverse causality, i.e. where cardiovascular symptoms may have prompted testing, a pre-exposure period of two weeks before testing was excluded.

The risk for a first-time hospitalization for heart attack and stroke during the first week after testing positive for influenza was found to be significantly higher than for any other period before or after; it increased threefold for a stroke and fivefold for a heart attack.

This increased risk was reduced by half for people who were infected but had been vaccinated against influenza for that influenza season.

The study did not account for differences in effectiveness between influenza vaccines, which can vary depending on how well the vaccine formulation matches the viral strains circulating in that season. It could also not assess whether vaccination timing or gender affected outcomes. Results might also not apply directly to populations or settings with different influenza epidemiology, healthcare systems, or vaccination strategies.

Protective effect could have implications for vaccination strategies

According to Croci et al., "highlighting the dual protection offered by vaccination, against both infection and its cardiovascular complications, could have a substantial public health impact."

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