Flu vaccine protects against heart attack – even in people infected by influenza

Flu 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 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 flu infection in the first place. The new study, which included 1,221 adults aged 40 or older in Denmark, looked at whether cardiovascular risk was reduced even if people were infected with flu despite being vaccinated.

‘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,’ says Dr Roberto Croci, Statens Serum Institut in Copenhagen.

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 a year after an influenza virus infection. It included all lab-confirmed flu virus infections that occurred during nine consecutive influenza seasons.

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.

The risk of a first-time hospitalisation 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.

However, the results add to the growing importance of protecting against infectious diseases in people at risk of cardiovascular events. ‘Highlighting the dual protection offered by vaccination, against both infection and its cardiovascular complications, could have a substantial public health impact,’ the authors said.

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 programmes.