A Danish nationwide study published in Eurosurveillance found that influenza infection sharply increases the short-term risk of heart attack and stroke, but prior vaccination is linked to a significantly lower excess risk for those who still contract the virus. The findings were released on Apr. 6.
This research is important because cardiovascular diseases remain a leading cause of disability worldwide, with conditions like ischemic heart disease and stroke accounting for thousands of disability-adjusted life-years per 100,000 people. Understanding how influenza vaccination affects post-infection risks could influence future public health policies.
The study analyzed data from over 1,200 Danish residents aged 40 years or older who experienced their first hospital admission for acute myocardial infarction or stroke within a year after confirmed influenza infection. Researchers used national health registries to track outcomes across multiple flu seasons between 2014 and 2025. They found that during the first seven days following infection, the incidence rate ratio (IRR) for cardiovascular events was 3.5 compared to baseline periods. Notably, individuals who had received a seasonal flu vaccine before infection showed about half the excess risk compared to unvaccinated individuals (IRR = 2.4 vs IRR = 4.7).
The results also indicated that women experienced a higher relative incidence of these events than men after influenza infection; however, vaccination appeared protective in both groups. The researchers used Campylobacter spp., another infectious agent known to increase cardiovascular risk, as a control exposure and found no protective effect from flu vaccination against this pathogen.
According to the authors, "these findings suggest that influenza vaccination may attenuate severe cardiovascular complications after breakthrough influenza infection, even when the vaccine fails to prevent the primary viral infection." They caution that subgroup analyses should be interpreted carefully but highlight the specificity of vaccine protection against post-influenza cardiac events.
Future studies are expected to integrate annual vaccine effectiveness data in order to further refine these estimates and guide more targeted recommendations.
