Key result
Influenza infection linked to ~3.5-fold higher 7-day AMI or stroke risk, attenuated by vaccination.
Why the study?
Influenza infection is a recognised trigger of AMI and stroke, but whether influenza vaccination modifies this risk remains unclear.
Does prior seasonal influenza vaccination attenuate the short-term risk of first-ever acute myocardial infarction or stroke during the 7 days following a PCR-confirmed breakthrough influenza infection in adults aged ≥40 years?
Observational (n=1,221)
Yes
Does prior seasonal influenza vaccination attenuate the short-term risk of first-ever acute myocardial infarction or stroke during the 7 days following a PCR-confirmed breakthrough influenza infection in adults aged ≥40 years?
Prior seasonal influenza vaccination cuts in half the transient excess risk of first-time acute myocardial infarction and stroke during the week following a breakthrough PCR-confirmed influenza infection.
Supports seasonal influenza vaccination to attenuate post-infection AMI/stroke risk in adults ≥40; extends observational data but leaves causal benefit open pending RCTs.
BACKGROUND Influenza infection is a recognised trigger of acute myocardial infarction (AMI) and stroke, but whether influenza vaccination modifies this risk remains unclear. AIM We aimed to quantify the short-term cardiovascular risk after laboratory-confirmed influenza infection and assess whether vaccination attenuates it. METHODS We conducted a nationwide self-controlled case series study using Danish health registries (2014–2025) and included individuals aged ≥ 40 years with a first-ever hospital admission for AMI or stroke within ± 365 days of a PCR-confirmed influenza infection. We defined days 1–7 after specimen date as the risk period and excluded a 14-day pre-exposure period to reduce reverse causality, where influenza testing might be prompted by cardiovascular disease symptoms. We linked testing, hospitalisation, vaccination and mortality data deterministically via unique personal identifiers. We estimated incidence rate ratios (IRRs) and 95% confidence intervals (CI) with conditional Poisson regression. RESULTS Among 1,221 individuals with a first-ever AMI (n = 429; 35%) or stroke (n = 792; 65%), median age was 75 years (interquartile range: 66–82); 561 (46%) were female. After calendar-month adjustment, the IRR for cardiovascular events during the risk period was 3.5 (95% CI: 2.6–4.7), higher for AMI (IRR = 4.7; 95% CI: 3.1–7.4) than stroke (IRR = 2.9; 95% CI: 2.0–4.2). Prior influenza vaccination during the same influenza season, recorded in 610 (50%) episodes, reduced the excess risk of AMI or stroke associated with influenza infection (interaction p = 0.020). CONCLUSIONS Influenza infection conferred a transiently increased risk of first-time AMI and stroke. Vaccination substantially attenuated this risk, supporting its role in preventing cardiovascular complications after breakthrough infection.
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Croci et al. (2026) conducted an observational in PCR-confirmed influenza infection and first-ever hospital admission for acute myocardial infarction or stroke (n=1,221). PCR-confirmed influenza infection (risk period days 1-7 post-test) and seasonal influenza vaccination vs. Within-person baseline control period (+/- 365 days around test date excluding days -14 to +7) and unvaccinated influenza episodes was evaluated on First-ever hospital admission for acute myocardial infarction or stroke during days 1-7 after PCR-confirmed influenza infection (IRR 3.5, 95% CI 2.6-4.7, p=<0.0001). PCR-confirmed influenza increased 7-day AMI or stroke risk (IRR 3.5, 95% CI 2.6-4.7), which was halved by prior seasonal vaccination (IRR 2.4 vs 4.7; interaction p=0.020).
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