In a nationwide Danish self-controlled case series study of 1,221 adults aged 40 years or older with a first-ever myocardial infarction or stroke and a PCR-confirmed influenza infection, cardiovascular risk was markedly elevated in the first week after infection (adjusted incidence rate ratio [IRR], 3.5), peaking within 3 days and returning to baseline within 2–4 weeks; the increase was greater for myocardial infarction (IRR, 4.7) than for stroke (IRR, 2.9). Prior influenza vaccination, recorded in 50% of infection episodes, was associated with a substantially lower excess risk (IRR, 2.4 in vaccinated vs 4.7 in unvaccinated episodes), with a statistically significant interaction indicating attenuation of postinfection cardiovascular risk, although differences were not significant when outcomes were analyzed separately.