Objective:

To investigate the association between prior influenza vaccination and the risk of acute myocardial infarction and stroke following breakthrough influenza infections.

Key Findings:

• Cardiovascular events clustered in the 7 days following a positive influenza test, with an adjusted incidence rate ratio (IRR) of 3.5 for acute myocardial infarction and stroke combined.
• The risk was higher for acute myocardial infarction (IRR, 4.7) than for stroke (IRR, 2.9).
• Prior vaccination was associated with about half the excess cardiovascular risk, with an adjusted IRR of 4.7 in unvaccinated episodes vs 2.4 in vaccinated episodes, indicating a significant interaction between vaccination status and post-infection risk.
• The highest risk occurred in the first 3 days following infection (IRR, 5.2), declining to baseline by days 15 to 28.

Interpretation:

Vaccination against influenza significantly reduces the risk of cardiovascular events following infection, particularly in the immediate days post-infection, highlighting its importance in public health strategies.

Limitations:

• The study used specimen collection date rather than symptom onset, potentially underestimating risk.
• It could not account for yearly variation in vaccine effectiveness and likely underrepresented milder influenza infections.
• Residual confounding from co-circulating pathogens or environmental factors could not be excluded.

Conclusion:

Prior influenza vaccination is associated with a significant reduction in the risk of acute myocardial infarction and stroke following influenza infection, underscoring the importance of vaccination in preventing serious cardiovascular events.