Even though several studies have suggested an association between influenza infections and acute cardiovascular events, most of them had inconsistent findings. A self-controlled case-series study by Canadian researchers has evaluated the association between laboratory-confirmed influenza infection and acute myocardial infarction (MI). The results were published in The New England Journal of Medicine. Using a linked laboratory database from 19 laboratories and a separate database of all admissions to acute care hospitals, investigators identified 332 Ontario residents over 35 years of age who were hospitalised for acute MI within one year before or after a positive influenza laboratory test.

Researchers compared the incidence of hospitalisation for acute MI during the risk interval (days 1 to 7 after a positive influenza detection) with that during the control interval (the 52 weeks before and the 52 weeks after the risk interval). Of the 364 hospitalisations that were due to acute MI, 344 occurred during the control interval (i.e. 3.3 admissions per week). 20 hospitalisations occurred during the risk period (i.e. 20 admissions per week) which was six times higher than that occurred during the control interval. No increased incidence was observed after day 7. Authors also found a smaller but significantly increased incidence of hospitalizations for acute MI after a positive test for respiratory syncytial virus and other viruses.

The study findings thus justify the need for influenza immunization especially for those patients who are at a risk of developing coronary artery diseases.

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