Acute Myocardial Infarction after Laboratory-Confirmed Influenza Infection

In a recent retrospective cohort study published in the New England Journal Medicine, a significant association was found between acute respiratory infections, particularly influenza, and the occurrence of acute myocardial infarction (MI) in the following week. The study also highlights the increased risk in older patients and in those who are hospitalized for the first time for an MI.

Previous studies have shown that upper respiratory tract infections might increase the risk of developing MI. However, these studies showed increased risk within the first three days (not the first week) after the infection. Moreover, the methods for detecting respiratory infections were less reliable such as serologic testing or documentation of the physician visit as a respiratory tract infection.

When asked to comment Dr. Kwong, the principal investigator and the associate professor at the University of Toronto, states “Our study was the first to use modern laboratory methods for detection of respiratory viruses (i.e., PCR) to confirm the association between influenza viruses and MI. By applying the self-controlled case series design to large databases of laboratory test results linked to hospitalization records, we confirmed that there is a 6-fold increased risk of MI during the 7 days after testing positive for influenza. This risk seems to be concentrated only during that time, and settles back to baseline after 1 week.”

The study included all the patients in Ontario who were hospitalized for acute MI between the years 2008 and 2015 and those who have undergone testing for the respiratory viruses between 2009 and 2014. The incidence of acute MI was found to be significantly higher in the week following the infection when compared to its incidence in a year before and after the infection. This observation was true after the infection with influenza A, influenza B, respiratory syncytial virus, and other viruses.

The authors noted the following important limitations associated with this study: One limitation is the uncertainty regarding the onset of influenza infection and of acute MI. A second limitation is the possibility of confounding due to time-varying factors (e.g., since both acute MI and influenza exhibit seasonal patterns, another seasonally varying factor could be a confounder). A third limitation is that these results might apply only to respiratory infections that are of sufficient severity to result in laboratory testing. Since most patients with milder symptoms do not undergo testing for respiratory viruses, these findings may not be generalizable to milder infections.

When asked about the implications of the study on the clinical practice, Dr. Kwong said “Hopefully, the findings of this study will persuade more patients – especially those who are at high risk of MI – to get vaccinated against influenza. Frequent hand-washing is also recommended, in order to prevent infection by influenza and other respiratory viruses.”