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Influenza is linked to myocardial infarctions


A new study shows that an influenza infection was associated with an increased risk of myocardial infarctions (heart attacks), especially for those without prior hospitalization for coronary artery disease (CAD)—another good reason to get the flu vaccine.

There is a belief that the flu is not a very serious disease, but there is plenty of evidence that it’s much more serious than many believe. This study provides evidence that it can be serious.

Let’s look at the study and determine if it establishes a correlation between influenza and myocardial infarctions.

a patient whose blood pressure and pulse are measured in the hospital influenza myocardial infarction
Photo by Engin Akyurt on Pexels.com

Influenza and myocardial infarctions paper

In an article published on 25 June 2024 in NEJM Evidence, Patricia Bruijning-Verhagen, MD, PhD, of Utrecht University in the Netherlands, and colleagues used a self-controlled case series design and included adults ages 35 and older who were tested for respiratory viruses from January 2008 through December 2019. The researchers identified 158,777 PCR (polymerase chain reaction) tests for influenza from 16 laboratories across the Netherlands and data were linked to national medication, hospitalization, mortality, and administrative databases. Of the PCR tests, 23,405 were positive for influenza and represented unique illness episodes.

Here are the key results of the study:

  • The relative incidence of acute myocardial infarctions (MI) in the 1 to 7 days after PCR-confirmed influenza infection (called the risk period) was 6.16 times greater than the control period of 52 weeks before and 51 weeks after the risk period
  • Among patients with influenza but without prior hospitalization for CAD, the relative incidence of acute myocardial infarctions was 16.60 compared with 1.43 for those who had been previously hospitalized for CAD. In other words, the risk of acute MI was 11.6 higher for those who were not previously hospitalized for CAD.

C. Raina MacIntyre, MBBS, PhD, of the University of New South Wales Sydney in Randwick, Australia, and colleagues wrote in an accompanying editorial that the finding that ambulatory adults with underlying, undiagnosed CAD were most at risk for acute MI was crucial.

Even though the study from Bruijning-Verhagen and colleagues did not include vaccination data, a randomized, double-blind, placebo-controlled, multicenter clinical trial examined the differences in risk between vaccinated and unvaccinated groups for coronary artery disease. MacIntyre and colleagues stated that it demonstrated that getting a flu shot early after an MI or with high-risk coronary heart disease resulted in a lower risk of a composite of all-cause death, MI, or stent thrombosis, and a lower risk of all-cause death and cardiovascular death, at 12 months compared with placebo.

Bruijning-Verhagen and colleagues hypothesized that the increased risk of acute MI after acute influenza and other respiratory infections is caused by increased metabolic demand and effects on inflammatory and coagulation pathways that lead to atherosclerotic plaque destabilization and subsequent occlusion. This biological plausibility provides us with evidence that there is a causal link between influenza and myocardial infarctions. Furthermore, it may show us why the flu vaccine is so useful in reducing the risk of heart attacks.

Summary

There are a couple of weaknesses to this study. First, it did not specifically examine whether the seasonal influenza vaccine had any effect on the risk of myocardial infarctions. However, we do have pretty powerful evidence that the flu vaccine is linked to a lower risk of heart attacks.

Second, there might be some other factor that is linked to both severe flu infection and heart attacks. In that case, the flu may not be linked to heart attacks.

However, this study is large and provides very good evidence that the flu is correlated with heart attacks. This means that the flu vaccine may be one of the easiest ways to reduce the risk of myocardial infarctions.

Citations

Michael Simpson
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