top of page


October 2020

The risk for myocardial infarction (MI) substantially increases within the first few days of the onset of influenza, while influenza vaccination has been shown to offset the increased risk.

Caldeira et al. conducted a systematic review and meta-analysis of self-controlled case studies (SCCS), published in July 2019 in Expert Review of Vaccines. Three studies of influenza infection and two studies of influenza vaccination were included, involving 5,480 and 32,676 patients, respectively.

Only SCCSs were included because they can “mostly overcome the problem of confounding bias in the control arm in transient exposure (influenza infection or vaccination), after and before the outcome,” the researchers wrote. Studies focusing on specific populations, such as those with immunodeficiency, heart failure or respiratory diseases were excluded.

The researchers stated that both the influenza vaccination and influenza infection are associated with inflammation, and that acute inflammation can trigger cardiovascular events. Regarding vaccination inflammation, researchers referred to the findings of a study from The Netherlands, which reported on adverse effects ranging from injection-site inflammation to limb swelling. 

The study reviewed the risk of MI in the context of vaccination versus infection. Among those with influenza or influenza-like illness who did not get vaccinated, the risk of MI was 5.8 times higher during the first three days of infection; it was 4.5 times higher between four and seven days.

Among the vaccinated cohorts, the study found the vaccine was safe regarding the short-term risk for MI and “the pooled estimate showed a significant risk reduction of MI risk in the first month after vaccination.”

While protective cardiovascular effects from flu shots are increasingly recognized, the duration of protection is unknown, according to Dr. Jacob Udell, associate professor of medicine at the University of Toronto. Protection may wane over time and is “likely just for that immediate season, with variation year to year in match and virulence of circulating strains,” he said.

On the plus side, studies suggest that flu vaccines may afford mild protection to strains of flu similar but not identical to those contained in the vaccine.

People who get the flu, especially more severe cases, can develop systemic inflammatory response syndrome which can lead to lower oxygen delivery to the myocardium. Inflammation contributes to plaque rupture and thrombus formation. However, many studies have not included milder influenza patients.

In addition to inflammation, influenza and influenza-like illnesses increase MI risk because “an already vulnerable patient doesn’t thrive with an infection that causes pneumonia, low oxygen, stress hormone release, faster pulse, low blood pressure. Alternatively, there is accumulating evidence of direct flu invasion in heart tissue in patients with heart complications, similar to lung tissue with severe pneumonia,” said Udell.

He added that studies suggest there are many similarities with COVID-19, “but COVID-19 is more stealthy and contagious.”

Dr. Udell is co-author of a new study describing mechanistic links between influenza and COVID-19 infection and the risk of acute cardiovascular events, published in the Journal of the American College of Cardiology.

October 5, 2020by Pippa Wysong on 
Categories: Influenza

Links between influenza, vaccination and heart attack: News
bottom of page