Myocarditis after COVID-19 and influenza

What is the difference in the risk of myocarditis after SARS-CoV-2 and influenza infection? A new cohort study provides clear answers.

COVID-19 and myocarditis:

• Viral diseases such as influenza and COVID-19 are the most common causes of myocarditis.
• According to studies, acute myocarditis occurs in 2.4 to 4.1 out of 1,000 hospitalised patients with COVID-19.
• Although the course is usually mild, it can lead to heart failure and sudden cardiac death.
• Myocardial damage has also been observed in mild or asymptomatic infections with SARS-CoV-2, although its clinical significance is unclear.

Primary endpoint: myocarditis within one year of infection

In a large-scale retrospective cohort study, US health data were used to compare the rates of myocarditis after infection with SARS-CoV-2 or influenza. The researchers also wanted to know which diagnostic measures were taken as a result.

Patients with either infection were included, with different time periods chosen for each to clearly distinguish COVID-19 and influenza cases. The primary endpoint was clinically diagnosed acute myocarditis within 12 months of infection. Secondary endpoints included various instrumental and laboratory tests.

Myocarditis rate three times higher in COVID-19 than in influenza

Of the more than 1 million adult COVID-19 patients included and the approximately 440,000 influenza patients, 0.06% and 0.02%, respectively, were diagnosed with myocarditis. The myocarditis rate per 1,000 person-years was 0.73 (95% CI 0.67-0.78) for COVID-19, three times higher than for influenza at 0.24 (95% CI 0.19-0.28). The risk was highest in COVID-19 in the age group between 18 and 29 years, with a rate of 1.16, while influenza mainly affected older patients over 70 years (rate of 0.42). Gender only played a role in COVID-19, where the rate was higher in men than in women (0.84 versus 0.63), whereas there was no gender difference in influenza.

The following were found to be independent risk factors for myocarditis:

• a diagnosis of COVID-19 (compared to influenza)
• cardiac comorbidities
• male gender
• age < 30 years

The most common diagnostic tests were ECG and echocardiography, with troponin levels dominating among laboratory values. In contrast, cardiac magnetic resonance imaging (CMR) was only performed in about a quarter of patients.

Young men under 30 particularly at risk

The study confirms what has already become apparent during the pandemic: COVID-19 infection is a significant risk factor for subsequent myocarditis, apparently even more so than influenza infection. In the opinion of the authors, it is therefore important to identify patients at risk early and to subject them to adequate diagnostics. Particular attention should be paid to young men under the age of 30.