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About two people per 100,000 who received an mRNA COVID-19 vaccine experienced myocarditis or myopericarditis afterward, according to a new study from Denmark.
Researchers found these rare cardiac conditions occurred more frequently after Moderna vaccines than Pfizer-BioNTech and most cases were mild.
“This is again highlighting the fact the vaccine is still very low risk for everybody compared to SARS-CoV-2,” said Yvonne A. Maldonado, M.D., FAAP, chair of the AAP Committee on Infectious Diseases. “And also, even this very very low risk … demonstrated that the disease was mild and did not have serious sequelae.”
Researchers studied about 5 million Danish residents from October 2020 to October 2021, collecting data on their vaccination status and hospital diagnosis of myocarditis or myopericarditis within 28 days of vaccination. They reported their findings Thursday in “SARS-CoV-2 vaccination and myocarditis or myopericarditis: population-based cohort study,” Husby A, et al. BMJ. Dec. 16, 2021, https://www.bmj.com/content/375/bmj-2021-068665.
The study found the Moderna vaccine was associated with a statistically significant increased risk of myocarditis or myopericarditis, but the numbers were low. Among about 499,000 Moderna recipients there were 21 cases of myocarditis or pericarditis, a rate of about 4.2 per 100,000. For people ages 12-39, the rate was 5.7 per 100,000.
There were 48 cases of myocarditis or myopericarditis among about 3.5 million Pfizer-BioNTech vaccine recipients, a rate of about 1.4 per 100,000. The rate for people ages 12-39 years was 1.6 per 100,000 and for ages 12-17 years it was one per 100,000.
The Pfizer vaccine was associated with a statistically significant increase in myocarditis or myopericarditis in women, but not in the overall population or in the 12-39 years cohort. For both vaccines, women had lower rates of myocarditis per 100,000 than men, but higher adjusted hazard ratios.
Most cases of myocarditis were mild. Researchers also found people infected with SARS-CoV-2 had a 14-fold higher risk of cardiac arrest or death compared to people who were not infected.
In the U.S., young males have been thought to be at highest risk for myocarditis or myopericarditis compared to other age groups who have received mRNA vaccines. The Denmark study’s finding that rates are higher after Moderna than Pfizer vaccines are consistent with other studies. However, the Pfizer vaccine doses were given to Denmark residents at a median of 35 days apart while in the U.S. the recommended interval is 21 days, which authors discussed could impact the myocarditis rates.
Authors said they believe the low risk of myocarditis after vaccination “supports the overall benefits of such vaccination on an individual, societal, and global level.”
“Given the worldwide spread of the highly contagious SARS-CoV-2 delta and omicron variants, future infection is the undesirable alternative to vaccination against SARS-CoV-2,” they wrote.
- Information from the CDC on myocarditis after COVID-19 vaccination
- CDC clinical considerations for COVID-19 vaccines
- AAP guidance on providing COVID-19 vaccines to children and adolescents
- Information for parents from HealthyChildren.org on myocarditis after vaccination and preparing children and adolescents for COVID-19 vaccination