By Landon Gray
The risk for myocarditis is 11 times higher for unvaccinated people after COVID-19 infection compared with those who develop the condition after receiving the COVID-19 vaccination or first booster dose, according to a large detailed analysis.
Nearly 43 million people, 13 years of age and older, who received at least one dose of the COVID-19 vaccine in England between Dec. 1, 2020 and Dec. 15, 2021, were analyzed in a self-controlled case series study to determine the risk for myocarditis after COVID-19 infection and vaccination. The researchers were particularly interested in younger people and those who received sequential vaccine doses.
By using England’s National Immunization database of COVID-19 vaccinations, researchers could evaluate people at least 13 years old, who had received at least one dose of the ChAdOx1 vaccine developed by the University of Oxford and AstraZeneca (a two-dose, adenovirus-vector COVID-19 vaccine, most similar to the Janssen vaccine in the United States), the Pfizer-BioNTech or the Moderna COVID-19 vaccine, between the periods cited above.
The database detailed the types of vaccines received, administration date and dose sequencing, while also providing individual demographic information, which included the age and sex of each person. Almost 43 million people were counted in the database’s records, including more than 21 million people who had received a booster dose of any of the COVID-19 vaccines—bringing their total count to three doses. Nearly 6 million people tested positive for COVID-19 either before or after vaccination during the study period.
The records from the database were then cross-referenced and matched to records in England’s national offices that house data on COVID-19 infection, hospital admission and death certificates from Dec. 1, 2020 through Dec. 15, 2021.
Individuals were categorized by age and sex to evaluate which groups had the highest risk for developing myocarditis after either COVID-19 vaccination or infection and hospitalization. The researchers estimated the relative incidence of an acute event in a predefined post-vaccination risk period of one to 28 days, and analyses were controlled to adjust for any fixed characteristics, including sex, race or ethnicity, or chronic health conditions.
The researchers found that fewer than 3,000 (0.007%) people were hospitalized or died due to myocarditis during the one-year study period. Approximately 615 of these myocarditis cases occurred during days 1 through 28 after receiving a COVID-19 vaccination–and 514 were hospitalized. The researchers also found that people who had a COVID-19 infection before receiving a dose of any vaccine had an 11 times higher risk for myocarditis during days 1 through 28 after a positive test.
“We found that across this large data set, the entire COVID-19?vaccinated population of England during an important 12-month period of the pandemic when the COVID-19 vaccines first became available, the risk of myocarditis following COVID-19 vaccination was quite small compared with the risk of myocarditis after COVID-19 infection,” said Martina Patone, PhD, a statistician at the Nuffield Department of Primary Health Care Sciences at the University of Oxford, in England, and first author of the study. “This analysis provides important information that may help guide public health vaccine campaigns, particularly since COVID-19 vaccination has expanded in many parts of the world to include children as young as 6 months old.”
The risk for COVID-19?related myocarditis decreased dramatically, by nearly half, for people who received a COVID-19 vaccination but had a breakthrough infection. Conversely, the risk increased after the first dose of the ChAdOx1 vaccine, and after a first, second or booster dose of any of the available mRNA vaccines. However, the risk for COVID-19 vaccine-related myocarditis was lower than that for COVID-19 infection-related myocarditis, except after a second dose of the Moderna vaccine, according to the researchers.
The risk for myocarditis was found to be higher during the first 28 days after a second dose of the Moderna vaccine for people of all ages and sexes, with the risk persisting even after a booster dose. The researchers noted that the people who received a booster dose of Moderna were, on average, younger than those who received a booster dose of the ChAdOx1 or Pfizer-BioNTech vaccine, so the results may not be generalizable to all adults, the researchers said.
“It is important for the public to understand that myocarditis is rare, and the risk of developing myocarditis after a COVID-19 vaccine is also rare. This risk should be balanced against the benefits of the COVID-19 vaccines in preventing severe COVID-19 infection,” said Nicholas Mills, PhD, the Butler British Heart Foundation Chair of Cardiology and a professor at the University of Edinburgh, in Scotland, and a co-author of the study.
“It is also crucial to understand who is at a higher risk for myocarditis and which vaccine type is associated with increased myocarditis risk,” Dr. Mills said.
