The risk of myocarditis (inflammation of the heart muscle) in unvaccinated individuals after the COVID infection was at least 11 times higher compared to people who developed the heart condition after receiving a vaccine or booster dose, a detailed analysis of nearly 43 million people, ages 13 and older, has revealed.

Several previous studies and reports from public health agencies, including the US Centers for Disease Control and Prevention (CDC), have highlighted a possible connection and potentially increased risk of myocarditis after receiving an mRNA COVID-19 vaccine, generating considerable scientific, policy and public interest.

"We found that the entire COVID-19-vaccinated population of England during an important 12-month period of the pandemic when the vaccines first became available, the risk of myocarditis following vaccination was quite small compared to the risk of myocarditis after infection," said Martina Patone, a statistician at the University of Oxford.

"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," Patone said in the study published in the American Heart Association's peer-reviewed journal Circulation.

In this study, Patone and colleagues evaluated a database of COVID-19 vaccinations for all people ages 13 or older who had received at least one dose of the University of Oxford-AstraZeneca vaccine, the Pfizer-BioNTech or the Moderna vaccine between December 1, 2020, and December 15, 2021. This dataset totalled nearly 43 million people, which included more than 21 million who had received a booster dose of any of the COVID-19 vaccines.

Nearly 6 million people tested positive for COVID-19 infection either before or after COVID-19 vaccination during the study period. The database records were then cross-referenced and matched to the national offices with data on COVID infection, hospital admission and death certificates for the same time period. The analyses found that less than 3,000 people were hospitalised or died from myocarditis during the one-year study period.

People who were infected with COVID-19 before receiving any doses of the vaccines were 11 times more at risk for developing myocarditis during days 1-28 after a COVID-19 positive test, the findings showed.

"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 vaccines in preventing severe infection," said Professor Nicholas Mills, the Butler British Heart Foundation Chair of Cardiology at the University of Edinburgh and a co-author of the paper.

It is also crucial to understand who is at a higher risk for myocarditis and which vaccine type is associated with increased myocarditis risk, Mills added. The SARS-CoV-2 virus continues to shift, and more contagious variants arise. "Our hope is that this data may enable a more well-informed discussion on the risk of vaccine-associated myocarditis when considered in contrast to the net benefits of vaccination," said co-author Julia Hippisley-Cox.