Myocarditis risk significantly higher after COVID-19 infection vs. after a COVID-19 vaccine

In a detailed analysis of nearly 43 million people, ages 13 and older, who received at least one dose of a COVID-19 vaccine in England, the risk of myocarditis in unvaccinated individuals after COVID-19 infection was at least 11 times higher compared to people who developed myocarditis after receiving a COVID-19 vaccine or booster dose between December 1, 2020 and December 15, 2021, according to new research published today in the American Heart Association’s flagship, peer-reviewed journal Circulation.

Several previous studies and reports from public health agencies around the world including the U.S. Centers for Disease Control and Prevention 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 .

Typically trigged by a viral infection, myocarditis is the inflammation of the middle layer of the wall of the heart muscle, the myocardium. This condition is uncommon and may temporarily or permanently weaken the heart muscle and the heart’s electrical system, which keeps the heart pumping normally. An episode of myocarditis may resolve on its own or with treatment, and may result in lasting damage to the heart. In the general population not during a global pandemic, it is estimated that approximately 10 to 20 people per 100,000 are diagnosed with myocarditis each year, according to the American Heart Association’s 2021 scientific statement on myocarditis.

“We found that across this large dataset, 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 to the risk of myocarditis after COVID-19 infection,” says first author of the study Martina Patone, Ph.D., a statistician at the Nuffield Department of Primary Health Care Sciences at the University of Oxford in Oxford, England. “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.”

In this study, Patone and colleagues evaluated England’s National Immunization database of COVID-19 vaccinations for all people ages 13 or older who had received at least one dose of the ChAdOx1 (a two-dose adenovirus-vector COVID-19 vaccine developed by the University of Oxford and AstraZeneca, most similar to the one-dose Johnson & Johnson/Janssen COVID-19 vaccine available in the U.S.), the Pfizer-BioNTech or the Moderna COVID-19 vaccine (the same mRNA vaccines available in the U.S.) between December 1, 2020 and December 15, 2021. This dataset totaled nearly 43 million people, which included more than 21 million who had received a booster dose of any of the COVID-19 vaccines (meaning they had received a total of 3 doses of a COVID-19 vaccine). The database detailed the type of COVID-19 vaccines received, dates received and dose sequencing, along with individual demographic information including age and sex for each individual. Nearly 6 million people tested positive for COVID-19 infection either before or after COVID-19 vaccination during the study period.

England’s National Immunization database records were then cross-referenced and matched to the national offices with data on COVID-19 infection, hospital admission and death certificates for the same time period, December 1, 2020 through December 15, 2021. Individuals were classified based on age and sex to reveal which groups had the highest risk of myocarditis after a COVID-19 vaccine or after COVID-19 infection and hospitalization. The authors used the self-controlled case series (SCCS) method, which was developed to estimate the relative incidence of an acute event in a pre-defined post-vaccination risk period (1-28 days), compared to other times (pre-vaccination or long after vaccination). Being a within-person comparison, the analyses were controlled to adjust for any fixed characteristics, including sex, race or ethnicity, or chronic health conditions.

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