Overall, the risk of myocarditis is greater after COVID-19 infection than after COVID-19 vaccination said UK researchers.
In their new study, published in Circulation, the authors explained that myocarditis is more common after COVID-19 infection than after COVID-19 vaccination. However, the risks in younger people and after sequential vaccine doses were less certain.
For the self-controlled case series study, the researchers used the National Immunisation Database of COVID-19 vaccination to identify vaccine exposure – which includes vaccine type, date, and doses for all people vaccinated in England – and linked this data at the individual level to national data for mortality (Office for National Statistics), hospital admissions (Hospital Episode Statistics and Secondary User’s service data), and SARS-CoV-2 infection data (Second Generation Surveillance System). The authors explained that the self-controlled case series design was originally developed to examine vaccine safety, and that analyses are conditional on each case, so any fixed characteristics during the study period, such as sex, ethnicity, or chronic conditions, are inherently controlled for.
They included almost 43 million people aged 13 years or older who had received at least one dose of COVID-19 vaccine in England between December 1, 2020 and December 15, 2021, and evaluated the association between vaccination and myocarditis, stratified by age and sex. They then estimated the incidence rate ratio and excess number of hospital admissions or deaths from myocarditis per million people for the 1 to 28 days after sequential doses of AstraZeneca, Pfizer-BioNTech, and Moderna vaccines, or after a positive COVID-19 test.
Risk of Myocarditis Following COVID-19 Vaccination Quite Small
The researchers found that the risk of myocarditis in unvaccinated individuals after COVID-19 infection was at least 11 times higher than that in people who developed myocarditis after receiving a COVID-19 vaccine or booster dose. They also highlighted that for those who had received at least one dose of a COVID-19 vaccine, the risk of COVID-19 infection-related myocarditis was reduced by half, which suggested that the "prevention of infection associated myocarditis may be an additional longer-term benefit of vaccination", said the authors.
Martina Patone, PhD, statistician at the Nuffield Department of Primary Health Care Science, University of Oxford, and first author of the study, said: "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."
The researchers explained that the risk of myocarditis "increased" in the 1 to 28 days after a first dose of the Astra-Zeneca COVID-19 vaccine (incidence rate ratio 1.33 [95% CI 1.09 to 1.62]), and after a first dose of Pfizer-BioNTech vaccine (IRR 1.52 [95% CI 1.24 to 1.85]). In addition, there was an increased risk of myocarditis at 1 to 28 days after a second dose of Pfizer-BioNTech vaccine (IRR 1.57 [95% CI 1.28 to 1.92]) and Moderna vaccine (IRR 11.76 [95% CI, 7.25 to 19.08]), and after a booster dose of Pfizer-BioNTech vaccine (IRR, 1.72 [95% CI, 1.33 to 2.22]) and Moderna vaccine (IRR, 2.64 [95% CI, 1.25 to 5.58]), they said. The risks of myocarditis after a positive COVID-19 test was IRR 11.14 [95% CI, 8.64 to 14.36] before vaccination, and IRR 5.97 [95% CI, 4.54 to 7.87]) after vaccination.
"The risk of myocarditis is substantially higher after COVID-19 infection in unvaccinated individuals than the increase in risk observed after a first dose of Astra-Zeneca vaccine, and a first, second, or booster dose of Pfizer-BioNTech vaccine," said the authors.
"In contrast with other vaccines, the risk of myocarditis observed 1 to 28 days after a second dose of Moderna vaccine was higher and similar to the risk after infection," they pointed out, and added that this risk "persisted after a booster dose".
However, the authors pointed out that people receiving a booster dose of Moderna were, on average, "younger" in comparison with those who received a booster dose of the Astra-Zeneca or Pfizer-BioNTech vaccines, therefore, results "may not be generalisable to all adults".
Vaccine-associated myocarditis was largely restricted to men younger than 40 years with one exception - both younger men and women were at increased risk of myocarditis after a second dose of Moderna vaccine.
Myocarditis is Rare
The authors said: "Myocarditis is an uncommon condition". They explained that the risk of vaccine-associated myocarditis is "small", with up to an "additional two events per million people" in the 28-day period after exposure to all vaccine doses other than the Moderna vaccine. "This is substantially lower than the 35 additional myocarditis events observed with COVID-19 infection before vaccination," they said.
Professor Nicholas Mills, PhD, Butler British Heart Foundation chair of cardiology, University of Edinburgh, and a co-author of the paper, emphasised: "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." He explained that the risk should be "balanced against the benefits of the COVID-19 vaccines in preventing severe COVID-19 infection".
He highlighted the importance of understanding who is at a higher risk for myocarditis and which vaccine type is associated with increased myocarditis risk.
Help Shape Future Vaccination Programmes
Julia Hippisley-Cox, FRCP, professor of clinical epidemiology and general practice, University of Oxford, and a co-author of the study, commented: "These findings are valuable to help inform recommendations on the type of COVID-19 vaccines available for younger people and may also help shape public health policy and strategy for COVID-19 vaccine boosters."
Dr Patone said that the 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".
Prof Hippisley-Cox pointed out that the SARS-CoV-2 virus continues to shift, and more contagious variants arise. She expressed hope that the new findings may enable a more well-informed discussion on the risk of vaccine-associated myocarditis when considered in contrast to the "net benefits" of COVID-19 vaccination.
"Our findings will inform recommendations on the type of vaccine offered to younger people and will help to shape public health policy on booster programmes, enabling an informed discussion of the risk of vaccine associated myocarditis when considering the net benefit of vaccination," the authors said.
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