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Study Questions:
What is the risk of myocarditis, pericarditis, and myopericarditis associated with the Pfizer BNT162b2 compared to the Moderna mRNA-1273 severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine?
Methods:
This is a population-based cohort study from British Columbia which integrates clinical, administrative vaccinations and surveillance data sets surrounding COVID-19, and includes data from adults (≥18 years) who received a first dose of mRNA vaccine between January 1, 2021 and September 9, 2021. Individuals who had a history of myocarditis or pericarditis within a year of receiving the first vaccine dose were excluded. The primary exposure was second dose of an mRNA vaccine. The outcome was diagnosis of myocarditis, pericarditis, or myopericarditis, as defined using International Classification of Diseases codes during a hospitalization or an emergency department visit within 21 days of the second vaccination dose.
Results:
A total of 3,095,175 individuals met eligibility criteria for this analysis. The majority of second doses were BNT162b2 (75%). Gender, age, and time between administration of the first and second dose were similar between both groups. In the 21 days following the second dose of mRNA vaccine, a total of 59 myocarditis (28 BNT162b2, 31 mRNA-1273) and 41 pericarditis (21 BNT162b2, 20 mRNA-1273) events were reported.
The rates of myocarditis and pericarditis per million second doses were higher for mRNA-1273 (n = 31, rate of 35.6 per million doses; 95% confidence interval [CI], 24.1-50.5 and n = 20, rate of 22.9 per million doses; 95% CI, 14.0-35.4, respectively) than BNT162b2 (n = 28, rate of 12.6 per million doses; 95% CI, 8.4-18.2 and n = 21, rate of 9.4 per million doses; 95% CI, 5.8-14.4, respectively). mRNA-1273 vs. BNT162b2 had significantly higher odds of myocarditis (adjusted odds ratio [aOR], 2.78; 95% CI, 1.67-4.62), pericarditis (aOR, 2.42; 95% CI, 1.31-4.46), and myopericarditis (aOR, 2.63; 95% CI, 1.76-3.93). The association between mRNA-1273 and myocarditis was stronger for males (aOR, 3.21; 95% CI, 1.77-5.83) and younger age group (<40 years; aOR, 5.09; 95% CI, 2.68-9.66). History of SARS-CoV-2 infection and time of year of vaccination had no impact on the association.
Conclusions:
While overall rare, the risk for myocarditis or pericarditis was 2- to 3-fold higher for the Moderna mRNA-1273 vaccine compared to the Pfizer BNT162b2 vaccine, and was highest among young males.
Perspective:
This is yet another large study confirming that myocarditis post-vaccination using BNT162b2 and mRNA-1273 is rare, but more common with the mRNA-1273 vaccine. The association was only statistically significant in the young (<40 years) and men. The strengths of this study are that it is population based and includes over 3,000,000 individuals. These data, beyond reassurance, have little clinical significance at this time given these specific vaccines are no longer in use, and we cannot extrapolate safety to future vaccines, given the immunogenicity likely depends on the mRNA product rather than the specific nature of the vaccine (mRNA or non-mRNA).
