Risk of myocarditis requiring hospitalization following Covid-19 mRNA booster vaccinations

Abstract

Abstract Background Reports have suggested an association between the development of myocarditis and the receipt of the primary series of messenger RNA (mRNA) Covid-19 vaccines. This association seems to decrease in the first monovalent booster. However, whether this association exists between the second monovalent and first bivalent boosters is unknown. Purpose To evaluate the association of the BNT162b2 mRNA booster vaccines and myocarditis events requiring hospitalization. Methods We analyzed the medical records of all CHS members aged 12 and above of Clalit Healthcare Services who received the first, second monovalent and bivalent BNT162b2 mRNA COVID-19 vaccine boosters between July 30, 2021, and November 15, 2022. We evaluated the risk differences per 10,000 subjects of myocarditis between 28 days before vaccination (baseline period: 35 days until 8 days before vaccination) and after vaccination (post-vaccination: the day of inoculation until day 27) in a pairwise method using the non-parametric percentile bootstrap. The results were stratified by sex and age. Results Among 1,965,200 subjects vaccinated with the first booster, 13 events were recorded post-vaccination versus 2 pre-vaccination (risk difference: 0.46; 95% CI: 0.06-0.10). Among 420,888 subjects vaccinated with the second booster, 4 events were recorded post-vaccination versus 1 pre-vaccination (risk difference: 0.07; 95% CI: -0.02-0.19). Among 131,312 subjects vaccinated with the bivalent booster, 0 events were recorded post-vaccination versus 1 pre-vaccination (risk difference -0.08; 95% CI: -0.23-0.00). Table 1 details the results by sex and age group. Conclusions The first BNT162b2 booster vaccine was associated with an increased but rare risk for hospitalization events for myocarditis. This association was not observed in the second and bivalent boosters.