• Mun Yee Tham, Cheng Leng Chan, Dorothy Toh, Jalene Poh, Adena Lim, Sally Soh, Li Fung Peck, Belinda Foo, Amelia Ng, Patricia Ng, Pei San Ang, Sreemanee Dorajoo, Desmond Teo, Toon Wei Lim, Yean Teng Lim, Jonathan Choo, Zee Pin Ding, Khung Keong Yeo, Jonathan Yap, and Hui Xing Tan.
• Vigilance and Compliance Branch, Health Sciences Authority, Singapore.
• Singapore Med J. 2024 Feb 16.

IntroductionMessenger ribonucleic acid (mRNA) severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccines have been associated with myocarditis/pericarditis, especially in young males. We evaluated the risk of myocarditis/pericarditis following mRNA vaccines by brand, age, sex and dose number in Singapore.MethodsAdverse event reports of myocarditis/pericarditis following mRNA vaccines received by the Health Sciences Authority from 30 December 2020 to 25 July 2022 were included, with a data lock on 30 September 2022. Case adjudication was done by an independent panel of cardiologists using the US Centers for Disease Control and Prevention case definition. Reporting rates were compared with expected rates using historical data from 2018 to 2020.ResultsOf the 152 adjudicated cases, males comprised 75.0%. The median age was 30 years. Most cases occurred after Dose 2 (49.3%). The median time to onset was 2 days. Reporting rates were highest in males aged 12-17 years for both primary series (11.5 [95% confidence interval [CI] 6.7-18.4] per 100,000 doses, post-Dose 2) and following booster doses (7.1 [95% CI 3.0-13.9] per 100,000 doses). In children aged 5-11 years, myocarditis remained very rare (0.2 per 100,000 doses). The reporting rates for Booster 1 were generally similar or lower than those for Dose 2.ConclusionsThe risk of myocarditis/pericarditis with mRNA vaccines was highest in adolescent males following Dose 2, and this was higher than historically observed background rates. Most cases were clinically mild. The risk of myocarditis should be weighed against the benefits of receiving an mRNA vaccine, keeping in mind that SARS-CoV-2 infections carry substantial risks of myocarditis/pericarditis, as well as the evolving landscape of the disease.Copyright © 2024 Copyright: © 2024 Singapore Medical Journal.

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