RESUMEN
INTRODUCTION: Due to the data scarcity in low- and middle-income countries, we aimed to examine the incidence rate of myocarditis and pericarditis within 30 days after each dose of homologous (3 × BNT162b2) and heterologous prime-boost (2 × BBIBP-CorV/BNT162b2) vaccine regimen among individuals younger than 40 years. METHODS: We conducted a historical control cohort using routinely recorded data from Thai national vaccine and insurance claims databases. Sex-specific incidence rate ratios (IRRs) for myocarditis and pericarditis were calculated for each vaccination strategy and contrasted with incidence rates among the non-immunised population in the pre-COVID-19 period. From August 2021 to September 2022, we tracked the incidence of myocarditis and pericarditis within 30 days after vaccinations using < 40-year-old national population databases. Our reference was the average monthly incidence of these conditions in the non-immunised population from August to October 2019. The exposure of interest was immunisation against the SARS-CoV-2 virus, incorporating the following vaccination strategies: three-dose 3 × BNT162b2 regimen, three-dose 2 × BBIBP-CorV/BNT162b2 regimen, and non-immunisation. RESULTS: For myocarditis, a total of 215 cases were identified among 7,594,965 individuals in the 3 × BNT162b2 cohort, 5 cases among 2,914,643 individuals in the 2 × BBIBP-CorV/BNT162b2 cohort, and 115 cases among 32,424,780 non-immunised individuals. The sex-specific IRRs (95 % confidence intervals) of myocarditis and pericarditis after the homologous vaccination were 3.09 (1.61, 5.93) and 1.84 (0.72, 4.73) for females and 7.43 (3.11, 17.73) and 10.48 (3.90, 28.15) for males, respectively. Conversely, the IRRs of myocarditis after the heterologous vaccination were not significant (females: 2.24 (0.70, 7.17); males: 1.99 (0.48, 8.21)). IRRs could not be obtained for pericarditis after the heterologous vaccination because of the small number of observed events. CONCLUSIONS: The study observed a significantly increased risk of myocarditis and pericarditis following homologous 3 × BNT162b2 vaccination but had insufficient power to confirm an increased risk for myocarditis following the heterologous prime-boost 2 × BBIBP-CorV/BNT162b2 vaccination. The incidence of pericarditis following the heterologous vaccination was too rare to evaluate.