Risk of carditis after three doses of vaccination with mRNA (BNT162b2) or inactivated (CoronaVac) covid-19 vaccination: a self-controlled cases series and a case–control study

Background: Large-scale comparative research exploring the risk after the third dose and after inactivated covid-19 vaccination is limited. This study aimed to assess the risk of carditis following three doses of BNT162b2 or CoronaVac. Methods: We conducted a self-controlled case series (SCCS) and a case–control study using electronic health and vaccination records in Hong Kong. Carditis incidents within 28 days of covid-19 vaccination were included as cases. In the case–control study, up to 10 hospitalized controls were selected with stratified probability sampling by age, sex, and hospital admission (±1 day). The incidence rate ratios (IRRs) were reported from conditional Poisson regressions for SCCS, and adjusted odds ratios (ORs) were reported from multivariable logistic regressions. Findings: A total of 8,924,614 doses of BNT162b2 and 6,129,852 doses of CoronaVac were administered from February 2021 to March 2022. The SCCS detected increased carditis risks after BNT162b2: 4.48 (95%confidence interval [CI]:2.99–6.70] in 1–14 days and 2.50 (95%CI:1.43–4.38) in 15–28 days after first dose; 10.81 (95%CI:7.63–15.32) in 1–14 days and 2.95 (95%CI:1.82–4.78) in 15–28 days after second dose; 4.72 (95%CI:1.40–15.97) in 1–14 days after third dose. Consistent results were observed from the case–control study. Risks were specifically found in people aged below 30 years and males. No significant risk increase was observed after CoronaVac in all primary analyses. Interpretations: We detected increased carditis risks within 28 days after all three doses of BNT162b2 but the risk after the third doses were not higher than that of the second dose when compared with baseline period. Continuous monitoring of carditis after both mRNA and inactivated covid-19 vaccines is needed.