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Vaccines

5C-05 - Two-Year Prognosis of mRNA Vaccine-related Myocarditis and Multimorbidity Incidence Compared with Historical Conventional Myocarditis: A Population-Based Cohort Study

Tuesday, August 26, 2025
9:30 AM - 9:45 AM ET
Background: Patients with myocarditis following SARS-CoV-2 mRNA vaccination generally exhibit a less severe prognosis compared with naturally acquired myocarditis, but longer-term prognosis has yet to be fully understood.

Objectives: Our study aimed to examine and compare the two-year prognostic outcomes and multimorbidity incidence of postvaccine myocarditis with conventional myocarditis.

Methods: A retrospective cohort study was conducted using a territory-wide electronic public health care database in Hong Kong, linked with population-based vaccination records. Since the roll-out of BNT162b2 (Pfizer-BioNTech), patients aged 12 years or older hospitalized with myocarditis within 28 days after mRNA vaccination were compared with historical conventional myocarditis recorded before the pandemic (2000-2019). The two-year follow-up period commenced from the diagnosis of myocarditis. All-cause mortality, heart failure, dilated cardiomyopathy, heart transplant, and multimorbidity were examined using Poisson regression with robust variance estimation.

Results: A total of 799 patients were included for analysis. Over the follow-up period, two deaths (1.87%) of 107 patients with postvaccine myocarditis and 74 deaths (10.69%) of 692 patients with conventional myocarditis were observed. Two cases (1.87%) of heart failure and two cases (1.87%) of dilated cardiomyopathy were identified in the postvaccine group, compared to 100 (14.45%) and 29 (4.19%), in the conventional group, respectively. Multimorbidity was diagnosed in 5 individuals (4.67%) with postvaccine myocarditis and 118 (17.05%) with conventional myocarditis. Adjusted analysis showed that the postvaccination myocarditis group had a lower risk of mortality (incidence rate ratios [aIRR]=0.23, 95% confidence interval [95%CI]: 0.06-0.93), heart failure (aIRR=0.15, 95%CI: 0.04-0.65), and multimorbidity (aIRR=0.34, 95%CI: 0.13-0.87). No significant differences in the risk of dilated cardiomyopathy was seen.

Conclusions: This study found a significantly lower risk of mortality, heart failure, and multimorbidity among the myocarditis cases following mRNA vaccination compared with those with conventional myocarditis at two years. Prognosis of mRNA vaccination-related myocarditis may be milder than naturally acquired conventional myocarditis.