Mortality rates among patients who contracted myocarditis following mRNA COVID-19 vaccination were found to be significantly lower than in people with viral infection-related myocarditis in a new Hong Kong observational study. Myocarditis developed as a side effect of vaccination could also be less severe than naturally acquired viral infection-related myocarditis, said the researchers, led by Francisco Tsz Tsun Lai, PhD, and Edward Wai Wa Chan, MPH, from The University of Hong Kong and the Laboratory of Data Discovery for Health (D2 4H). The findings, published Monday online ahead of the Dec 13 issue of the Journal of the American College of Cardiology – follow recent “public concern” raised over accruing evidence that myocarditis is a rare potential adverse event following messenger RNA (mRNA) COVID-19 vaccines. There have been few data to facilitate a fair comparison of the prognosis between myocarditis cases following mRNA vaccination and those related to a viral infection, since “the incidence of otherwise acquired myocarditis, mainly from a viral infection, has essentially dropped to near zero in many developed countries with strict [social distancing] measures,” the researchers stressed. They, therefore, set out to run such a comparison in 866 myocarditis patients in the Hong Kong public health care database linked with population-based vaccination records. This included 104 patients aged ≥12 years hospitalized with myocarditis within 28 days after receiving the Pfizer/BioNTech BNT162b2 mRNA vaccine and 762 patients with viral infection–related myocarditis recorded before the pandemic (2000-2019). At baseline, none of the vaccine group had a history of heart failure, transplant and dilated cardiomyopathy, compared with 101 in the viral-related myocarditis group. Three of the vaccinated group had previously been diagnosed with COVID-19 vs. none of the viral infection group. Meanwhile there was an absence of elevated troponin in 12 of the vaccinated patients vs. 620 of the viral infection group. The vast majority (79%) of younger patients with myocarditis were male (89.4% of the post-vaccination myocarditis group aged 12 to 17 years and 75.2% of those with viral infection-related myocarditis). In those aged 18 to 59 years, 59.7% of those with viral infection-related myocarditis were male vs. 75% of the vaccinated patients, and in the ≥60 years patients, 51% of the viral infection and 20% of the vaccinated groups were male. The biggest number of patients (548) were aged 18 to 59 and had viral infection-related myocarditis. The patients were assessed over a 180-day follow-up period starting from diagnosis of myocarditis, and all-cause mortality, heart failure, dilated cardiomyopathy, heart transplant, and post-discharge health care utilization were examined with Cox proportional hazards models. One death (1%) occurred in the 104-patient post-vaccination myocarditis group vs. 84 deaths (11%) in the 765 patients with viral infection-related myocarditis. According to an adjusted analysis, the post-vaccination myocarditis group had a 92% lower mortality risk than those who contracted myocarditis from viral infections (adjusted hazard ratio [HR]: 0.08; 95% confidence interval [CI]: 0.01-0.57). “In this territory-wide retrospective cohort study, we found a significantly lower rate of mortality among the myocarditis cases after mRNA vaccination within 180 days of diagnosis, compared with viral infection– related cases in a historical cohort,” the researchers added. “We observed very low incidence rates (≤1 per 10,000 person-days) of mortality, heart failure, and dilated cardiomyopathy following myocarditis after mRNA vaccination, in contrast with incidence rates of 7, 8, and 2 per 10,000 person-days, respectively, among the viral infection–related myocarditis patients.” The finding that around 2.61 myocarditis cases per 100,000 people vaccinated were also in line with previous population studies, such as in Israel, the researchers added. The demographic features were also consistent with other research, they added, namely that the cohort of vaccinated patients were generally younger and had a higher proportion of males, and that “the prognosis was typically mild among those with myocarditis following mRNA vaccination, with very few deaths and other adverse prognostic outcomes recorded”. In an accompanying editorial, the University of Ottawa’s Peter P. Liu, MD, and Tahir S. Kafil, MD, said the results overall are “reassuring” for patients hospitalized with vaccine myocarditis due to BNT162b2. They highlighted limitations including the absence of standardized case definition criteria. “Further, the inability to confirm exclusion of other potential causes of myocarditis can lead to heterogeneity of disease inclusion and subsequent prognosis,” they said, adding that prepandemic viral myocarditis is “not an ideal comparator group” given said heterogeneity. Instead, COVID-19-induced myocarditis could have been a better comparator, the editorialists said. They and the study’s authors concluded by calling for more work to capture the longer-term outcomes of COVID-19 mRNA vaccines, including the identification of potential risk factors for myocarditis. Sources: Lai FTT, Chan EWW, Huang L, et al. Prognosis of Myocarditis Developing After mRNA COVID-19 Vaccination Compared With Viral Myocarditis. J Am Coll Cardiol 2022;80:2255–2265. Liu PP, Kafil TS. COVID-19 Vaccine Myocarditis: Cautious Reassurance in an Era of Dynamic Uncertainty. J Am Coll Cardiol 2022;80:2266–2268. Image Credit: freshidea – stock.adobe.com