Discussion

COVID vaccine-associated myocarditis was first described in March 2021 and has subsequently been established in all age groups above 12 years of age. ^{Reference Shimabukuro2,Reference Marshall, Ferguson and Lewis5} We present this case of an 8-year-old patient with COVID vaccine-associated myocarditis after the second dose of his Pfizer-BioNTech mRNA vaccine.

Our patient was only mildly symptomatic and was stable for discharge within 3 days of hospitalisation with down-trending troponins and resolution of symptoms using only ibuprofen. This disease course and rapid recovery seen in our patient parallels additional case reports detailing COVID vaccine-associated myocarditis in other paediatric and adult age groups. Myocarditis in the young has a bimodal age distribution, with peaks during infancy and adolescence, and an incidence gradually rising starting in preschool age children. According to currently available data, the age distribution for COVID vaccine-associated myocarditis in children older than five appears to follow the same trend as that of viral myocarditis. ^{Reference Jain, Steele and Fonseca7–Reference Lin, Yaser Hashemi and Wang9} Whether the age distribution of COVID vaccine-associated myocarditis continues to follow that of viral myocarditis, with lower incidence in younger children as compared to adolescents, remains to be investigated. These similarities indicate the possibility that host factors, in addition to a viral or immunological insult, play a role in the development of myocarditis.

It should also be noted that patients in the 5–11 age group receive only half of the adult dose of the Pfizer-BioNTech COVID-19 vaccination, and one report raised concerns about possible decreased efficacy of this lower dose in preventing disease in the 5–11 year age group. ^{Reference Dorabawila, Hoefer, Bauer, Bassett, Lutterloh and Rosenberg10} It remains to be seen if the risk of myocarditis is reduced in this age group owing to the lower dose of the vaccine. Similarly, the long-term effects of COVID vaccine-associated myocarditis are still unknown. We recently reported a high prevalence of myocardial injury and scarring in adolescents with COVID vaccine-associated myocarditis as evidenced by cardiac magnetic resonance imaging with late gadolinium enhancement, which is in contrast to the relative mild disease course. ^{Reference Jain, Steele and Fonseca7} This patient did not undergo cardiac MRI as the history, serological data, and lack of other associated symptoms were indicative of myocarditis per the most recent American Heart Association guidelines. Despite the clinical diagnosis, it would have been interesting to know whether there was any myocardial scarring on cardiac MRI. ^{Reference Law, Lal and Chen6}

The current case suggests that children less than 12 years of age can also be affected by COVID vaccine-associated myocarditis. The COVID vaccine is a crucial intervention in mitigating the SARS-CoV-2 pandemic. Further studies are needed to determine the safety profile of vaccinations in any age group. The information regarding the long-term effects of COVID vaccine-associated myocarditis in the paediatric and young-adult population is critical to the ongoing efforts to mitigate the COVID-19 pandemic, as vaccinations are a cornerstone to the prevention of disease spread and alleviation of disease burden on the healthcare system.