The world has crossed five million deaths from Covid-19, but resistance to vaccination continues to persist, with many citing concerns over side effects and potential unknown long-term effects. Among the adverse side effects of Covid-19 messenger RNA (mRNA) vaccines that have received some attention is myocarditis, an inflammation of the middle layer of the heart wall caused by an immune response to some viruses, certain medications and some autoimmune diseases. The mechanism for vaccine-induced myocarditis is as yet unknown, but could be caused by the SARS-CoV-2 spike protein or the immune response following the vaccination.

Myocarditis has received recent attention after a pre-print study from the University of Ottawa Heart Institute was withdrawn due to a calculation error that erroneously increased the rate of contracting the condition as a side effect of Covid-19 vaccination. This paper incorrectly reported a one in 1,000 risk of myocarditis as a side effect, which was subsequently promoted on blogs and social media sites as a reason to distrust Covid-19 vaccines. It also received some attention after Taiwan halted its administration of the second dose of Pfizer’s vaccine for people aged 12–17 years due to myocarditis concerns – the country is awaiting further data before proceeding with the vaccine rollout in this age group.

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While cases of myocarditis have been reported in the Vaccine Adverse Event Reporting System (VAERS) following mRNA Covid-19 vaccination, these rates have been lower than those reported in the retracted paper. The majority of events have occurred in male adolescents and young adults after the second dose. The side effect has a prevalence of around 12.6 cases for every million second doses administered. For males aged 12–29 years, the group with the highest rates of myocarditis, there are an estimated 39 to 47 cases for every million second doses given. Most vaccine-related myocarditis patients who receive care respond well and recover. An Israeli MRI study has found that most of the damage to the heart is mild with favourable outcomes, and no long-term effects are expected.

Both Pfizer’s and Moderna’s Covid-19 vaccines have been associated with myocarditis, but the rates are higher for Moderna’s vaccine than for Pfizer’s vaccine. There are ten additional cases of the condition for every 100,000 Moderna inoculations among males aged 12 to 29 years. Consideration should, however, be given to the fact that Moderna’s vaccine has lower rates of severe disease after vaccination than Pfizer’s vaccine, resulting in fewer hospitalisations and deaths, which outweighs the risk of myocarditis.

It is important to note that Covid-19 also causes myocarditis, with an estimated 28% of hospitalised patients showing signs of heart muscle injury. The risk of the condition is around 16 times higher in Covid-19 patients than in the uninfected, with inpatient visits for myocarditis 42% higher last year compared with 2019. In addition, the risk of myocarditis is 37 times higher for children with Covid-19 than in their uninfected peers. These rates indicate that the small risks of myocarditis from vaccination are still outweighed by the risks of Covid-19.