The two reports published in JAMA Cardiology include observations that are broadly similar to those presented in a growing number of small case series from the US, Europe and Israel.

As such, the clinical features that these cases share do contribute to the accumulating circumstantial evidence for a link between mRNA vaccines and a type of heart muscle inflammation – also known as an acute myocarditis.  These shared features include a relatively mild clinical course in the majority, a predisposition for young men, and an onset that most typically follows the second of two vaccine doses.

However, aside from contributing to the weight of circumstantial evidence, these reports do not otherwise include much that is new.  Specifically, there is little that helps establish if or how the association could be causal.  In particular:

1. There is little or no new data that can shed light on potential causal mechanisms – this, for example, could come from more detailed immunologic studies including analyses of heart muscle biopsies.

2. All the case reports share a potential for a reporting bias, and this is difficult if not nearly impossible to completely adjust for.  Very similar clinical presentations of myocarditis are relatively common in the absence of COVID-19 and vaccination, have a seasonal incidence, are most frequently seen in young men and may often follow uncharacterized mild chest or gastrointestinal infections that are frequently of uncertain cause.

To counter the possibility of bias, at least two key questions are as yet unanswered by these and prior reports.  First, could the reports reflect the frequency of myocarditis cases that would occur in that community regardless of vaccination?  Second, could the increased rates of diagnosis and proximity to vaccination be an artefact of high vaccination rates delivered in a short period of time and the heightened awareness and surveillance for post-vaccine complications?

In summary, these reports do not contribute much that is new.  The most compelling evidence for a causal association remains the observation that cases most commonly follow a few days after a second dose of either of the two mRNA vaccines.  It is also evident that myocarditis, in all of its various forms, remains poorly understood in terms of cause, diagnosis and treatment.  We need large-scale association studies to tell us if there is an association between vaccine and myocarditis, as well as more nuts and bolts basic research to tell us why.  Until then, balancing the value of vaccination against the rarity/mildness of this form of myocardial inflammation seems very much in favour of vaccination.

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