Modulation of immune responses
Details about the immune processes underlying mRNA-vaccine myocarditis, hormone modulated or not, have been elusive. The complication doesn’t resemble serum sickness, nor does it seem to be a reaction to infection by other cardiotropic viruses, such as coxsackie virus B, a cause of viral myocarditis, Dr. Bozkurt said. The latter had been a compelling possibility because such hypersensitivity to smallpox vaccination is well recognized.
“We don’t know the mechanism, that’s the short answer. But there are many hypotheses,” she said. One candidate widely proposed in the literature: autoantibodies driven by molecular mimicry between the SARS-CoV-2 spike protein targeted by the mRNA vaccines and a structurally similar myocardial protein, possibly alpha-myosin, noted Dr. Bozkurt and colleagues in a recent publication.
But elevations in specific “antiheart antibodies” have not been documented in recipients of the two mRNA-based vaccines, said Dr. Cooper. “So, I would say that – although molecular mimicry is a well-established mechanism of, for example, rheumatic carditis after a streptococcal A infection – that has not been demonstrated yet for COVID-19 mRNA vaccination–related myocarditis.”
“We probably won’t know, ever, with a huge level of certainty, the exact mechanisms,” Dr. Cooper added. There is no animal model for vaccine-induced myocarditis, and “We’re still talking very, very small numbers of patients. The vast majority of them recover,” and so don’t generally provide mechanistic clues.
Prospects for younger children
Vaccination against SARS-CoV-2 has now been authorized by the Centers for Disease Control and Prevention for kids as young as 5-11 years, using the Pfizer-BioNTech vaccine. Experience so far suggests the immunization is safe in that age group with negligible risk of myocarditis or other complications. But with prospects of possible authorization in children younger than 5, should myocarditis be a concern for them?
Probably not, if the complication is driven primarily by sex hormones, Dr. Cooper proposed. “One would predict that before puberty you would have a lower – much, much lower – rate of myocarditis in males than you would in the 16- to 19-year-old range, and that it would be roughly equal to females.” Dr. Ammirati and Dr. Bozkurt largely agreed.
It remains to be seen whether the vaccine-related myocarditis risk applies to children younger than 12, “but I doubt it. I think it’s going to be puberty-related,” Dr. Bozkurt said. Still, “I don’t want to hypothesize without data.”
A version of this article first appeared on Medscape.com.