The risk of myocarditis after immunization with mRNA-based vaccines against SARS-CoV-2 raised concerns when it came to light in early 2021. But as report after report showed such cases to be rare and usually mild and self-limited, focus has turned to the “how and why.”
The mechanism linking the BNT162b2 (Pfizer-BioNTech) and especially mRNA-1273 (Moderna) vaccines to the occurrence of myocarditis is unclear for now, but one potential driver may be tied to a peculiarity that became apparent early: It occurs overwhelmingly in younger males, from 16 to perhaps 40 or 50 years of age. Excess risk has not been consistently seen among women, girls, and older men.
That observation has led to speculation that higher testosterone levels in adolescent boys and young men may somehow promote the adverse vaccine effect, whereas greater levels of estrogen among girls and women in the same age range may be cardioprotective.
Unlikely, brief, and ‘benign’
“Most of the myocarditis is benign, by which I mean that maybe the patients are admitted due to chest pain, but without reduction in ventricular function,” Enrico Ammirati, MD, PhD, a myocarditis expert at De Gasperis Cardio Center and Transplant Center, Niguarda Hospital, Milan, said in an interview.
In a Nov. 14 address on this topic at the annual scientific sessions of the American Heart Association, Dror Mevorach, MD, described the typical case presentation as “mild” and one that clears in fairly short order based on resolution of “clinical symptoms, inflammatory markers and troponin decline, EKG normalization, echo normalization, and a relatively short length of hospital stay.”
Dr. Mevorach, of Hadassah Hebrew University Medical Center, Jerusalem, subsequently published the findings in a report in the New England Journal of Medicine that described 136 confirmed myocarditis cases among more than 5 million people in Israel immunized with the Pfizer-BioNTech vaccine. Myocarditis was considered “mild” in 129 cases, or 95%.
And the risk is tiny, compared with myocarditis from infection by SARS-CoV-2, not to mention the possibility of nasty clinical COVID-19 complications such as pneumonia and pulmonary embolism, Dr. Mevorach observed.
Many other reports agree that the incidence is minimal, especially given the rewards of vaccination. In a separate NEJM publication in September 2021 – from Noam Barda, MD, Clalit (Israel) Research Institute, and colleagues on 1.7 million people in that country, about half unvaccinated and half given the Pfizer-BioNTech vaccine – there were an estimated 2.7 cases of myocarditis per 100,000 vaccinated persons. There were also 11 cases of myocarditis per 100,000 persons who were positive for SARS-CoV-2 infection.
And in a recent case series of vaccinated people aged 16 or older, the myocarditis rate after a first or second Pfizer-BioNTech or Moderna injection was estimated at 1 or fewer per 100,000. The corresponding estimate was 4 such cases per 100,000 after a positive SARS-CoV-2 test among the same population, notes a report published Dec.14, 2021, in Nature Medicine.
In general, “the risk of any kind of cardiac injury is vastly lower with a vaccine than it is with the actual viral infection,” Leslie T. Cooper Jr., MD, a myocarditis expert and clinical trialist at the Mayo Clinic, Jacksonville, Fla., said in an interview. With the mRNA-based vaccines, “we do not have any conceivable danger signal that would outweigh the benefit of vaccination.”