Recent literature features new descriptions of myocarditis linked to the two available mRNA vaccines against SARS-CoV-2. They tell a story largely consistent with experience to date, and support what might be its most useful public health message: The associated myocarditis is usually mild and self-limiting, and is far less likely to occur than myocarditis or death in unvaccinated people with COVID-19.
In line with previous research, the new analyses suggest the myocarditis – with onset usually a few days to a week after injection – has an overall incidence that ranges from less than 1 to perhaps 3 per 100,000 people who received at least one of the full mRNA-vaccine regimen’s two injections. Also, as in earlier studies, the incidence climbed higher – sometimes sharply – in certain groups by age and sex, particularly in young men and older male teens.
The new studies “are confirmatory, in terms of the risk being low,” but underscore that clinicians still must be wary of myocarditis as a potential complication of the mRNA vaccines, Biykem Bozkurt, MD, PhD, Baylor College of Medicine, Houston, told this news organization.
Dr. Bozkurt, a leading heart failure specialist and researcher, did not contribute to any of the new reports but does study the myocarditis of COVID-19 and was lead author on a recent review of the potential vaccine complication’s features and possible mechanisms.
In the new myocarditis reports, she observed, more than 90% of the cases were mild and “resolved on their own without a major adverse outcome.” Dr. Bozkurt emphasized the need for perspective regarding the risk. For example, the myocarditis associated with SARS-CoV-2 infection is not only more likely than the vaccine-related myocarditis, but it’s also usually far more severe.
Dr. Bozkurt pointed to a recent study in which the mRNA vaccines, compared with no vaccination, appeared to escalate the myocarditis risk by a factor of 3, whereas the risk for myocarditis in SARS-CoV-2 infection was increased 18 times.
In contrast, she observed, the new myocarditis cases reported this week feature a few that are novel or are at least very rare, including the case of a patient who developed cardiogenic shock and another with fulminant myocarditis who died.
The Centers for Disease Control and Prevention in May publicly described the apparent link between myocarditis and the two available mRNA vaccines against SARS-CoV-2: BNT162b2 (Pfizer-BioNTech) and mRNA-1273 (Moderna). The next month, the Food and Drug Administration added a warning about the risk to the labeling.
Less than 1 case per 100,000
Fifteen confirmed cases of myocarditis were identified among about 2.4 million members of Kaiser Permanente Southern California aged 18 or older who received at least one injection of the Pfizer or Moderna mRNA vaccines between December 2020 and July 2021, in a report published in JAMA Internal Medicine. The study counted cases up to 10 days after the first or second injection, of which there were 2 and 13, respectively.
All eight patients who received the Pfizer BNT162b2 vaccine and the eight given the Moderna mRNA-1273 vaccine were male with a median age of 25 years (interquartile range, 20-32 years).
“The main takeaway messages from our study are that the incidence of myocarditis after COVID-19 mRNA vaccinations is very low, that this condition is primarily observed in young men within a few days after the second dose, and that most patients recover quickly,” senior author Mingsum Lee, MD, PhD, Kaiser Permanente Los Angeles Medical Center, told this news organization.
“The incidence of vaccine-related myocarditis was significantly lower than rates of COVID-19 hospitalization during the same period and population area,” she added.
The group saw a per-million incidence of 0.8 and 5.8 myocarditis cases in the 10 days after first and second injections, respectively. That made for an incidence of 0.58 per 100,000, or 1 case per 172,414 fully vaccinated adults.
The group also considered a cohort of 1,577,741 unvaccinated people with a median age of 39 years (interquartile range, 28-53 years) during the same period. Of the 75 cases of myocarditis, 52% were in men, they reported.
Comparing the vaccinated and unvaccinated cohorts, they saw a 10-day vaccine-associated myocarditis incidence rate ratio of 0.38 (95% confidence interval, 0.05-1.40; P = .15) after the first dose, and 2.7 (95% CI, 1.4-4.8; P = .004) after the second dose.
In a comparison of the vaccinated group with itself using data from a 10-day period in the previous year, the corresponding myocarditis IRRs were 1.0 (P > .99) and 3.3 (P = .03), respectively.
Dr. Lee said none of the 15 patients required admission to an intensive care unit. “All patients with myocarditis responded well to treatment and felt better quickly,” she noted.
Myocarditis after an mRNA vaccine injection is rare and, Dr. Lee said emphatically, and “the benefits of the COVID-19 vaccine greatly outweigh the risks.”