Why some COVID-19 vaccines seem occasionally to cause a distinctive form of myocarditis, and why adolescent boys and young men appear most vulnerable, remain a mystery. But the entity’s prevalence, nuances of presentation, and likely clinical course have come into sharper view after recent additions to the literature.
Two new publications all but confirm that the rare cases of myocarditis closely following vaccination against SARS-CoV-2, primarily with one of the mRNA-based vaccines from Pfizer-BioNTech and Moderna, is a clinically different creature from myocarditis physicians were likely to see before the pandemic.
A third report unveils rates of hospitalization for myocarditis linked to Pfizer-BioNTech vaccination in the 12- to 15-year age group, based on active surveillance across Israel. Of note, the rates were lower than corresponding numbers among the country’s 16- to 19-year-olds published in late 2021 by the same authors.
No link with CoronaVac
Acovering almost the entire population of Hong Kong from February to August 2021 confirms a slight but significant excess risk for myocarditis and, to a lesser degree, pericarditis, after injections of the Pfizer-BioNTech vaccine. As consistently reported from other studies, the risks were highest in adolescent and young adult males and after a second dose.
The study estimated an overall carditis incidence of 5.7 cases per million doses of Pfizer-BioNTech, for a risk 3.5 times that in the unvaccinated Hong Kong population. Carditis rates after a first dose were about 2.5 per million and 10 per million after a second dose.
Hong Kong launched its public SARS-CoV-2 immunization program in late February 2021 with the Chinese-made CoronaVac (Sinovac) inactivated-virus vaccine, and introduced the mRNA-based alternative several weeks later. By August 2021, the vaccines had reached about 3.3 million people in the region – 49% of the Hong Kong population at least 12 years of age.
In a novel finding, there were no excesses in carditis cases after CoronaVac vaccination. The difference between vaccines likely isn’t caused by chance, because three-fourths of the carditis-associated Pfizer-BioNTech injections arose within a week, whereas “71% of cases following the use of CoronaVac occurred more than 30 days after vaccination,” senior author Ian Chi Kei Wong, PhD, University of Hong Kong, said in an interview.
“This onset distribution for cases having received CoronaVac demonstrates that it is highly unlikely the carditis cases are related to the vaccine,” he said. And that “plausibly implies a specific underlying mechanism between vaccination and carditis that may only be applicable to mRNA vaccines.”
That inference is in line with case reports and other research, including large population-based studies from Israel and Denmark, although a recent study from the United Kingdom hinted at a potential excess myocarditis risk associated with the adenovirus-based AstraZeneca-Oxford vaccine.
The Hong Kong study identified 160 patients age 12 or older with a first diagnosis of carditis during February to August 2021, in electronic health records covering nearly the entire region.
“We used laboratory test results of troponin levels to further eliminate unlikely cases of carditis,” Dr. Wong said. The health records were linked to a “population-based vaccination record” maintained by the government’s department of health.
About 10 control patients from among all hospitalized patients without carditis were matched by age, sex, and admission date to each of the 160 carditis cases. About 83% of cases and 92% of the controls were unvaccinated.
Among those who received the Pfizer-BioNTech vaccine, representing 12.5% of cases and 4.2% of controls, the estimated carditis incidence was 0.57 per 100,000 doses. For those who received CoronaVac, representing 4.4% of cases and 3.9% of controls, it was 0.31 per 100,000 doses.
In adjusted analysis, the odds ratios for carditis among Pfizer-BioNTech vaccine recipients, compared with unvaccinated controls, were 3.57 (95% confidence interval, 1.93-6.60) overall, 4.68 (95% CI, 2.25-9.71) for males, 2.22 (95% CI, 0.57-8.69) for females, 2.41 (95% CI, 1.18-4.90) for ages 18 and older, and 13.8 (95% CI, 2.86-110.4) for ages 12-17
Myocarditis accounted for most of the excess cases, with an overall OR of 9.29 (95% CI, 3.94-21.9). The OR reached only 1.06 (95% CI, 0.35-3.22) for pericarditis alone.
The case-control study is noteworthy for its design, which contrasts with the many recent case series and passive or active surveillance studies, and even the more robust population-based studies of vaccine-related myocarditis, observed Dongngan Truong, MD, University of Utah and Primary Children’s Hospital, both in Salt Lake City, who wasn’t part of the study.
Among its strengths, she said in an interview, are its linkage of comprehensive hospital and vaccination data sets for two different vaccines; and that it corroborates other research suggesting there is “something in particular about mRNA vaccination that seems to be associated with the development of myocarditis.”