Myocarditis seems rare after COVID-19 vaccination

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The overall risk of developing myocarditis after vaccination against COVID-19 is low, according to a new report from British Columbia, Canada.

However, due to certain risk factors, the researchers recommended the preferential use of the Pfizer-BioNTech vaccine over the Moderna vaccine for patients 18 to 29 years of age.



Dr Naveed Janjua

“Overall, myocarditis after SARS-CoV-2 vaccination was low but higher in younger men, especially after Moderna’s second mRNA vaccine. After the third dose, myocarditis rates were lower,” said study author Naveed Janjua, MBBS, DrPH, an epidemiologist. and executive director of data and analytical services at the British Columbia Center for Disease Control in Vancouver, said Medscape Medical News.

“Although the observed myocarditis rates were higher than expected, the benefits of the vaccine in reducing COVID-19 severity, hospital admissions, and deaths far outweigh the risk of developing myocarditis,” he said.

The study was published online on November 21 in Journal of the Canadian Medical Association.

British Columbia cohort

The researchers analyzed administrative data from the British Columbia COVID-19 cohort from December 2020 to March 2022. They focused on the mRNA-1273 vaccine manufactured by Moderna and the BNT162b2 vaccine manufactured by Pfizer-BioNTech and accounted for differences. in age, sex, dose number and type of vaccine.

The research team examined the incidence of hospital admissions or emergency department visits for myocarditis or myopericarditis within 7 to 21 days after vaccination, calculated as rates of myocarditis per 100,000 doses of mRNA vaccine. They also compared the number of observed cases with an estimated number of cases that would be expected if there was no association between the vaccine and myocarditis. They calculated the expected rates using data on hospital admissions and emergency department visits from 2015 to 2020 and used the 2019 rates for the observed-to-expected analysis.

Overall, more than 10.2 million doses of mRNA vaccines were administered in British Columbia during the study period, including nearly 7 million doses from Pfizer-BioNTech and 3.2 million doses from Moderna. Nearly 4 million were first doses, about 3.9 million were second doses, and 2.4 million were third doses.

The investigators observed 99 incident cases of myocarditis within 7 days of vaccination, compared with the expected seven cases. The myocarditis rate was 0.97 cases per 100,000 vaccine doses, compared with an expected rate of 0.13 per 100,000 population. The observed versus expected ratio was 14.81.

In addition, they observed 141 cases within 21 days of vaccination, compared to the expected 20 cases. The myocarditis rate was 1.37 cases per 100,000 vaccine doses, compared with an expected rate of 0.39 per 100,000 population. The observed versus expected ratio was 7.03.

In general, most cases of myocarditis developed among men and after the second dose. Among patients with myocarditis, men were younger than women in cases within 7 days of vaccination (28 years vs. 45 years) and in cases within 21 days of vaccination (31 years). vs. 49 years).

age and risk

When analyzed by age, myocarditis rates were highest among patients 12 to 17 years of age and patients 18 to 29 years of age and lowest among patients 70 to 79 years of age. Myocarditis rates were about 2.6 cases per 100,000 doses in people younger than 30 years.

By sex, myocarditis rates were higher for men than for women. Myocarditis rates were 1.64 cases per 100,000 among men, compared with 0.35 per 100,000 among women.

By number of doses, myocarditis cases were higher for those who received the primary vaccine series. Myocarditis rates were approximately 2.5 per 100,000 doses for the first and second doses, compared with 0.76 per 100,000 for the third dose.

By type of vaccine, cases of myocarditis were higher among those who received the Moderna vaccine. Myocarditis rates were 1.4 per 100,000 doses among Moderna recipients, compared with 0.74 per 100,000 doses among Pfizer recipients.

The highest observed to expected ratio was observed after the second dose among men 18-29 years of age who received the Moderna vaccine. The risk was nearly 3 per 100,000 cases, compared with an overall expected rate of less than 1 per 100,000 cases.

The investigators also identified 179 incident cases of myopericarditis within 7 days of vaccination and 308 cases within 21 days of vaccination. The rate of myopericarditis was 1.75 per 100,000 doses for the 7-day window, with an observed to expected ratio of 5.18, and 3 per 100,000 doses for the 21-day window, with a relationship between the observed and the expected of 2.97.

Like myocarditis rates, myopericarditis rates were highest among men, patients 12 to 29 years of age, those who received Moderna, and after the second dose of the vaccine.

Janjua noted that future studies should monitor the long-term intermediate outcomes of the development of myocarditis and myopericarditis, particularly among children and young adults. The researchers are also interested in the risks after booster doses and catch-up vaccinations, as well as potential causal associations between mRNA vaccines and myocarditis.

Low absolute numbers

“At the end of the day, the absolute number of myocarditis cases after vaccination is very low, although higher than expected,” C. Buddy Creech, MD, MPH, told Medscape. Creech is director of the Vanderbilt Vaccine Research Program and a professor of pediatrics at the Vanderbilt University School of Medicine in Nashville, Tennessee.



Dr C Buddy Creech

Although he was not involved in this study, Creech has led clinical trials of the COVID-19 vaccine during the pandemic. Pfizer, Moderna, the Centers for Disease Control and Prevention and the National Institutes of Health have launched large studies to understand potential links between vaccines and myocarditis risks, she noted.

“Perhaps most importantly, fortunately, cases of myocarditis after vaccination have been very mild and often did not require treatment,” he said. “This should provide parents with a measure of confidence as they seek to protect their families from COVID illness, including cases of myocarditis that are often not mild after COVID illness.”

Establishing a causal link between a vaccine, food or drug and a side effect is challenging, Creech said. “This is due, in part, to the natural occurrence of certain things that, if they happen after a vaccine, food or drug, we would call them side effects.”

For example, Creech explained, a person might develop diarrhea and vomiting after eating at a restaurant and believe they have food poisoning. But they may actually have contracted a stomach virus from interacting with people in another public setting. With COVID-19 vaccines, researchers have focused especially on looking at myocarditis risks to determine causes and related factors.

“It is important to this issue that people, especially young men ages 15 to 30, develop myocarditis after other viral infections, such as the common cold,” Creech said. “Sometimes it happens that there is a vaccine involved shortly before the diagnosis is made.”

The study was supported by the Canadian Immunization Research Network through a grant from the Public Health Agency of Canada and the Canadian Institutes of Health Research, as well as funding from the Public Health Agency of Canada through the Group Reference for Vaccine Surveillance and COVID-19. Immunity Working Group. Janjua reported fees from AbbVie and Gilead outside of this study. Creech did not report any relevant financial conflicts of interest.

CMAJ. Published online November 21, 2022. Full text

Carolyn Crist is a health and medical journalist who reports on the latest studies for Medscape, MDedge, and WebMD.

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