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

Because of certain risk factors, however, researchers recommended preferential use of the Pfizer–BioNTech vaccine over the Moderna vaccine for patients aged 18-29 years.

Dr Naveed Janjua

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

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

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

British Columbia Cohort

The investigators 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 vaccine type.

The research team examined the incidence of hospital admissions or emergency department visits for myocarditis or myopericarditis within 7-21 days after vaccination, calculated as myocarditis rates per 100,000 mRNA vaccine doses. They also compared the number of observed cases to a calculated number of cases that would be expected if there were no association between the vaccine and myocarditis. They calculated the expected rates by using data on hospital admissions and emergency department visits from 2015-2020 and used the rates from 2019 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 Pfizer–BioNTech doses and 3.2 million Moderna doses. Nearly 4 million were first doses, about 3.9 million were second doses, and 2.4 million were third doses.

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

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

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

Age and Risk

Analyzing by age, myocarditis rates were highest among patients aged 12-17 years and patients aged 18-29 years and lowest among patients aged 70-79 years. Myocarditis rates were about 2.6 cases per 100,000 doses in people under age 30 years.

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

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

By vaccine type, myocarditis cases were higher for 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 seen after the second dose among men aged 18-29 years 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 researchers also identified 179 incident cases of myopericarditis within 7 days after vaccination and 308 cases within 21 days after 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 an observed-to-expected ratio of 2.97.

Like myocarditis rates, the myopericarditis rates were higher among men, patients aged 12-29 years, Moderna recipients, and after the second vaccine dose.

Janjua noted that future studies should monitor the intermediate to long-term outcomes of developing myocarditis and myopericarditis, particularly among children and young adults. Researchers are also interested in risks after booster doses and updated vaccines, 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, though higher than we would expect,” C. Buddy Creech, MD, MPH, told Medscape. Creech is director of the Vanderbilt Vaccine Research Program and professor of pediatrics at the Vanderbilt University School of Medicine in Nashville, Tennessee.

Dr C. Buddy Creech

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

“Maybe most importantly, the cases of myocarditis following vaccination have thankfully been very mild, often requiring no therapy,” he said. “This should provide parents a measure of confidence as they seek to protect their families from COVID disease, including the often not-mild cases of myocarditis following COVID disease.”

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 occur after a vaccine, food, or drug, we would call them a side effect.”

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

“Important to this issue is that people, especially young men of 15 to 30 years of age, develop myocarditis after other viral infections, such as common colds,” Creech said. “Sometimes there just happens to be 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 Vaccine Surveillance Reference Group and the COVID-19 Immunity Task Force. Janjua reported honoraria from AbbVie and Gilead outside of this study. Creech reported no relevant financial relationships.

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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