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Risk of heart inflammation following the second dose of an mRNA COVID-19 vaccine was substantially higher in younger men with Moderna's product (Spikevax) versus Pfizer/BioNTech's (Comirnaty), though some of this risk was reduced with longer spacing between doses, a population-based study from Canada found.

Cases of myocarditis or pericarditis were highest in men ages 18-24, reaching 299.5 cases per million second doses of Moderna's vaccine (95% CI 171.2-486.4), as compared to 59.2 cases per million second doses of Pfizer's vaccine (95% CI 19.2-138.1), reported Sarah Buchan, PhD, of Public Health Ontario in Toronto, and colleagues in .

In this group of men, an 8-week interval or longer lowered the second-dose risk, but it was still markedly higher with Moderna's product:

• Moderna: 132.5 cases per million second doses (95% CI 27.3-387.2)
• Pfizer: 11.1 cases per million (95% CI 0.3-61.6)

"Although myocarditis or pericarditis following receipt of mRNA vaccines is rare, the findings of this study suggest that modifying mRNA COVID-19 vaccination programs to incorporate age-based product considerations and longer interdose intervals may reduce the risk of these events," Buchan and coauthors concluded.

Data on boys ages 12-17 years were only available with Pfizer's product (Moderna's vaccine was not authorized in Canada for this age group during the study period), and this group saw an even higher rate of myocarditis following the second doses (97.3 cases per million, 95% CI 60.3-148.8) than older males who received Pfizer's vaccine.

Following last week's FDA authorization of the Moderna vaccine in children ages 6 to 17 years, and CDC's discussion with its Advisory Committee on Immunization Practices, the CDC said it plans to recommend an 8-week spacing for Moderna's vaccine in boys ages 12-17.

"Although the absolute numbers were small, there was a consistent reduction in the rates of myocarditis or pericarditis with increasing intervals between doses, with the lowest rates occurring among individuals with interdose intervals of 56 days or more," noted Eric S. Weintraub, MPH, of the CDC, and coauthors, writing in an . "In addition, data from other countries indicate that vaccine effectiveness may be higher with an interdose interval for mRNA vaccinations of 6 to 8 weeks compared with the 3- to 4-week interval that is recommended in the U.S."

With Pfizer's vaccine, the overall rates of myocarditis or pericarditis across age groups were 15.6 cases per million first doses and 29 cases per million second doses. Rates following both the first and second doses were lower in females (8.9 and 11.9 cases per million) than in males (21.8 and 45.3 cases per million).

For Moderna's vaccine, rates of myocarditis or pericarditis overall were 23 cases per million first doses and 62.5 cases per million second doses. Again, females had lower rates for both the first and second doses (9.5 and 22 cases per million doses) than males (33.7 and 96.8 cases per million).

When the time before the second vaccine dose was extended to a minimum of 56 days, overall rates of reported myocarditis or pericarditis cases decreased dramatically, to 9.6 cases per million doses of Pfizer's product (95% CI 6.5-13.6) and 16.2 cases per million doses of Moderna's (95% CI 10.2-24.6).

Weintraub's group also pointed to another novel finding from the study, which was that a heterologous series with Pfizer's vaccine for the first dose and Moderna's for the second was associated with a higher rate of myocarditis than two doses of Moderna's product alone.

"The reasons for and significance of this finding are unclear, but it merits further study and replication in other data systems," the editorialists noted.

Buchan and colleagues' population-based cohort study included a total of 297 reports of myocarditis or pericarditis from December 2020 to September 2021 among 19.7 million mRNA doses administered in Ontario, Canada. Data were derived from the Ontario COVID-19 vaccine registry and passive vaccine-safety surveillance system.

Most patients (97.6%) required emergency department visits, 70.7% needed hospitalization, and 14 cases required intensive care (4.7%).

Average age of the myocarditis/pericarditis patients was 24 years, and 76.8% were males. In females, the highest risk following the second dose was also among those 18-24 years, at 69.1 cases per million (95% CI 14.2-201.9) with Moderna's vaccine and 27.4 per million (95% CI 3.3-99.0) with Pfizer's.

Over two-thirds (69.7%) of cases overall followed the second dose of the two-dose mRNA vaccine series, and 97.1% of the patients had symptoms arise within 30 days of the second dose.

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