According to a preprint research study, rates of myocarditis or pericarditis across all ages and sexes combined were found to be lower for those who had a longer interval between the two doses of an mRNA vaccine.

This was a population-based study of people in Ontario, Canada, who were administered at least one dose of a COVID-19 mRNA vaccine from December 14, 2020 through September 4, 2021. A total of 19.7 million doses of Moderna or Pfizer/BioNTech mRNA vaccines were identified to have been administered during the study period in the Ontario Ministry of Health′s COVaxON database. Overall, 417 cases of myocarditis or pericarditis were reported. Of these, 297 reports met the inclusion criteria. Among all ages and sexes combined, myocarditis or pericarditis rates appeared to be higher among people who had a shorter interval between the two doses (≤30 days vs. ≥56 days). Moreover, the rates were similar for the Moderna (rate ratio [RR] 5.2) and Pfizer (RR 5.5) vaccines… (Medscape)