https://www.cdc.gov/covid/hcp/vaccine-considerations/routine-guidance.html
> An 8-week interval between the first and second mRNA COVID-19 vaccine (Moderna, Pfizer-BioNTech) doses and between the first and second doses of Novavax COVID-19 Vaccine might be optimal for some people as it might reduce the rare risk of myocarditis and pericarditis associated with these COVID-19 vaccines.
> While absolute risk remains small, an elevated risk for myocarditis and pericarditis has been observed among mRNA COVID-19 vaccine recipients, particularly in males ages 12–39 years (see COVID-19 vaccination and myocarditis and pericarditis for additional information). Cases of myocarditis and pericarditis were identified in clinical trials of Novavax COVID-19 Vaccine and through passive surveillance during post-authorization use outside the United States.
So it is recognized that these vaccines can cause myocarditis and pericarditis. For now they are saying it is a rare risk. But couple years ago they said these don't cause myocarditis and pericarditis. So should prepare for updates...