CDC is investigating several reports that teenagers and young adults may have developed myocarditis after receiving mRNA vaccines. Most cases were mild, more often in males than females, more often following dose 2 than dose 1, and typically, within 4 days after vaccination.

COVID-19 VaST Work Group Technical Report – May 17, 2021 – Centers for Disease Control and Prevention

Commentaries: C.D.C. Is Investigating a Heart Problem in a Few Young Vaccine Recipients – The New York Times AND U.S. CDC looking into heart inflammation in some young vaccine recipients – Reuters

Related: Israel said probing link between Pfizer shot and heart problem in men under 30 – The Times of Israel