The Covid-19 vaccine by Pfizer-BioNTech carries a very small risk of inflammation of the heart muscle, known as myocarditis, new data from Israel has revealed. A separate cohort, however, found that the risk of this heart problem was substantially higher after infection with the virus than after vaccination.
Myocarditis generally occurs around three to four days after vaccination and typically after the second dose. The rare side effect has most commonly been seen in adolescent boys and young men.
"In this study in a nationwide mass vaccination setting, the (Pfizer) vaccine was… associated with an excess risk of myocarditis" – an extra 2.7 cases of myocarditis for every 100 000 people in the vaccinated group, compared with the unvaccinated one, they wrote. Most of these events were mild and temporary.