How Safe Are Covid Vaccines? Here’s the Rundown on Reactions

Covid vaccines are designed to prevent a disease that has so far killed more than 5 million people worldwide. No pharmaceutical, however, is completely benign. The goal is to ensure the benefits outweigh the risks. So far, the vaccines have been connected to a relatively small number of cases of heart inflammation, blood clots, a rare immune-system disorder, and a serious -- but treatable -- allergic reaction called anaphylaxis. A concern that vaccinations could be linked to temporary facial paralysis has not been confirmed.

Cases of myocarditis, inflammation of the heart muscle, have been reported in people, especially male adolescents and young adults, who received vaccines developed by the Pfizer Inc.-BioNTech SE partnership and Moderna Inc. These shots use an innovative technology called messenger RNA that temporarily turns the body’s cells into tiny vaccine-making factories. A study published in October put the risk of the reaction at 0.8 cases per 1 million after a first dose and 5.8 cases per 1 million after a second one. The U.S. Centers for Disease Control and Prevention said most people recover fully but that potential long-term effects were unknown. A number of European countries have advised that younger people not receive the Moderna vaccine; so-far unpublished data from Nordic countries suggest the side effect is more common with that formulation than with Pfizer-BioNTech’s.