According to the results of Pfizer’s trial study, the overwhelming majority of reactions to the booster shot ranged in severity from mild to moderate, with the top side effect being pain at the injection site (via Insider). Among the nearly 300 trial participants, 83% experienced injection site pain or arm soreness, 63.7% experienced fatigue, 48.4% had a headache, 39.1% developed muscle and joint pain, and 29.1% had chills. Only one participant experienced a severe adverse reaction to the shot. The participant in question experienced lymphadenopathy, also known as lymph node swelling, which went away in five days.
Generally speaking, the side effects of the booster shot should not last more than three days, and the likelihood that the third dose will yield a more severe reaction than the second dose is very slim. “If you’ve tolerated two doses of a vaccine, you’re far less likely to have an anaphylactic reaction to a third dose,” Dr. Kawsar Talaat, an infectious disease physician at Johns Hopkins Medicine, told STAT.