Side effects from COVID-19 vaccines typically begin within a few hours to a couple of days after vaccination. Most commonly, mild side effects such as pain at the injection site, headache, fatigue, muscle aches, and chills start within 6 to 48 hours post-vaccination. Injection site reactions often begin within 6 to 24 hours. Some delayed reactions, such as redness or rash near the injection site, can appear around 7 to 8 days after the first dose but are less common. Severe allergic reactions, if they occur, usually happen within the first 15 to 30 minutes after the shot, which is why monitoring after vaccination is standard. Serious side effects like myocarditis typically appear within the first week, mostly within a few days after vaccination.
