More than 13 billion doses of COVID-19 vaccines have been administered since December 2020, with very few serious side effects reported. Although severe reactions to a COVID-19 vaccine are rare, mild side effects are common. Many people experience pain and discomfort at the injection side, such as a sore arm, redness, or swelling. Less commonly, people experience tiredness, headache, muscle pains, chills, fever, or nausea. These side effects are normal and usually last only a few days. They mean your body is learning to fight off the disease. Serious side effects, such as severe allergic reactions, swelling of the heart muscle, thyroid issues, or blood clots, are very rare, and are tracked and reported on by the government. In almost all cases, these side effects were treatable and less harmful than severe COVID-19.