Why do vaccines sometimes cause a sore arm or mild fever?

Vaccines work by prompting immune activation, and that immune response can cause local redness, soreness, or swelling at the injection site. Some vaccines can also cause mild fever, fatigue, or muscle aches as the body produces cytokines and builds immunity. These effects are usually short-lived and are a normal sign that the immune system is doing its job. Serious side effects are rare, which is why vaccine safety is monitored continuously by agencies such as the CDC and WHO. The benefit of preventing severe disease far outweighs these temporary reactions.