How do vaccines sometimes cause mild side effects like fever or sore arms?

Common side effects such as sore arms, mild fever, tiredness or headache usually reflect the immune system’s normal response to a vaccine. Immune cells release signaling molecules that create temporary inflammation, which helps activate defenses and build memory. These symptoms are generally short-lived and are much less serious than the diseases the vaccines prevent. Severe adverse events are rare and are carefully monitored by health systems worldwide.