Why do some people get side effects after COVID-19 vaccines?

Temporary side effects after COVID-19 vaccines are normal and a sign your immune system is revving up. Your immune system has two main parts, and the first kicks in as soon as the body detects a foreign intruder by prompting the inflammation that can cause chills, fatigue and other common side effects. But everyone reacts differently and it doesn’t mean the shot didn’t work if you don’t feel anything within a day or two. The shots also set in motion the other part of your immune system, which will provide the real protection from the virus by producing antibodies.