Why do some people get side effects after Covid-19 vaccines? Temporary side effects including headache, fatigue and fever are signs the immune system is revving up — a normal response to vaccines. And they’re common.

“The day after getting these vaccines, I wouldn’t plan anything that was strenuous physical activity,” said Dr Peter Marks, the US Food and Drug Administration’s vaccine chief, who experienced fatigue after his first dose.

Here’s what’s happening: The immune system has two main arms, and the first kicks in as soon as the body detects a foreign intruder. White blood cells swarm to the site, prompting inflammation that’s responsible for chills, soreness, fatigue and other side effects.

Behind the scenes, the shots also set in motion the second part of your immune system, which will provide real protection from the virus by producing antibodies.

Read the full AP story here.