WASHINGTON : 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.