This is critical. Most people get side effects, which is good. It's your immune system reacting to this vaccine. A pain in the arm in the injection site, fever, chills, fatigue — all of which are common. All of which go away after 24 to 48 hours. So it is critical that you are aware of this. If you feel bad the next day, that's pretty normal, and it will get better. And Tylenol is a remarkable antidote for most of the side effects related to this vaccine.