Why Your Arm Hurts The Day After You Get A Flu Shot

It doesn’t take a rocket scientist to know getting a shot hurts, but it turns out the reason your arm hurts the next day is a bit more complicated. University of Pittsburgh professor of family medicine Richard Zimmerman says the pain you feel is due to an immunological response, and actually means the vaccination is working. It means your immune system recognizes the injected inactivated virus as an invader and begins to attack it. This causes inflammation, which is why you feel pain the next day. However, only about one in five people have this local reaction, and that number varies by vaccine– researchers are not sure why. (PopSci