And why it's actually a good thing.

By Meghan Overdeep
October 26, 2019

If you've gotten a flu shot recently (and if you haven't, get a move on!), there's a good chance you experienced an uncomfortable ache in your arm for a few days afterward.

It makes sense that a needle shoving a liquid into your arm is painful, but why does the discomfort increase the following day?

Simply put, it's because your immune system is reacting to the vaccine. As Richard Zimmerman, a professor of family medicine at the University of Pittsburgh and director of PittVax, explained to Popular Science, it's not the act of injecting the vaccine that makes you sore the next day—it's how your body responds to the injection.

This also means that your immune system is making antibodies in response to the virus. That's a good thing! Those antibodies should protect you from the actual flu if you become exposed to it.

"The flu shot is made to produce an immune response, so soreness is a sign that it's working," Nate Favini, the medical lead at Forward, explained to HuffPost. "Your body has an inflammatory response, and that's what gives you immunity against the flu."

But a lack of soreness shouldn't be cause for concern either. You may have a higher pain threshold than you realize, or perhaps you were more relaxed while getting your shot. According to Favini, it can hurt more if you tense your arm muscles during the shot.

Fortunately, there are ways to limit post-shot pain.

"The best ways to minimize injection site pain is to move the arm around after the shot," Michael Grosso, chief medical officer at Huntington Hospital, told HuffPost. "This will help circulation in the injection area by giving the muscle the blood flow and oxygen it needs to recover."

Other effective pain remedies include placing a cool compress on the injection site and taking pain medications like acetaminophen or ibuprofen.

But no matter how uncomfortable it might be, we know one thing for sure: it can't be worse than the flu.