While the flu shot cannot cause you to become sick with the flu, your immune system still recognizes what's been injected into you as foreign. As a result, it produces an immune response, which leads to the soreness or swelling that occurs near the injection site.

Why does your arm get sore after a flu shot?

Soreness in your arm after getting a flu vaccine typically lasts no longer than one or two days. The pain and inflammation is your body's natural response to a foreign invader. It's a sign that your immune system is making antibodies, which is what offers you the protection from getting the actual virus.

What muscle hurts after flu shot?

Here's the simple answer: The flu shot hurts because someone put a needle into your shoulder muscle and inserted fluid. The complete explanation is a bit more complicated, and it has more to do with your body's response to the shot than with the shot itself.