Why do shots hurt?



Not the needle part, because whatever. But like…why does my arm feel like I got smacked with a baseball bat for a day after I get a vaccine?

In: Biology

Muscles soreness happens when you work out. This is because you’re literally ripping it apart so it can heal and become stronger. Needles are puncturing your muscle to deliver the vaccine which makes the muscle have to heal, making it sore.

That’s the vaccine working.
The needle causes only very minor damage unless the person giving it to you REALLY screwed it up.
The vaccine tricks your immune system into thinking it’s fighting the sickness.
The main thing is the production of antibodies so you don’t get infected for real, but it also turns on a lot of the other things the body uses to fight a sickness.
Stuff like causing the place to swell and to hurt, like you got a bad wound there.
If it was a real wound, it would be the body telling you to not get the wound hurt worse, but your body can’t tell it’s not a real wound.

Some vaccines are intramuscular, which means you’re sticking the muscle itself with the needle. So the soreness in the muscle makes sense there.

Other vaccines contain ingredients designed to stimulate your immune system so that it’s more likely to detect and respond to the antigens (proteins, pieces of dead virus, or live virus depending on the vaccine) while they’re still intact in your blood. So they’ll cause general inflammation for a little while afterwards – a minor fever or cold-like illness is a common-ish side effect in cases where this reaction’s more vigorous than normal.

The key to making a vaccine is you want to trigger the immune system to respond enough to make memory cells of the virus you are presenting it with.

You don’t want to make the person sick so you don’t use the real virus, but either a weakened version of it or just parts of it. If you weaken it too much or don’t use enough small parts, you won’t trigger enough of a response. If you don’t weaken it enough the person gets sick.

So you’re introducing just the right amount of foreign particles for your body to think “somethings wrong here, I must do something about it”. And the response is an inflammatory reaction, which comes with swelling, pain, heat and redness (inside the muscle you were injected with so you may not feel/see it on the surface).

All this makes your muscle ache when you move your arm until your body efficiently takes care of the problem