Why do vaccines cause your arm to hurt?

Question

When you get a shot for a vaccine (for example, the annual flu vaccine), the nurse frequently indicates that your arm will ache for a day or two, maybe more. This ache is typically not just a pain from having your skin punctured, but is actually an ache of the entire arm.What mechanism causes the entire arm to ache from a vaccine? Why doesn't that ache extend to other parts of the body, like the hands or the shoulders? Why do non-vaccine arm punctures (such as drawing blood, injecting drugs) not cause the ache?

Answer 1

Hey there,

The basic reason the these injections are painful is due to the irritative nature of intramuscular injections and the pursuant inflammatory cascade in the area. The "ache of the entire arm" is due to localized signaling effects down the length of the entire muscle into which the vaccine is injected.

Non-vaccine punctures are usually given subcutaneously (right under the skin) or intravenously (into the blood stream) and these bring about very different effects. Vaccines are delivered IM in order to ensure they are maximally effective.

Other IM injections cause the same pain, including intra-gluteal steroid and testosterone injections, among others. Here are some interesting papers about the factors involved so you can learn more:

https://www.clinicaltherapeutics.com/article/S0149-2918(09)00263-X/pdf

http://www.asiaandro.com/news/upload/20130621-aja20101a.pdf

Answer 2

Hi! The reason your arm aches after a vaccine is because of the pressure from the liquid being squeezed into the muscles.