yak-broker t1_j0snmu4 wrote on December 19, 2022 at 2:47 AM

Here's an article that goes into the history of vaccine sites a little. TLDR is that injecting into a muscle gives a localized area for the vaccine and its adjuvant to sit and cause an immune response, but not so concentrated that you get scarring and such. However, that article notes we're not actually entirely sure why intra-muscular injection seems to work better or even if it's really that much better than hitting a vein. A longer article on the subject.

Muscles have a lot of blood flow, so the effect does spread through your body eventually.

Oral vaccines are tricky because your stomach/gut are pretty good at breaking down and neutralizing stuff we eat. (Not 100% perfect at it obviously.) Some vaccines are taken orally: there are oral vaccines for polio, typhoid, and cholera (and some others). You'll note that those are all also diseases that are transmitted by consuming contaminated water - so in this case it also helps that the tissue the vaccine touches (the gut lining) is also the first tissue the disease touches. That bonus doesn't apply to infections you get via the lungs though.

Your arm is sore because of the immune reaction - it causes minor inflammation.

TonyJPRoss t1_j0tgvwu wrote on December 19, 2022 at 7:45 AM

From that article, it looks like if you were to inject a harmless, dead virus into your body, your body would largely ignore the harmless thing. An inflammatory adjuvant is added to the injection to purposely cause inflammation around the injection site, so the body associates the harmless virus with harm, thus developing immune memory.

So injecting most vaccines intravenously would spread the harmful adjuvant widely throughout your body, which is a risk you wouldn't want to take unnecessarily. Injecting into muscle is known to work well and be relatively safe.