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They're meant to remain in place for more than a few hours to better stimulate an immune response. Most vaccines have an adjuvant - something to help it stick around longer and further stimulate the immune system, like an oil or protein-based antigen.

Some vaccines are oral (rotavirus, poliovirus), others can be intranasal (some flu vaccines) or even intradermal (I believe the mpox vaccine was given intradermal to get more vaccinations from a single vial).

E: to expand

1. Probably not a good idea to give most vaccines in a vessel. Some are modified live and best kept locally. There are some places you may not want them to go directly. That's why the vaccinator will often pull back on the syringe - to make sure it's not in a vessel.

2. The adjuvant can help keep it around, but most of it is cleared within ~48hrs.

3. Your arm is sore because of the immune response happening there - white blood cells coming in and breaking up the vaccine components and updating the OS as you say. Sometimes, your local lymph nodes may be sore, too (ie in the armpit), because the white blood cells have migrated there to update the OS more centrally.

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.

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All great questions. I’m a med student and a grad student in immunology, but I’ll try to answer as simply as I can. Your questions in order:

• Different vaccines have different routes of administration (they are oral vaccines for example, or many studies on intranasal vaccines), but, in general, you want local inflammation at the injection site. This inflammation recruits immune cells that eventually will lead to antibody production.

-Somewhat answered before, but there are oral vaccines. Most of them, however, would not survive the harsh environment of gastrointestinal tract.

-I still remember how this blew my mind. The immune system doesn’t “update” your OS. Your OS comes with all of the antigens pre-installed, if you wish. What the vaccine does is tell the cells that just happened to have the correct receptor to expand. How we get to a point where we effectively have cells that are able to recognize pretty much any antigen out there (while also not recognizing self) is a complicated process and involves genetic rearrangements and extensive T and B cell education.

-every part of your body is perfused with blood vessels that get progressively smaller. So if you inject into muscle, it will get picked up by the blood, it won’t typically be as fast as putting it into the vein, but it is a similar process.

-soreness/pain: the pain is not exactly the same as when you stub a toe, for example. Pain from most vaccines is mostly caused by the local inflammation that is occurring at the injection site, and is typically very helpful for the development of a good vaccine response. A classic medical way of describing this pain is “Rubor, calor, dolor and tumor”, which translates to redness, warmth, pain and swelling and is typically for indicative of an inflammatory response.

You’ve asked a bunch of good and complicated questions, let me know if you want me to elaborate in any way, I love immunology :)

If I may please piggyback on this thread to ask: I got my second shingles shot a couple of months ago (left shoulder), and my elbow has hurt ever since. Stiff and sore. What's up with that? Is this forever? Full disclosure, I got a COVID booster and a flu shot at the same time about 10 days beforehand.

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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.

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>-why aren't vax shots done in the veins and arteries? Why the arm muscle? If it's in the vein, wouldn't that carry the "product" around faster?
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>-if the vax/needle is put in the arm/muscle, how does that product move around the body? Does our immune system just update our OS with the new info and its filed away?

Your immune cells move around your body and find anything it doesn't recognise. There's no need for the vaccine to move, it'll be found.

Once it is - antigen presenting cells like macrophages eat the peptides and go back to the lymph nodes to alert your other immune cells. It does this by literally presenting it to them. Then only the specific immune cells that react with the exact peptide shape will respond, and start to generate an immune response (antibody production, B & T cell expansion).

Once this has happened, your body has essentially been taught how to react if it sees the same thing in future. Some cells remain in a dormant state (called memory cells), waiting for the next time they come across what they "remember". This drastically reduces the time needed for your immune system to react, which in turn reduces the damage the pathogen can do.

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>-why the arm soreness? Is it because a needle pierced the muscle? If so, wouldn't that pain happen right away like stubbing a toe? Or does the arm hurt from the product? And why?

There's a few typical signs of infection/inflammation - from latin: calor, dolor, rubor, tumor. Heat, pain, redness, and swelling. These are by-products of what happens when you have an active immune response. Your body will widen your blood vessels to bring more immune cells to the area, which in turn allows more fluid to get there. This extra fluid causes the swelling, and the swelling causes the pain by putting pressure against local nerves.

In this case, your vaccine shot is like a localised pseudo-infection, so it invokes the same response.

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Likely inflammation caused some nerve pinching. The elbow is known to do that often. Take some anti inflammatory and maybe apply some topical steroid cream