Why Do Vaccines Go in the Arm?

(Last Updated On: September 20, 2022)

Your flu shot might be on the horizon as the fall kicks off and children go back to school, or you’re scrambling to find booster shots under shortage or vaccines under shortage, because there are shortages for both COVID-19 and Monkeypox vaccines. Nice. Anyway, most of the time you get shots you get them in the arm. Then your arm gets all sore and you get annoyed about it for a bit. So why do vaccines go in the arm? Do they all go there?

Further Reading: What’s the Difference Between Inoculation, Vaccination, and Immunization?

Your Muscles

Even if you have noodle arms, you have the muscles for your vaccines. 

Your muscles have their own immune cells which will get your immune response going. Vaccines deliver antigens into your system, typically in the form of virus or bacteria pieces. The human immune system is an incredibly complicated beast with lots of ongoing research, but the basics behind vaccines is they use your immune system’s memory for your benefit. You can imagine every disease has a perfect countermeasure that can be used against it. Floating around in your body you have cells constantly mixing and matching their genetic code to make these weapons–most of the time they might not get used but when it’s relevant you’ll have that silver bullet ready. Somewhere. 

That somewhere is the key, because it often takes a while for your body to scrounge up those weapons when it faces a new disease. It’s like finding a needle in a haystack. But when your body does have to fight off a disease, your immune system remembers it so it can fight it off faster next time. A vaccine gets your body to remember the disease without having to fight it off the first time.

So what does this have to do with the muscles in your arm? For starters, your muscles are pretty good at self-repair. The muscles vaccines are injected to are also pretty close to some of your lymph nodes, you can think of these as little command outposts for your immune system if we’re keeping the weapons analogy. Getting your vaccine near them just makes it faster for your body to get its response going. Specifically vaccinations go in your deltoid, which is close to lymph nodes in your armpit. Also getting vaccines into fattier tissues can see more irritation and even a less effective immune response depending on the vaccine.

Do They All Have to Go in the Arm?

The arm is common for the reasons we just talked about, but also because it’s pretty easy. For children with less developed arm muscles, vaccines can go in the thigh muscles. But rolling up your sleeve is a lot easier as an adult. 

There are also subcutaneous vaccines that go into your fattier tissues. These can go in your thigh or the fattier tissue near your tricep. While intramuscular vaccines in your arm are typically injected with the needle at a 90 degree angle to you, subcutaneous ones might be done at a smaller angle to prevent injection into your muscles. Why fattier tissues if they can result in greater irritation? Well there’s less blood flow in your fatty tissue, which means the medicine will be released more slowly into your body. For vaccines, this is often done with live attenuated vaccines (the ones with live, but weakened, viruses or bacteria. Outside the vaccine realm, subcutaneous injection is also commonly used for insulin. 

There are also intradermal vaccines, which are uncommon but go under the skin–and can still be injected into your arm. These are more common with allergy tests though; like when they stick a bunch of tiny needles into your forearm and you get all itchy.

Some vaccines can be administered orally, and have been explored as an alternative to intracolorectal vaccines. They definitely sound more comfortable. 


See if you know what we have a vaccine for here.

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About Kyler 728 Articles
Kyler is a content writer at Sporcle living in Seattle, and is currently studying at the University of Washington School of Law. He's been writing for Sporcle since 2019; sometimes the blog is an excellent platform to answer random personal questions he has about the world. Most of his free time is spent drinking black coffee like water.