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From MMR boosters to tetanus, you'll probably get a lot of shots in your life. And one thing you might notice is that some of them don't feel like much, but some of them can make your arm reallllly sore! Why is that?

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You’ve probably gotten a lot of shots in your lifetime… the ones you probably freaked out over as a baby, all the way up to your last flu vaccine. Either way, you might remember some shots being nastier than others.

Like, some aren’t that bad at all, hardly more than a pinch. But others hurt like no one’s business, and can make your arm sore for days afterward. In general, a little pain after a shot isn’t a bad thing.

It just means your body is learning to recognize and fight off a pathogen, like the flu virus, so that you don’t get really sick if it shows up again. But it doesn’t mean that the pain is, well, any less of a pain. How much a shot hurts depends on a lot of things, including where you get jabbed, what’s in the vaccine, and of course, your personal feeling about needles.

You might think that a bigger needle and a deeper shot would hurt more. But in most cases, those deeper injections into muscle, or what doctors call intramuscular shots, actually pose less of a problem than the ones given just under the skin. Scientists aren’t totally sure why this is, but they think part of it has to do with the way that each tissue absorbs fluid.

If you inject a bunch of fluid into fatty skin, it doesn’t have much of a place to go and can get trapped, which can be painful. Muscles, meanwhile, are full of blood vessels and can clear out the liquid much more quickly, so they tend to cause less pain, swelling, and redness. As a bonus, muscles also tend to have a better mix of immune cells, so intramuscular shots are often more effective.

Besides where the shot is given, how much a shot hurts also depends on what’s in it. Some vaccines contain adjuvants, which are materials added to vaccines to make it easier for your immune system to recognize the pathogen you want to avoid. They’re inflammatory, and made of things like aluminum, or small parts of bacteria.

They’re harmless, but since they aren’t normally found in your body, adjuvants cause more immune cells to come to the scene of the injection to check out what’s there, and to begin processing it. It turns out that some of the more painful vaccines, including those that protect against tetanus or the human papillomavirus, also called HPV, tend to have adjuvants in them. And in the case of HPV, the version of the vaccine that patients report as being more painful actually has two adjuvants in it.

If that sounds terrible, remember, they’re there because they make the vaccine work better. In fact, so far, it looks like the double-adjuvant vaccine might help people have stronger and longer-lasting protection. Scientists are still working on making vaccines less painful, but for now, it’s no pain, no gain, as they say.

Although, of course, if you have severe pain after a shot, or if it’s lasting a long time, you should consult your doctor. We are definitely no substitute for professional medical advice. SciShow is produced by Complexly, a group of people who believe the more we understand, the better we get at being human.

If you want to learn more about vaccines, shots, or anything else related to healthcare, you can go check out one of our other channels, Healthcare Triage, over at ♪ OUTTRO.