So, when you’re getting a vaccination,  which arm do you get your shots in?

Are you always a non-dominant kind of person just in case it puts that arm out of commission? Or do you leave it up to the  dealer’s choice and take the shot in whichever arm your healthcare  provider sidles up to?

Every time you get a shot, like  the vaccines for the flu or COVID, you get to choose which arm it goes into. And maybe you’re thinking, “I  honestly don’t know what arm I get shots in because it  can’t matter at all that much. I mean, it’s all going to the  same place anyway, right?” But it turns out that might be wrong.

A study published in 2023 found that  people who got their COVID doses in the same arm were more protected  against the pandemic-maker than people who switched  between the left and right arms. So when getting multiple shots  to fight the same bad guy, there might be benefits to  keeping things consistent. But when you’re getting multiple  shots for different vaccinations, a left jab and a right jab might  knock them out just as well.

So I’m here today to tell you everything you never wondered about which  arm you get vaccinated in. [♪ INTRO] Right off the bat, let’s talk about how  few studies there are on this topic. Generally, people who get vaccinated  do better at fighting off that nasty pathogen than people  who don’t get vaccinated. So usually researchers leave it at that.

As long as you’re vaccinated,  they don’t really care whether it happened on your left or right side. And they’re absolutely right. The most important thing you can do  is get the vaccine in the first place.

But for some killers like  COVID, experts say pretty much everyone in the world should get  multiple doses of protection. So one group of researchers  took it upon themselves to peel back the layers of that  left-is-just-as-good-as-right assumption. And as they say, “Ogres are like  onions, or vaccines.” Layers.

On the outermost layer, just get the vaccine. Get it in your left arm, get it in your right  arm, get it in a car, get it at the pharmacy. Everyone in this 2023 study got  several doses of the COVID vaccine, and their bodies made antibodies to protect them against the SARS-CoV-2 pathogen.

But when these scientists went  deeper to the next layer of vaccination effectiveness, they  found that people who got each dose in the same arm had more protection  against COVID than participants who got the first dose in one arm  and the second dose in the other. Let’s walk through the numbers. Both people who got their shots in  one arm and people who got it in two made more of the immune cell called CD8 T-cells during the two weeks after their vaccination.

CD8 Ts kill cells that have  been infected by an intruder. So you’re going to want a lot  of those if something nasty like COVID is making the rounds. But people who split their  doses between their arms didn’t make as many COVID-specific CD8 T-cells as people who got both doses in the same arm.

To the point where only 43% of both arm  participants had detectable levels of CD8 T-cells, as compared to 67.2%  of the single arm participants. In a more subtle but important comparison,  antibodies neutralized the bad guys 65% of the time in participants  who got shots in both arms and 69% of the time in  participants who stuck to one side. So getting multiple doses of the COVID  vaccine in the same arm helped people make more of their body’s protective  cells and use them more effectively.

The authors of this study  think that the difference between one arm and two arms  comes from the lymph nodes. See, your immune cells, like those CD8 T-cells, distribute to the parts of your  body that need them the most. They’re spewed out by the lymph  node closest to the pathogen.

That’s why the area on your  neck and behind your ears feels swollen when you get a cold. Those lymph nodes by your nose, mouth, and throat, where you’re fighting off the  virus, are filled with immune cells. It’s also one reason you usually  get shots in the shoulder.

It’s really close to the  lymph node in your armpit. So when you get a vaccination in your  left shoulder, the lymph node in your left armpit starts sending out immune  response cells to address the situation. That lymph node keeps up its anti-COVID  response for at least 12 weeks.

Which is why these scientists think that  getting a second dose of the same vaccine near the same lymph node might give  your immune system a head start and make it more likely to mount a  stronger attack against the COVID invader. Now, this study was only  conducted with COVID vaccinations. And like I said before, there aren’t  many other similar studies out there.

But if these scientists are right about  why the results came out the way they did, then we’re talking about basic  principles of the immune system that should apply to other multi-dose  vaccines, like measles, as well. And speaking of the measles vaccine, you usually get that at the same time  as a lot of other vaccines. But the idea behind multiple shots  in the same arm may or may not hold up when we’re talking  about different vaccinations.

The same kind of thorough study,  measuring different immune cells, just hasn’t been conducted. But we do have some research on people getting multiple shots of different vaccinations at once. There was a 2015 study that seems to  suggest there isn’t much difference in getting shots in one or two limbs for infants, but it depends on what vaccines they’re getting.

Like, tetanus might be a little  less flexible in terms of where the shot goes than pneumococcal vaccines. And the Centers for Disease Control  and Prevention say that young people, who get a bunch of shots at once,  usually get them in multiple limbs. So it sounds like we need a bit more research before we can make any solid conclusions here.

And in the meantime, most  of us are probably choosing where to get our shots based  on how they make us feel. So let’s take a look at the data on that: In the COVID study that started this  video, more people said they had a fever after the second dose on the same side, while more people said they  had swollen lymph nodes after the second dose on the other side. But they were generally similar across  the board… or I guess across the body.

And a study on pre-teens getting both  Meningococcal C and Hepatitis B vaccines found that there was slightly  more redness, tenderness, and drowsiness in the two arm group. But that might have been because, in this study, the people who got the  vaccines in a single arm always got them in their non-dominant arm. So they might not have been using the  arm and noticing the pain as much.

Ultimately, this study also concluded  that getting both injections in the same arm is a good option. But there’s no real wrong choice. Either way, getting a second dose of a multi-dose shot significantly boosts immunity.

If you put it by the same lymph node both times, you’re probably increasing  your chance of protection. And, if you’re like me, about to  head into cold and flu season, it’s not too late to protect  yourself and the people around you. … whether you’re fighting the  viruses with one arm or two. This SciShow video is supported by you!

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