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As Covid-19 vaccines make it into more and more people's arms, you may be hearing that the second dose can be a little rough. But, while it may be unpleasant, these intense side-effects are actually a sign that the vaccines are working. And there may be even more good news when it comes to people who've already had COVID-19 getting the vaccine.

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Why Your Second COVID Shot Might Be a Doozy
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This episode was filmed on February 9th, 2021.

If we have any updates about COVID-19  vaccinations or the pandemic in general, they’ll be in the playlist  linked in the description. [♪♪♪]. With the COVID-19 vaccines finally  getting into people’s arms, you might’ve heard that your second dose  might feel a little rougher than the first.

And it’s true. Participants in clinical  trials, plus the recently vaccinated public, saw more frequent and more intense side  effects on their second jab than their first. This is actually a sign that the  vaccines are working properly.

So while you should prepare to feel crummy  for a couple days, it’s a good thing overall! And it might hint at the  possibility of more good news: that people who’ve recovered from  COVID-19 could only need one shot! The key thing to understand is  that these reactions to vaccines, things like fever, chills, and other side-effects,  aren’t really from the vaccine itself.

They’re natural byproducts of your  immune system learning to fight the germ the vaccine is teaching it to spot. And that response is stronger  after the second shot because the first shot did  what it was supposed to do! To understand what I mean, let’s have  a quick refresher on how vaccines work.

In general, vaccines teach your  immune system to spot a pathogen by giving it some harmless version  or just part of it to study. That way, if ever encounters a real  deal, it’ll know how to fight it. We call this adaptive immunity —  but it doesn’t happen immediately.

It takes weeks to fully develop the strong,  targeted response that vaccines are aiming for. So the first time a vaccine introduces  your immune system to a new pathogen, any discomfort you feel is something  else: your innate immunity kicking in. This is a generalized first line of defense that  attacks anything the body perceives as a threat.

And it can definitely have some  uncomfortable side effects. For instance, within a few hours,  you can have local reactions, which are things like pain and  redness at the injection site or tenderness in the armpit of the  arm that received the injection. You can also have systemic  reactions that point to a more widespread activation of your immune system.

These are symptoms like fatigue,  headache, and all over muscle pain. But often, these side-effects are  minor, or maybe not noticeable at all. What matters is what’s  happening in the background.

While your innate immunity is doing its thing, the immune cells that do the  heavy lifting in adaptive immunity   are studying the foreign material and building up targeted weapons like antibodies. That way, by the time you get  your second dose of the vaccine, your adaptive immunity has the  tools it needs to fight the invader. And it does.

That, too, can set off local  and systemic reactions. So following your second shot, you get both  innate and adaptive immunity acting at once! Which is why the side effects  are often more severe.

This strong, dual immune  reaction also ensures your body takes the threat seriously and ramps  up its defenses against it even more. So as much as we dislike  the potential side-effects, the result is lasting protection from the virus. Now, some people get the  short end of the stick here.

Thanks to their unique  experiences and genetic makeup, they really feel it when their immune  system gets riled up, while others don’t. And we don’t totally know why. But, if you’re going to feel crummy, the odds  are higher it’ll happen in dose number two.

And that’s not news—that’s exactly what was  seen in the clinical trials for these vaccines. For example, across all age  groups in the Moderna trial, more participants reported some  kind of local or systemic reaction on their second dose of the vaccine. And something similar was seen with the BioNTech  vaccine, now being distributed by Pfizer.

While pain at the injection site was  more common after the first dose, the researchers noted more redness  and swelling after the second dose, as well as more body-wide reactions  like fatigue, fever, and muscle pain. Thankfully, in trials, these effects almost  always went away after two or three days. There’s also nothing really special  about the side effects occurring now.

You’re hearing about them because lots of  people are getting vaccinated all at once, and this is a really big deal,  and everyone is talking about it. But these side effects are on  par with what’s seen following other multi-dose vaccines given to adults. Take Shingrix, for example—a shingles vaccine  you might get if you’re over 50 years old.

Most people get a sore arm, while some  feel fatigue, headaches, or muscle pain that lasts a couple of days. That said, some people who’ve already had  COVID-19 are reporting something different: they seem to be having those stronger,  “second-dose” side effects after dose one. That may be because the disease itself  can act kind of like a first vaccine dose, in that it teaches the immune  system to spot the virus.

So, the first dose of the vaccine may be having  similar effects to everyone else’s booster. What’s nifty about that is that it  could mean they only need one dose. To be clear, the vast majority of the  clinical trials for these vaccines have only used a two-dose vaccination strategy.

So no one has tested this one-vaccine hypothesis! But two recent preprint articles  concluded that we might be able to reallocate the limited doses of the  mRNA vaccines being distributed by. Pfizer and Moderna without compromising safety.

One found that people who had  recovered from COVID-19 developed at least ten times as many  antibodies after one dose of an mRNA vaccine as previously  healthy people who got two. And the other found that healthcare  workers who had previously had COVID-19 had antibody levels on par with folks  who got two doses of the mRNA vaccines but had never been infected. That might mean that previously  infected individuals might only need one shot to protect them long term!

And if that’s the case, we may be able to  give that second vaccine dose to someone else, and hopefully get people vaccinated even faster. Now, each of these studies only  had a few hundred participants, and neither has been peer-reviewed, so we  can’t start making sweeping statements yet. And there are no changes in policy or anything.

So, in the meantime, keep doing what your  medical professionals tell you to do! Still, if you have had COVID-19, be warned  your first vaccine shot could be the rough one. And for everyone else, it might be helpful  to set your expectations ahead of time.

Yes, there’s a good chance you’ll feel  bad after your second dose in particular, and need to take a day or  two to rest and recuperate. But it’ll be worth it, because these  expected temporary side effects are way less intense and debilitating  than getting a bad case of COVID-19, and we all just want to get through this  pandemic so we can relax a little bit. Thanks for watching this episode of SciShow News!

We’re here every Friday with an  update from the world of science. So if you want to stay informed, be sure to  subscribe and click on that notification bell. And if you have further questions about  the vaccines being rolled out in the US, you might want to check out our deep  dive into the history of mRNA vaccines.

You can find it and all our  latest episodes regarding the COVID-19 pandemic in the  playlist linked in the description.