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Here's Why COVID-19 Vaccines Are Safe

Editor's Note: This is an excerpt from a COVID-19 vaccine special edition of WBUR's weekly coronavirus newsletter. This is part one of three. You can get the other special edition emails, in addition to our weekly vaccine and COVID reporting, in your inbox when you sign up here.

En español, traducido por El Planeta Media.


This week, grab your hard hat and shiny vest because we're talking safety. We're going to explain why, when it comes to the development of these vaccines, "fast" does not mean "shoddy." We’ll also talk about side effects, and why the type of vaccine made by Moderna and Pfizer-BioNTech isn't going to change your DNA (or whatever other false claims are making the rounds on your Facebook and Twitter feeds).

What vaccines are out there?

So far, two companies have received emergency authorization to distribute vaccines in the U.S.: Pfizer-BioNTech (which we'll shorten to "Pfizer" to keep things simple) and Moderna. Both vaccines use messenger ribonucleic acid (mRNA) to teach your body how to respond to the coronavirus.

That's very different from many other vaccines, which often use weakened or inactivated versions of viruses. That’s good news: These are the first mRNA vaccines to be authorized for widespread use — a breakthrough that could have profound effects on the future of medicine.

There are other vaccines in development, of course. Johnson & Johnson has requested authorization for its candidate, which uses a copy of a piece of the virus' DNA to get your body to develop an immune response. The J&J candidate, which is about to get reviewed and voted on for emergency use on Friday, offers some practical advantages, too: Unlike the mRNA vaccines, J&J's requires just one shot and does not need the frozen storage required for Moderna and Pfizer. As of Wednesday, new analysis from the FDA shows it provides strong protection against disease and death. AstraZeneka also has a DNA-based candidate and may seek emergency use in the U.S later this year. (It's already gotten the green light in the U.K. and European Union).

There are more in the pipeline, too. This Bloomberg vaccine tracker does a great job of explaining them all. (Fair warning: Bloomberg requires free registration to see the tracker. The New York Times also has a good tracker, sans registration.)

This mRNA process is new, you say? Is it safe?

Yeah, it's safe.

It's not a crazy question to ask. This process happened faster than anyone imagined, and people want to make sure this wasn't a rush job, like a kid "cleaning" their bedroom by stuffing everything under the bed. Even medical professionals wanted the details explained before they felt confident getting inoculated.

Manufacturers didn't cut safety corners to speed through steps. Instead, they layered steps as much as they could. So while they were lining up trials, they were setting up manufacturing, selling future doses to governments around the world, etc. Scientists more generally had been studying the mRNA technology for over a decade.

For a longer explanation, here's a 22-minute video from Cambridge Health Alliance physician Gerard Coste, who said learning about the process helped allay his fears over safety and speed. We'll get into a lot more detail about this process and the clinical trials involved in the next of these vaccine edition emails.

And since this is something people have asked: No, the mRNA-based vaccine will not change your DNA. Not at all.

What side effects do we know about?

So far, very few. There have been very rare cases of allergic reactions (we'll get to that in a bit), most notably with the Moderna and Pfizer vaccines. The most common side effects are soreness at the injection point, headaches, chills and fatigue. It seems these effects are more common after the second of the two doses, and may be more noticeable than similar reactions to, say, a flu shot.

“We should anticipate that if you got vaccinated that day, you may not want to go to work,” Peter Hotez, dean of the National School of Tropical Medicine at Baylor College, told Vox in explaining the side effects.

These side effects are signs your immune response is learning from the vaccine, as Katherine Wu lays out in excellent, understandable detail in The Atlantic.

When it comes to long-term side effects, honestly we don't know. Literally no one has been vaccinated long enough for us to know about long-term effects. But long-term adverse effects from vaccines are exceedingly rare.

What about those allergic reactions I read about?

They happened, and they're serious, but they are extremely rare. We're talking 5 per million doses administered for Pfizer, and 2.8 per million for Moderna, according to the CDC. Researchers are still investigating, but one possible culprit is something called polyethylene glycol, which sounds nasty but is actually the main ingredient in Miralax and other laxatives.

To make sure you aren't having an adverse reaction to the vaccine, you're going to have to sit around for about 15 minutes after getting your shot. That may stretch to 30 minutes if you're someone with a history of allergic reactions.

Can I choose which vaccine I get? Can I mix and match since they both use mRNA?

No, you can't as of now. You get what you get and you don't get upset.

These are not interchangeable products, even if they are based on the same mRNA science, and there's just not enough research to support using them interchangeably. Here's what the CDC says about that. 

Roberto Scalese Twitter Digital Producer
Roberto Scalese is a digital producer and editor. 

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