COVID-19 vaccine side effects and mRNA technology, explained

A man getting a vaccine

Editor’s note: As what we know about COVID-19 evolves, so could the information in this story. Find our most recent COVID-19 articles here and learn the latest in COVID-19 prevention at the Centers for Disease Control and Prevention. Some photos and videos on this site were filmed prior to the COVID-19 outbreak or may not reflect current physical distancing and/or masking guidelines.

With such an accelerated vaccine development process, the public hasn’t had long to learn how COVID-19 vaccines work, understandably leading some to be more wary than usual of vaccine risks.

Let’s break down how this vaccine technology works and why scientists and medical experts like me are confident in its safety.

It’s a new technology — but not that new

All COVID-19 vaccines that currently exist are messenger RNA vaccines, or mRNA vaccines. RNA stands for ribonucleic acid, a molecule that helps carry out and regulate many functions in the body.

mRNA vaccines are relatively new to science’s long vaccine history. But mRNA vaccine technology has been studied for decades in vaccines focused on other viruses, such as the flu, rabies and Zika. mRNA has even been previously studied outside of vaccine research, having been used in clinical studies involving treatments for cancer.

One of the benefits of mRNA vaccine technology is that scientists have the ability to quickly apply a standardized mRNA technology to new vaccines as new viruses are discovered, tailoring the mRNA vaccine template to an individual disease and creating vaccines faster than ever.

How mRNA functions in the COVID-19 vaccine

mRNA vaccines carry strands of mRNA that, in the body, function like an instruction manual. For COVID-19 vaccines, those instructions tell the body how to create a piece of the “spike protein” unique to SARS-CoV-2, the virus that causes COVID-19.

Because the mRNA encodes only for the spike protein — by itself a harmless protein found on the surface of the actual virus — the vaccine cannot cause COVID-19 infection. It is antigenic, though, meaning it can provoke an immune response in the body that helps create the antibodies necessary to fight COVID-19 effectively.

Once the spike protein is created in the body using the mRNA instructions, the body’s cells use enzymes to quickly break down the mRNA strands for removal by normal cellular processes. The mRNA doesn’t actually enter the nucleus of any cell (where your DNA is located) or affect genetic material in the body.

What we know about mRNA vaccine safety

Because of the research that has taken place for other mRNA vaccines, scientists already have been able to refine some of the science behind the vaccines and work around various challenges, such as ensuring the body doesn’t react too strongly to the mRNA.

Clinical trials for COVID-19 vaccines have been taking place since summer 2020, and they’ve been found to be very safe.

The most common side effect of the vaccine is pain/soreness at injection site. Other commonly reported side effects include fever, fatigue, muscle aches or headache, but these are usually mild and resolve in a few days. Serious reactions are uncommon and include rare cases of severe allergic reactions. Caution is advised for anyone who has a history of serious allergic reactions to vaccines or injections in the past.

Out of millions of doses administered to date, very few severe side effects, including allergic reactions, have been reported. No long-term adverse effects have been reported.

This is a new technology that we’re all still learning about, and millions of people have been vaccinated in the U.S., with the pace of vaccinations increasing daily. Over time, we’ll continue to track data, learn and be able to make even more-informed decisions about its use and potential effects.

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