Dr. Fauci Says These 2 Side Effects Mean Your COVID Vaccine Is Working

These side effects are signs your immune system is getting "revved up," says Fauci.

While COVID vaccine side effects can seem alarming, medical experts have cautioned that they are typically mild to moderate, only last up to 48 hours, and are actually an indication that your shot is working. During a Jan. 28 interview with MSNBC, chief White House COVID adviser Anthony Fauci, MD, recently name dropped two side effects in particular that he feels should be viewed as a welcome sign that your immune system is responding to your vaccination. Read on to find out which side effects are good news, and for Fauci's firsthand account of getting the COVID vaccine, check out Dr. Fauci Says He Had These Side Effects From His Second Vaccine Dose.

Fauci explained that the two unpleasant side effects in particular are not cause for worry, but for reassurance. "The vaccine, because you're giving it in the arm, it gives a systemic reaction. You know that because sometimes after the second dose you feel a little achy, a little chilly, which means the immune system is really getting revved up," Fauci explained.

According to the Centers for Disease Control and Prevention (CDC), that's because the two mRNA COVID vaccines currently approved in the U.S., from Pfizer and Moderna, do not inject recipients with inactivated virus. Instead, they teach our own cells to mimic certain features of the COVID virus so that our immune systems can train to fight against it later, if needed.

Specifically, the COVID vaccines work by instructing our cells to recreate their own version of the "spike protein" found on the outside of the SARS-CoV-2 virus. As more cells create these spike proteins, our immune systems "recognize that the protein doesn't belong there and begin building an immune response and making antibodies, like what happens in natural infection against COVID-19," the CDC says. So, if you feel those aches and chills that Fauci mentioned, rest assured: that's just your immune system firing up to take down a perceived threat.

Wondering what other side effects you might expect after getting your shot? Read on for the most common COVID vaccine side effects that Moderna patients reported, and for more essential vaccine news, check out If You're Over 65, You Shouldn't Get This New Vaccine, Experts Warn.

Joint pain

Man feeling joint pain in elbow
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Joint pain, known medically as arthralgia, is the fourth most common side effects reported by vaccine recipients: nearly half of those enrolled in the Moderna trials—46 percent, to be exact—experienced this particular side effect in the hours or days after receiving the shot. And for more on what you shouldn't do before you get your vaccine, check out If You Take These OTC Meds, You Have to Stop Before Getting the Vaccine.


Cropped shot of an attractive young businesswoman suffering with a headache while standing in her office

Headaches were reported slightly more frequently than joint pain, with 64 percent of patients in Moderna trials experiencing this particular side effect. According to the CDC, headaches tend to be more common after the second dose.

While a study published in the Journal of Virology warned that taking over-the-counter pain medication prior to getting the shot may blunt the efficacy of the vaccines, experts say it's fine to take them afterward to treat vaccine side effects, including a headache. And for another possible delayed reaction to the shot, check out This COVID Vaccine Side Effect Could Show Up a Week After Your Shot.


Woman experiencing fatigue at home

If you experience sudden exhaustion after getting your COVID shot, you're not alone: about 70 percent of Moderna vaccine recipients reported feeling fatigued after their vaccination, making it the second most common side effect.

For this reason, you may want to schedule your shot when you know you'll have time to rest afterward, if possible—for example, on a lighter workday, or at the end of your work week. In fact, after personally experiencing side effects from the COVID shot, infectious disease epidemiologist Saskia Popescu, PhD, recently urged, "We need to ensure folks have the ability to take time off post-shot." And for more regular COVID news delivered right to your inbox, sign up for our daily newsletter.

Pain at the injection site

Young woman has pain in left arm after getting vaccinated
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The most common side effect reported by Moderna recipients was pain at the injection site: 92 percent of those vaccinated experienced this sensation.

"There are two types of COVID-19 vaccine side effects," Teresa Bartlett, MD, senior medical officer at medical claims company Sedgwick, recently told Bustle. While systemic side effects affect broader body function (fever, chills, and aches are all prime examples), "local side effects are more common, and involve redness, swelling and perhaps some lymph node swelling in the vaccinated arm." And for more on that last side effect she mentioned, check out The Rare COVID Vaccine Side Effect Doctors Want You to Prepare For.

Lauren Gray
Lauren Gray is a New York-based writer, editor, and consultant. Read more
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