7 Potential Side Effects From the Updated COVID Vaccine to Anticipate

You just got your COVID-19 shot, and you’re feeling like crap. You know it’s worth it, but BOY, it doesn’t feel that way.

That’s a common reaction: A recent study found that 30-90% of people who got the COVID vaccine experienced some type of side effect, which can appear within 1–3 days after you get immunized.

But even they’re no fun, side effects can be an excellent sign your shot is starting to protect you. A brand new study (a pre-print, meaning it has yet to be peer-reviewed and so should not be seen as final or definitive) from researchers at the University of California San Francisco, found that if you have chills, fatigue, fever and/or headache after your vaccine, your body makes more antibodies against the virus compared to those who didn’t have side effects (these antibodies are detectable at both one month and six months following immunization.) The study reports that the more of these symptoms you have, the more antibodies you’ll have; also, if your heart rate increases and your skin feels warm, that’s a further indication that your immune system is revving up.

“Evidence does suggest that local and systemic reactions to the COVID shot may mean they are building more robust protection,” says Onyema Ogbuagu, MBBCh, FACP, FIDSA, a COVID vaccine/infectious disease expert and assistant professor of medicine at Yale School of Medicine in New Haven, CT.

Just as with all vaccines, a COVID shot mobilizes your body’s immune response. Your immune system’s job is to battle any germ or virus that enters your body, and when you get vaccinated (in this case, with mRNA or a protein subunit) the shot creates antibodies that recognize the latest strain of COVID. If you are exposed to the actual virus, your immune system will “remember” the germ or virus you were immunized against, and send out those antibodies to fight it.

Potential side effects of the 2023 vaccine:

This fall’s updated COVID vaccine is new, but it does not produce new, unknown or harsher side effects. “I get that people might be worried about getting new shots, including the new booster,” says Dr. Ogbuagu. “But it’s important to know that the COVID-19 vaccine, and COVID-19 boosters are very safe. We haven’t seen any serious problems with side effects. The new booster uses the same technology as the other COVID vaccines do – it’s simply been changed to target the newest strain of the virus, the same way that the flu shot is changed every year.” The shot has been rigorously tested.

According to Johns Hopkins School of Public Health, the new COVID vaccine is monovalent, which means it’s meant to target one strain of COVID, the XBB.15 variant, the latest “version” of Omicron likely to make you sick.

However, the Centers for Disease Control does note that there are common symptoms associated with this vaccine and previous ones, including:

• Pain, swelling or redness on the arm where you got the shot

• Tiredness

• Headache

• Muscle pain

• Chills

• Nausea

• Fever

But remember: “If you have side effects, they’re usually due to your immune system responding to the vaccine, which is how vaccines work,” says Shira Doron, MD, chief infection control officer for Tufts Medicine health system and hospital epidemiologist at Tufts Medical Center in Boston, MA.

But I don't have any side effects, is the vaccine still working?

If you don’t have side effects, you can still rely on the vaccine’s effectiveness, says Judith O’Donnell, MD, professor of infectious diseases at the Perelman School of Medicine at the University of Pennsylvania and section chief of infectious diseases at Penn Presbyterian Medical Center in Philadelphia. Although most people experience one or two side effects, “there are some people who don’t have any,” she says. “Side effects are related to your immune system recognizing the antigen or foreign protein in the vaccine and responding. However, everyone’s immune system is unique to them, and responds uniquely when presented with something foreign from a vaccine.”

As the research shows, side effects may indicate your system got a nice boost against COVID. Still, fewer side effects, less severe ones or a lack of any side effects doesn’t mean you're not well-protected – it simply shows that your bodies are different. “You don’t need to worry if you’re one of the lucky ones that doesn’t have side effects,” adds Dr. Doron. “The immune system is complex, and the vaccine is still working even if you don’t feel it.”

How long will the side effects from the shot most likely last?

Data from the University of Alabama at Birmingham notes that mRNA vaccines, like the COVID shot, deliver their payload and then quickly leave your body, so this eliminates the concern of any long-term side effects. “Most side effects last 24–48 hours,” says Dr. O’Donnell. “Arm soreness can sometimes last a little longer.”

How to get relief from potential side effects:

According to Yale New Haven Health, you can reduce any discomfort you have by using an ice pack or cool damp cloth on the injection site, or by taking a nice cool bath. You can also take an OTC pain medication if you don’t have any other conditions that preclude that. Drinking extra water for a day or two can also help you feel better.

As is the case after having any vaccine or taking any medication, Yale New Haven Health also advises that if you experience any emergency symptoms like trouble breathing, chest pain or pressure, trouble staying conscious, feeling confused, or having blue skin on your mouth or face, you should call 911. Again, however, this is very unlikely.

The bottom line: “Getting COVID-19 carries way more baggage than any side effects you might get from the vaccine,” Dr. Ogbuagu sums up. “We know that about two out of every 10 people who get COVID will develop long COVID. Plus, there could be many long-term effects COVID can have on a person’s health that we don’t even know about yet.” So get your shot, and try to look at the bright side of side effects — that they’re a good sign, and will be gone soon.

As more information about the coronavirus pandemic develops, some of the information in this story may have changed since it was last updated. For the most up-to-date information on COVID-19, please visit online resources provided by the CDC, WHO, and your local public health department.

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