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With COVID-19 vaccinations now available to all American adults, more people are getting their shots. And with more vaccinations come more people experiencing the vaccine’s side effects.

You may have heard from friends or family members — or maybe it happened to you — left feeling lousy after their shot. The side effects after the vaccine range in severity, with some feeling fine after their shot, some experiencing minor symptoms like soreness on the arm where they were injected, and others feeling more serious effects.

Now that vaccinations are open to a younger portion of the population, and with data showing that younger people are more likely to feel side effects, here are answers to questions you may have about side effects from experts and the Centers for Disease Control and Prevention.

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What kind of side effects are we talking about? What do they mean?

Side effects as a result of the COVID-19 vaccine mean that your immune system is responding how it should and generating a reaction that allows the body to retain memory of COVID-19 to make a response in the future if you are exposed, said Dr. Vandana Madhavan, clinical director of pediatric infectious disease at MassGeneral Hospital for Children.

People who have been vaccinated might experience pain, redness, and swelling on the arm where they got the shot, according to the CDC.

To help reduce pain and discomfort in the arm, the CDC recommends people apply a clean, cool, and wet washcloth over the area and try to use or exercise the arm after the shot.

Some may also feel worse, with symptoms like headaches, tiredness, muscle pain, chills, fever, or nausea, according to the agency.

Dr. Gabriela Andujar Vazquez, an infectious disease physician at Tufts Medical Center and the medical director for the hospital’s COVID-19 vaccine program, said the most common side effect is soreness at the site of the injection, and most symptoms resolve between one and three days.

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Andujar Vazquez said some people have also reported having enlarged lymph nodes after they’ve been vaccinated, which is expected because the vaccine goes to your lymphatic system, where we have cells that produce immunity, and enlarged lymph nodes indicate that your body is working to produce antibodies. That side effect is more common in younger people, she said.

Madhavan emphasized that the symptoms from the vaccine are reflective of the body’s immune system response and not the virus itself, because the vaccines approved for use in the United States don’t contain live virus.

Younger people are more likely to feel side effects than older people. Why?

According to CDC data, people between 18 and 49 are more likely to feel side effects from the vaccine than those who are older.

The data was collected through the V-safe, a CDC program that allows vaccine recipients to self-report any side effects they may be experiencing. Andujar Vazquez cautioned younger people may be more likely to report their symptoms to the CDC than older people, which might skew the numbers. However, there is a biological explanation for why younger people would be more likely to experience side effects than older people.

Younger people have a more robust immune system, she said.

“As opposed to an elderly patient with comorbidities and a weak immune system, side effects may not be prominent because the immune system is not young,” she said.

She noted that the symptoms do not differ between younger and older people.

What if I don’t feel any side effects?

If all you feel after getting vaccinated is arm soreness, it doesn’t mean the vaccine isn’t working.

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There are a range of factors that contribute to the way a person responds to a vaccine, Madhavan said, including their age, risk factors, other medications, and how active someone’s immune system might be on a given day. A person’s immune system doesn’t have to generate a response to be working.

“Individuals are different,” Andjuar Vazquez said. “Even though we’re made of the same materials, we all have different medical problems and immune systems. It doesn’t mean that if you don’t have chills and a fever that you don’t have immunity. Each of us respond differently.”

Andujar Vazquez noted that data show that the “robustness of side effects doesn’t necessarily correlate with the amount of immunity,” because the clinical trials for the Pfizer and Moderna vaccines looked at six months post-vaccination and found that people still had immunity against the virus, and the pool of participants in the trials included people of all ages who reported experiencing a range of side effects.

Is it true that I might feel worse after the second shot than the first? Why?

With the Pfizer and Moderna vaccines, which require two shots to be fully protected, side effects are more common after the second shot, according to the CDC.

In Massachusetts, more than 2.2 million people have received both doses of the Pfizer or Moderna vaccines, and about 3.4 million people have received only the first dose, according to the most recent vaccination numbers from the state, leaving about 1.2 million people who are between shots.

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Some may experience more side effects with the second shot because their body is trying to respond more robustly to the vaccine after recognizing it from the first dose, Andujar Vazquez said.

“You have some circulating antibodies against the synthetic, dummy virus and then when you get the second dose, your body responds even more because you already had antibodies from that first dose,” she said. “And that’s what creates the full immune response.”

The first dose operates as a primer to the second dose, she noted.

“The first dose creates good protection, but that second dose closes the deal and makes it more robust, which is probably why we respond more,” Andujar Vazquez said.

How can I treat the side effects at home?

Once you’re home from your shot, if you find yourself beginning to experience some of the more serious side effects, there are a number of things experts say you can do to mitigate the symptoms.

Madhavan said that those who just received their COVID-19 vaccine should try to rest, stay hydrated, and anticipate possibly being away from work for a day or two.

Andujar Vazquez said the side effects respond well to medicines like Tylenol, and the CDC’s website says you can take ibuprofen, acetaminophen, aspirin, or antihistamines for any pain or discomfort you feel after the shot if you take those medications normally. The agency also recommends people who have been vaccinated speak to their doctor about taking over-the-counter medication to treat side effects.

Enlarged lymph nodes can be treated by applying warm compresses to the area, Andujar Vazquez said, and the swelling typically goes away in a couple of days.

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The CDC advises that people should contact their doctor if the redness or tenderness where they got the shot gets worse after 24 hours or if their side effects are worrying and don’t appear to be going away after a few days.

Experts say the benefits of the vaccine outweigh the risks of COVID-19. Here’s why.

Despite reports of expected side effects after being vaccinated, the Food and Drug Administration has deemed the vaccines safe and effective in preventing serious illness and death due to COVID-19. Public health officials are urging all adults 16 and older to get vaccinated, noting that the benefits of getting vaccinated far outweigh the risks of becoming infected with the virus.

Even if it’s a mild infection, someone who tests positive for COVID-19 will need to stay home or be isolated from their family, either to recover from the symptoms of the virus, or to avoid spreading the virus to others. The quarantine period that comes with getting infected lasts longer than side effects from the vaccine, Madhavan said.

“We know that there’s so many risks to an individual and that individual’s family and community with active COVID-19 infection, and that those far outweigh the risks of being out for a day or two — or not even that — after getting the vaccine,” she said.

Andujar Vazquez also noted that the side effects from the vaccine are expected, and people are aware of how they might feel ahead of time, whereas if someone is infected with the virus, there is a greater degree of uncertainty.

“You have no predictability with COVID-19,” she said. “You know if you have the vaccine you will get x, y, and z symptoms, and they will last for a day and they’ll be done. But if you get COVID, you don’t know what’s going to happen. You don’t know if you’ll be asymptomatic, symptomatic, get very sick, or be hospitalized and die. There’s such a spectrum of different presentations of getting infected with COVID that to me, the short amount of side effects that you get from a vaccine that will prevent you from getting super sick from infection outweigh any risk.”


Amanda Kaufman can be reached at amanda.kaufman@globe.com. Follow her on Twitter @amandakauf1.