It's possible to experience side effects are receiving either one or two doses of any of the three coronavirus vaccines currently being administered in the U.S., experts have said. But not everyone experiences them -- and certainly not in the same way.
For those who do, experiencing side effects isn't necessarily a bad thing. In fact, it's a sign one's body is responding.
"The good news on our part is that a brisk response equals an effective response," Dr. Mark Loafman, chair of family and community medicine for Cook County Health in Illinois, told NBC 5. "It tells us that the vaccine is working. Our body's forming a robust immune response and we feel that that's a positive thing. So we tend to see the vaccines that have a higher efficacy rate also have more of the so-called side effects or the symptoms because they work so well."
So what makes someone more likely to experience them than others?
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Younger People Are More Likely to Report Side Effects
Chicago's top doctor, Chicago Department of Public Health Commissioner Dr. Allison Arwady, broke it down Thursday, saying in Facebook Live that younger people are more likely to experience side effects "because younger people have more robust immune system broadly."
And, according to Loafman, the body's immune system is what creates the symptoms.
"That's simply a reflection of the immune response, just the way we have when we get ill," he said.
Women Are More Likely to Report Side Effects
Arwady noted that women are more likely to report side effects than men.
"Some of this is because women may just be better reporters... but there probably is something real to this too because something else interesting for those who may not know as much about immunity is that autoimmune diseases? Much more likely in women, too," Arwady said. "And even the, like, more serious like the allergic reactions, the more serious allergic reactions? More likely in women."
Why is that?
Arwady said estrogen can elevate immune responses, while testosterone can decrease it. At the same time, she noted that "a lot of your immune modulating genes" can live on an "x" chromosome, which women have two of, while men have one.
"So there's all these reasons that sort of immunity in general goes up a little bit different in women than it does in men," she said. "And so we're seeing women, a little more likely to report some of the side effects."
Data from the CDC also reported women were more likely to experience side effects than men, according monitoring from the first month of vaccinations.
From Dec. 14 through Jan. 13, more than 79 percent of side effects were reported by women, the data showed. Meanwhile, women received roughly 61.2 percent of the doses administered during that same time.
Does Experiencing Side Effects Mean You Had COVID?
Side effects could also vary depending on whether or not you've had coronavirus.
"We have seen more likely that people will report some side effects because that is acting a little bit like a booster dose to your immune system," Arwady said. "Your immune system has already learned some of those lessons of how to protect itself, not in as long a way not as protective a way."
"That is also probably that booster effect," Arwady said.
Loafman agrees.
"If you had COVID a while ago or you've already got some immunity, it's more like a booster," he said. "And boosters for some people are completely asymptomatic, boosters for other people trigger their immune response against it so they have some inflammation with it."
Are Side Effects More Common With the Second Dose?
People are also more likely to report side effects after their second dose, Arwady said, echoing reports from the Centers for Disease Control and Prevention.
According to the CDC, side effects after your second shot "may be more intense than the ones you experienced after your first shot."
"These side effects are normal signs that your body is building protection and should go away within a few days," the CDC states.
In trials of both the Moderna and Pfizer vaccines, more people experienced side effects after the second dose.
But that doesn't mean that you shouldn't get your second shot if you get side effects after your first, experts say.
“When people receive that second dose, they are receiving the second booster to try and reach the maximum efficacy," said Dr. Edward Cachay, infectious disease specialist at UCSD.
The CDC also noted that both shots are needed.
"The Pfizer-BioNTech COVID-19 Vaccine and Moderna COVID-19 Vaccine both need 2 shots in order to get the most protection," the CDC states. "You should get the second shot even if you have side effects after the first shot, unless a vaccination provider or your doctor tells you not to get it."
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What If You Don't Get Side Effects?
But not getting side effects isn't negative, health experts say.
"If you don't get side effects it does not mean that you are not protected," Arwady said. "I want to be really clear about that."
According to Loafman, it simply means "your body didn't react with as much of an inflammatory response.
"You're still making antibodies," he said.
According to Loafman, every person's response is unique.
"It's really just kind of a reflection of how unique each of our systems are, what other immunities we have," he said. "You know, a lot of the antibodies cross react and we have cross reactivity so it's really a mosaic. Each of our immune systems is a mosaic composite of all that we've been through and all that we have and all we've recently been dealing with. Our individual response varies. Everybody gets gets the appropriate immune response."
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Data: City of New York, State of New Jersey • Nina Lin / NBC
What Are the Possible Side Effects?
The CDC reports the most common side effects for all three authorized vaccines is at the injection site. They include:
- Pain
- Redness
- Swelling
Common side effects in the body include:
- Tiredness
- Headache
- Muscle pain
- Chills
- Fever
- Nausea
The Centers for Disease Control and Prevention advises people to stick around for 15 minutes after vaccination, and those with a history of other allergies for 30 minutes, so they can be monitored and treated immediately if they have a reaction.
Do People Report More Side Effects With One Vaccine Than Others?
According to Pfizer, about 3.8% of their clinical trial participants experienced fatigue as a side effect and 2% got a headache.
Moderna says 9.7% of their participants felt fatigued and 4.5% got a headache.
Like Pfizer and Moderna's COVID-19 vaccines, the main side effects of the J&J shot are pain at the injection site and flu-like fever, fatigue and headache.
But Arwady noted that Johnson & Johnson trial participants did report fewer side effects.
"One thing that I've recommended is, if there are people who are very, very worried about side effects, you know, the Johnson & Johnson in the trials had a little bit less side effects," she said. "And that's, you know, that single dose, you know can be one thing there."
What Can You Do If You Experience Side Effects?
The CDC recommends people talk to their doctors about taking over-the-counter medicines like ibuprofen, acetaminophen, aspirin, or antihistamines, for any pain and discomfort after getting vaccinated.
"You can take these medications to relieve post-vaccination side effects if you have no other medical reasons that prevent you from taking these medications normally," the CDC states. "It is not recommended you take these medicines before vaccination for the purpose of trying to prevent side effects."
The CDC recommends you seek medical attention if you experience any of the following:
- If the redness or tenderness where you got the shot gets worse after 24 hours
- If your side effects are worrying you or do not seem to be going away after a few days
- If you get a COVID-19 vaccine and you think you might be having a severe allergic reaction after leaving the vaccination site, seek immediate medical care by calling 911.
