Side effects are possible after receiving either one or two doses of any of the three coronavirus vaccines currently being administered in the U.S., but not everyone experiences them.
So what makes someone more likely to experience them than others?
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."
Arwady noted that women are also more likely to report side effects than men.
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"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.
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.
Dr. Mark Loafman, chair of family and community medicine for Cook County Health in Illinois, 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."
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."
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.
The CDC reports the most common side effects for all three authorized vaccines is at the injection site. They include:
Common side effects in the body include:
- Muscle pain
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.
Read more about side effects here.