When Freedom Baird got her first dose of Moderna's vaccine in February, she wasn't sure what kind of side effects to anticipate.
Baird is a COVID-19 long-hauler she's had lingering shortness of breath and chest pain for roughly a year. Many people who've had a prior infection develop more side effects in response to the first vaccine dose than the second. On average, however, people typically feel more run-down after their second shot.
Baird's age complicated her expectations: She's 56, and clinical trials have shown that people over 55 often develop fewer vaccine side effects. As it turns out, she didn't feel much.
"It was really just that first day of feeling achy and flu-y," Baird told Insider.
While doctors can't predict exactly how someone will respond to a coronavirus vaccine, they've identified a few patterns based on a person's age, sex, health status, and which dose they're receiving. Clinical trials suggest that side effects are generally more pronounced among women and younger adults, especially after their second dose.
A vaccine vial. Jessica Rinaldi/The Boston Globe via Getty Images
The most common side effect for all three authorized US vaccines is pain or swelling at the injection site: Nearly 92% of participants in Moderna's clinical trial developed this side effect. in Pfizer's trial, 84% of participants reported that, as did 49% in Johnson & Johnson's.
Other common side effects include fatigue, headache, and body or muscle aches. About 65% of vaccine recipients in Pfizer's and Moderna's trials, and 38% in Johnson & Johnson's, developed fatigue.
For those who haven't had COVID-19 before, side effects tend to be more numerous and severe after the second dose.
Roughly twice as many participants in Pfizer's trial developed chills and joint pain after their second dose than after their first. In Moderna's trial, meanwhile, about five times as many participants developed chills after their second dose as did after their first. Fevers were also far more common among second-dose recipients than first-dose recipients in both trials.
A small study from the Icahn School of Medicine at Mount Sinai found that vaccine side effects such as fatigue, headaches, and chills were more common among people with preexisting immunity to the coronavirus than people who'd never been infected. About 73% of vaccine recipients who'd previously had COVID-19 developed side effects after dose one of Pfizer-BioNTech's or Moderna's shot, compared with 66% of vaccine recipients who'd never gotten infected.
"If you've already had a COVID-19 infection, you've developed memory cells from that infection," Dr. Vivek Cherian, an internal-medicine physician in Baltimore, told Insider.
"If you were ever to be exposed to the infection again, your body would basically be able to respond quickly and more robustly that second time around," he added. "That's why you tend to have more strong side effects from that initial vaccine."
A woman receiving a COVID-19 vaccine in Wales. Getty/Matthew Horwood
Our immune systems gradually deteriorate as we age, which means older people's bodies don't work as hard to defend them against foreign invaders including the protein introduced to the body via a vaccine.
"Younger individuals have a much more vigorous immune response, so it should make sense that they would also have more side effects," Cherian said.
After one dose of Moderna's shot, 57% of people younger than 65 developed side effects, compared with 48% of those older than 65. After the second dose, nearly 82% of people in the younger group developed side effects, compared with nearly 72% of older adults.
Pfizer broke down its data slightly differently: About 47% of people ages 18 to 55 developed fatigue after dose one, whereas 34% of people ages 56 and older reported that side effect. After dose two, the numbers rose to 59% and 51%, respectively.
After Johnson & Johnson's one-shot vaccine, nearly 62% of people ages 18 to 59 developed side effects, compared with 45% of people ages 60 and up.
A woman receiving a COVID-19 vaccine. John Moore/Getty Images
The Centers for Disease Control and Prevention analyzed Americans' reactions to nearly 14 million doses of the Pfizer and Moderna shots from December to January. The results showed that roughly 79% of instances of vaccine side effects reported to the CDC came from women, though just 61% of doses were administered to women overall.
Cherian said women tended to react more strongly to vaccines for polio, influenza, measles, and mumps as well.
"All of these vaccines in general, women tend to have greater side effects," he said. "They're even more pronounced for a pre-menopausal woman compared to a post-menopausal woman."
Scientists suspect the difference has to do with estrogen levels.
"Testosterone tends to be an immune-suppressive hormone and estrogen tends to be an immune stimulant," Cherian said. "So more than likely it's the estrogen hormone that's why females tend to have more side effects."
The CVS pharmacist Gina Glancy giving the second dose of Pfizer's COVID-19 vaccine to Bob Jensen, 89, in West Chester, Pennsylvania. Pete Bannan via Getty Images
People with weakened immune systems don't mount a strong defense against viral infections in general, so they're particularly vulnerable to severe COVID-19. For that reason, the CDC recommends that these groups get vaccinated right away.
But it's possible that immunocompromised people, such as patients with cancer, won't mount a strong immune response to the vaccine, either.
"Your immune response essentially dictates your side effects, so if you're immunocompromised, you may not necessarily be having as many side effects, but you should still absolutely get vaccinated," Cherian said.
The vaccines should provide immunocompromised people with at least some protection against severe COVID-19, even if they don't feel any side effects though the effectiveness may be lower than for the average person.
Cherian said that for people with autoimmune conditions, meanwhile, the side effects probably won't be any worse than for the average person.
"If you have those high-risk factors, you really, really want to get vaccinated," he said. "Dealing with a few side effects of some diarrhea or some muscle aches is a much, much better thing than some of those serious, potentially life-threatening side effects of the COVID-19 infection."
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