- All three US-authorized coronavirus vaccines can bring mild to moderate side effects.
- Johnson & Johnson's shot generally has fewer side effects than Pfizer's and Moderna's.
- Older people had fewer side effects than younger adults across all three clinical trials.
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If you're getting vaccinated, expect a sore arm.
Pain at the injection site is the most common side effect of all three coronavirus vaccines authorized in the US.
In general, the vaccines produce mild to moderate side effects that shouldn't last more than a few days. Side effects typically show up within 12 to 24 hours of getting the shot. They're often a sign that the body is building immunity to the virus.
"It's important to remember: When people get side effects from vaccines, it's not really because of the vaccine. It's more of the body's immune response to the vaccine," Dr. Vivek Cherian, an internal-medicine physician in Baltimore, told Insider.
Scroll through the chart below to see which side effects are common based on your age group, which manufacturer's vaccine you get, and whether you're on dose one or two. In general, older people experience fewer side effects than younger adults because immune responses gradually weaken with age. For Pfizer's and Moderna's two-shot vaccines, the second shot amplifies an existing immune response, so people typically feel more run-down after that dose.
Vaccines prompt our bodies to produce coronavirus antibodies. But since our immune systems can't distinguish between a real infection and a vaccine-induced response, they still release inflammatory chemicals to protect us. That's why people can develop a fever, muscle pain, fatigue, or headaches shortly after their shots.
Johnson & Johnson's shot has the fewest side effects
Across the board, Johnson & Johnson's shot has milder and fewer side effects than the other two. Some experts suspect that's because it's a single shot.
Nearly 62% of participants younger than 59 in Johnson & Johnson's trial reported side effects, compared with 45% of people ages 60 and up.
That's relatively similar to the reported side effects after one dose of Moderna's vaccine: About 57% of people younger than 65 in that trial developed side effects, compared with 48% of those older than 65. But after the second Moderna dose, nearly 82% of people in the younger group reported side effects, compared with nearly 72% of older adults.
But Johnson & Johnson's shot also seems to be less effective overall: Clinical trials suggest the vaccine is 66% effective at preventing COVID-19. Pfizer's and Moderna's vaccines are each more than 90% effective. But it's difficult to compare the companies' trials side by side because they happened at different stages in the pandemic and in different geographic regions.
Common side effects include fatigue and headache
Once a vaccine goes into your arm, your blood flow increases and immune cells rush to the scene. This can result in pain, redness, or swelling at the injection site.
The reaction is more common after Pfizer's and Moderna's vaccines than after Johnson & Johnson's. Across all age groups, less than 50% of participants in Johnson & Johnson's clinical trial reported pain at the injection site, compared with 92% of Moderna participants and 84% of Pfizer participants.
Headache and fatigue were also relatively common across all the trials. About 63% of Moderna participants reported headaches, compared with 55% of Pfizer participants and 39% of Johnson & Johnson's.
About 69% of people in Moderna's trial and 63% in Pfizer's reported fatigue. Just 38% of participants reported fatigue in Johnson & Johnson's trial, but the prevalence of those side effects varied by age.
Younger adults had more side effects, with a few exceptions
Just a few side effects appear to be equally as common among older participants as younger ones: After dose two of Pfizer's vaccine, for instance, joint paint was equally common in the two groups, with about 22 to 23% reporting the effect.
Though older adults tend to have fewer side effects overall, experts say there's no reason to believe vaccines won't work as well among them.
"For the COVID-19 vaccine, we've actually not seen decreased effectiveness as we get older, so that's actually a really good thing," Cherian said.
And younger people shouldn't worry too much about feeling strong side effects, he added.
"Dealing with a few side effects of some diarrhea or some muscle aches is a much, much better thing to get than some of those serious, potentially life-threatening side effects of the COVID-19 infection," Cherian said.