- All three US-authorized coronavirus vaccines can have similar side effects, though they vary slightly.
- Injection-site pain is the most common side effect of all three shots.
- More than 60% of participants in Moderna’s and Pfizer’s trials also reported fatigue.
- See more stories on Insider’s business page.
It’s normal to feel discomfort after a coronavirus shot.
Once a vaccine goes into your arm, blood flow increases and immune cells rush to the scene. This can result in pain at the injection site — the most common side effect of all three US-authorized coronavirus vaccines.
The reaction is more common after Pfizer’s and Moderna’s vaccines than Johnson & Johnson’s. 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.
When our immune systems detect the ingredients of a vaccine, they also release inflammatory chemicals to protect us. That’s why people can develop a fever, muscle pain, fatigue, or headaches shortly after their shots.
Fatigue was the second-most common side effect in Moderna’s and Pfizer’s trials. Nearly 69% of Moderna participants and 63% of Pfizer participants reported it.
But headaches were slightly more common than fatigue among Johnson & Johnson participants: 39% reported headaches, compared with 38% who reported fatigue.
Here’s a breakdown of how vaccine side effects differ by age and manufacturer:
Fatigue and headaches are more common after dose two
The Centers for Disease Control and Prevention released a report on Monday that examined side effects among more than 1.9 million Americans who’d received both doses of Pfizer’s or Moderna’s vaccines.
Overall, side effects were slightly more common after both of Moderna’s shots than Pfizer’s. And side effects across the board were more numerous and severe after the second dose of either vaccine.
Reports of injection-site pain rose from 68% after dose one of either vaccine, to 72% after dose two. Fatigue rose from 31% to 54% from the first to second shot, while headaches rode from 26% to 47%.
Nearly 82% of Moderna recipients reported some reaction at the injection site — pain, redness, itching, or swelling — after their second dose, while 69% of Pfizer recipients did. In addition, 60% of Moderna recipients reported fatigue and 53% reported headaches after dose two. After the second dose of Pfizer’s shot, 48% of vaccine recipients reported fatigue and 40% reported headaches.
Muscle pain and fever are more common than gastrointestinal issues
Across all three trials, muscle pain was the fourth most common side effect.
In Moderna’s trial, 60% of participants had muscle pain, while 38% of Pfizer participants reported the symptom. Around one-third of participants in Johnson & Johnson’s trial reported muscle pain as well.
Chills were less common but not rare: 43% of people in Moderna’s trial reported chills and 32% of Pfizer participants did. Just 2% of Johnson & Johnson participants felt that effect, though. In both Pfizer’s and Moderna’s trials, 15% of participants reported fever, compared with 9% in Johnson & Johnson’s trial.
A physician administers the Moderna COVID-19 vaccine.MediaNews Group/Boston Herald via Getty Images
For the most part, gastrointestinal issues like nausea, vomiting, and diarrhea weren’t commonly associated with Pfizer’s or Moderna’s shots, but around 14% of Johnson & Johnson participants reported nausea.
Side effects were fleeting across all three trials, though.
The majority of Moderna participants said their side effects started the day they got their shot and lasted two days after each dose. On average, Pfizer participants also experienced side effects one to two days after their shot, with the reaction usually lasting just one day.
Johnson & Johnson participants saw side effects within two days of their injection. On average, fatigue, headache, and muscle pain lasted two days, while nausea and fever lasted one day.
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