Experts say that it’s normal to feel discomfort after a coronavirus vaccine 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 coronavirus vaccines.
In AstraZeneca’s case, injection-site tenderness was most common, affecting 64% of trial participants. Around 54% of participants reported injection-site pain.
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.
In AstraZeneca’s trial, fatigue and headaches were equally as common: around 53% of participants reported these side effects.
Here’s a breakdown of how vaccine side effects differ by age and manufacturer. (Data for Pfizer, Moderna, and Johnson & Johnson are based on clinical trials. AstraZeneca’s data are based on a small study of nearly 130 vaccine recipients.)
Fatigue and headaches are more common after Pfizer’s and Moderna’s second dose – but not AstraZeneca’s
A recent Centres for Disease Control and Prevention report 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 two. 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 rose 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 said the same. 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.
AstraZeneca’s shot tends to have worse side effects after the first dose for reasons scientists don’t fully understand. Experts say the reaction may have to do with the vaccine’s technology, which uses a genetically engineered common-cold virus to introduce a coronavirus gene into the body. That common cold virus could potentially stimulate a stronger immune response right away.
Muscle pain and fever are more common than gastrointestinal issues, muscle pain was among the most common side effects across all four trials.
In Moderna’s trial, 60% of participants had muscle pain, while 38% of Pfizer participants reported the symptom. About one-third of Johnson & Johnson participants and 44% of AstraZeneca participants reported muscle pain as well.
Chills were less common but not rare: 43% of people in Moderna’s trial reported chills, as did 32% of Pfizer and AstraZeneca participants. Just 2% of Johnson & Johnson participants felt that effect. In both Pfizer’s and Moderna’s trials, 15% of participants reported fever, compared with 9% in Johnson & Johnson’s trial and 8% in AstraZeneca’s.
For the most part, gastrointestinal issues like nausea, vomiting, and diarrhea weren’t commonly associated with Pfizer’s or Moderna’s shots, but about 14% of Johnson & Johnson participants and 22% of AstraZeneca participants reported nausea.
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.
AstraZeneca’s side effects usually resolve within a few days as well.
By Joe Cusmano