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- Side effects from COVID-19 booster shots generally resemble those after a second dose.
- Fatigue, headaches, and muscle aches are common after both.
- But two shots of J&J's vaccine yields fewer side effects than three shots of Pfizer or Moderna.
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Second and third doses of COVID-19 vaccines generally produce similar side effects, but there are some slight differences.
A recent study from the Centers for Disease Control and Prevention (CDC) provides the first comparison between booster shots of all three of the vaccines available in the US: Pfizer and Moderna (both mRNA shots), and Johnson & Johnson (an adenovirus vaccine).
While the study includes far more data on mRNA boosters - 12,000 people, compared to 48 who received two J&J doses - it gives valuable insights into how people are responding to different boosters outside clinical trials so far.
Broadly, the CDC found that people who'd received three doses of either Pfizer or Moderna saw more local reactions - pain, itchiness, redness, or swelling at the injection site - after dose three compared to dose two. Other side effects such as fatigue, muscle aches, or headaches were less common, affecting 74% after their third dose, compared with 77% after dose two.
A second dose of J&J's vaccine yielded the fewest side effects of all. Just 10% of J&J booster recipients said their side effects prevented them from performing normal daily activities - usually on the day after their booster shot - while the rest said they were able to go about their day as normal. By comparison, 28% of Pfizer or Moderna booster recipients said the shots hindered their daily routine.
The following table shows how booster shots affected people who got three doses of Moderna, three doses of Pfizer, or two doses of Johnson & Johnson's vaccine, according to CDC data.
Aside from local reactions at the injection site, fatigue was most common among people who received a Pfizer or Moderna booster.
Overall side effects were more prevalent after Moderna's third dose: Half of Moderna booster recipients reported muscle aches and headaches, and around 60% reported fatigue.
Meanwhile, less than 40% of Pfizer booster recipients reported muscle aches and headaches, and around 50% reported fatigue.
Just one-quarter of people who got a J&J booster had injection site reactions, compared with 70% who got a Pfizer booster and 80% who got a Moderna booster. And 20% of J&J booster recipients reported muscle aches and headaches.
Fatigue, headaches, and muscles aches were common after first and second doses
The mRNA vaccines from Pfizer and Moderna instruct the body to produce a harmless viral protein, then develop antibodies against it. The first dose of these vaccines generally produces the mildest side effects, since our bodies are being introduced to the instructions and the resulting protein for the first time.
By the time we receive a second and third dose, our bodies have learned to recognize that protein quickly, so are ready to attack it. That's why side effects are generally more pronounced. (If you've had COVID-19 before, though, your first dose may yield more intense side effects than your second or third, according to one preprint.)
The following chart shows common vaccine side effects based on your age group, vaccine manufacturer, and which dose you received. Data comes from each vaccine's clinical trial.
As with a booster shot, fatigue, headaches, and muscles aches were common after doses one and two. In general, older people experience fewer side effects than younger adults because immune responses gradually weaken with age.
Boosters haven't been authorized for everyone yet
The Food and Drug Administration (FDA) first authorized Pfizer and Moderna boosters for people with severely weakened immune systems in August. That group includes people receiving cancer treatment, those with advanced HIV infections, or organ transplant patients. Third doses are necessary for this group, since they don't develop the same protection from two shots as others do.
More recently, the FDA and CDC recommended Pfizer boosters for other vulnerable groups as well.
The FDA said last month that people 65 years and older and others at high risk of severe COVID-19 should seek a booster shot. That includes people who are more likely to get sick because of their health status, as well those who are at high risk of exposure to the virus due to where they live and work - such as healthcare workers, teachers and daycare staff, grocery store workers, and residents of homeless shelters or prisons.
The CDC, on the other hand, recommends boosters for people 65 years and older, nursing home residents, and people ages 50 to 64 with underlying medical conditions. The agency says younger adults with underlying medical conditions or those at increased risk of COVID-19 exposure because of their job or living arrangement may consider a booster shot.
For most Americans, though, there's no need to run to the pharmacy for a booster right away. Vaccines are still highly effective in preventing severe disease, hospitalization, and death.
A recent CDC study suggests that unvaccinated Americans are 11 times more likely to die and 10 times more likely to be hospitalized from COVID-19 than people who are fully vaccinated.
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