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Which COVID-19 vaccine should you choose for your second booster? It’s a question again — Pfizer or Moderna — after the US Centers for Disease Control and Prevention signed off on second booster doses of both vaccines for all adults ages 50 and up, people who are immunocompromised (ages 12 and older) and those who’ve had two shots of Johnson & Johnson. If you’re eligible, you can get a second booster four months after your last shot.
Months ago, the CDC and US Food and Drug Administration authorized the use of heterologous booster doses, or a “mix and match” approach to COVID-19 boosters, which means people aren’t married to whichever COVID-19 vaccine they originally received.
What’s more, both mRNA vaccines (Pfizer and Moderna) are recommended over Johnson & Johnson’s vaccine. The one-dose vaccine is linked to very rare but serious side effects (which aren’t seen with Pfizer’s or Moderna’s vaccines), and it may also be less protective against COVID-19 emergency room visits, according to a CDC report published Tuesday. For this reason, most people should choose either mRNA vaccine for a booster or primary shot, the agency says. But which one?
Kids and teens between the ages of 12 to 17 are only eligible for a booster of the Pfizer/BioNTech vaccine, as it’s the only vaccine authorized for younger age groups. But for everyone else, US public health guidance is a little less clear and raises questions of the differences between the vaccines.
Here’s what we know about mixing and matching COVID-19 vaccines.
What’s the difference between Pfizer’s and Moderna’s booster?
Pfizer and BioNTech’s booster dose is the same size as its original vaccine, 30 micrograms. While Moderna’s booster dose is only half the size of the doses given as the first two shots, it’s slightly larger at 50 micrograms. Both are mRNA vaccines, which teach our cells to make a specific protein and build immunity against a virus.
Johnson & Johnson’s booster and vaccine are the same dose. It’s a viral vector vaccine that uses a harmless virus to activate an immune response that teaches our bodies what to fight in future infections.
Read more: Who Is Eligible for a 4th COVID Shot?
Why mix and match?
Mixing COVID-19 vaccines when you choose a different booster may enhance the immune response, the National Institutes of Health says, and it also allows for more flexibility when people are making a vaccine appointment or looking for a shot. Other countries have been allowing people to mix shots for their primary COVID-19 vaccine series.
“The most important reason is that mixing offers advantages in enhanced immune response and, therefore, anticipated enhanced protection,” Dr. William Schaffner, a professor of preventative medicine at Vanderbilt University, told UK health site Patient. He added, though, that the advantage of mixing two mRNA vaccines (Pfizer and Moderna) is smaller than the benefit someone who originally received Johnson & Johnson gets from an mRNA booster.
The CDC report published this week found that people who received two doses of Johnson & Johnson’s vaccine were less protected from emergency room or urgent care visits (54% effective) due to COVID-19 disease compared to people who received one dose of Johnson & Johnson followed by an mRNA booster (79% effective ). People who received three doses of an mRNA vaccine were 83% protected against emergency visits.
Because of the new data, adults who received two doses of Johnson & Johnson’s vaccine are included in the group of people who may receive another booster of Pfizer or Moderna, the CDC says.
During a mix-and-match COVID-19 vaccine trial funded by the National Institute of Allergy and Infectious Diseases (before omicron was the dominant variant), boosters from all three vaccine companies induced good immune responses in roughly 450 people who got different vaccines. In the study, Moderna’s booster gave the most robust response. However, that study examined a full dose of Moderna, rather than the authorized half-dose of the company’s booster, which could minimize Moderna’s edge over Pfizer, as The Atlantic reported.
If you’re looking to switch from Pfizer to Moderna, or vice versa, the answer on a benefit may be a little less clear than the data or recommendations for Johnson & Johnson. But Pfizer’s and Moderna’s vaccines are “not interchangeable” as Scripps Research Translational Institute Director Dr. Eric Topol said on Twitter, referencing a study on the different mRNA vaccines and suggesting a benefit to mixing.
Of course, if you’re most at risk of severe COVID-19 because of a medical condition or age, the best booster may be the one most convenient to you. People ages 50 and up with a medical condition, and adults age 65 and older, are most likely to benefit from a second booster at this time, CDC Director Dr. Rochelle Walensky said Wednesday.
If you have specific questions about which shot is best for you, talk with your health care provider.
The definition of “fully vaccinated” hasn’t changed. A person is considered fully vaccinated two weeks after their second dose of Pfizer or Moderna, or two weeks after a single dose of Johnson & Johnson.
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Can I mix and match the first two shots?
No, the health officials’ allowance for mixing COVID-19 vaccines applies only to boosters. As of now, the FDA has only authorized a mixed-series booster, meaning the first coronavirus vaccine series must be two doses of Moderna or Pfizer or one dose of Johnson & Johnson.
More data on mixing for boosters will inform decisions on primary coronavirus vaccine series being used together, which could make it easier to reach underserved communities and possibly reduce health care and vaccine inequity.
The information contained in this article is for educational and informational purposes only and is not intended as health or medical advice. Always consult a physician or other qualified health provider regarding any questions you may have about a medical condition or health objectives.