Mixing and Matching Covid-19 Vaccines Could Impact Immunity

The Food and Drug Administration recently authorized a mix-and-match booster shot strategy that allows fully-vaccinated Americans to choose a booster shot of one of three Covid-19 vaccines: Johnson & Johnson, Pfizer-BioNTech or Moderna, even if it’s different from the one they initially received. A mix-and-match option not only makes it more convenient for Americans to access boosters—studies show benefits from making the switch to another brand, especially for those who initially received the Johnson and Johnson Covid-19 vaccine.

Public health officials aren’t recommending one vaccine brand over the other, instead they are allowing individuals to weigh personal benefits and risks. Studies have shown that mixing vaccines prompts a strong antibody response regardless of the combination of brands and provides effective protection against the hyper-infectious Delta variant. Those currently eligible for a booster shot include roughly 70 million older and high-risk recipients who received Pfizer and Moderna jabs, and those who received the one-shot Johnson & Johnson vaccine. 

“Part of the beauty of the mix and match is it enables people no matter where they are—rural or in the city—to have a choice,” said Kirsten Lyke, a professor at the University of Maryland School of Medicine who presented study data to the Food and Drug Administration. “They’re all safe, they’re all going to give you a boost, and they’re all going to protect you against severe disease and death.”

In a study conducted by the National Institutes of Health (NIH) this summer, experts gave volunteers a different booster than their original inoculation to see if certain combinations proved particularly effective against Covid-19. The trials assessed individuals’ neutralizing antibodies—the specific antibodies that stop the virus and protect you from illness—and found that every combination prompted an antibody response. The biggest differences in antibody levels were seen in the Johnson & Johnson recipients, reports Kate Baggaley for Popular Science. Those that got a second Johnson & Johnson jab had a fourfold rise in neutralizing antibodies, while those who got the Pfizer booster had a 35-fold rise. The most dramatic jump was seen in individuals that switched to the Moderna booster; they experienced a 76-fold increase in neutralizing antibodies.

“I was delighted to see that [mixing and matching vaccine brands is] as effective as one would expect,” says immunologist Martina Sester at Saarland University in Homburg, Germany to Ewen Callaway for Nature. “This is really good news and this will certainly have influence on clinical practice.”

For many of the roughly 15 million Americans who received the Johnson & Johnson shot, another dose of the same vaccine is still effective at boosting immune response. One study of 30,000 people found that a second dose of the J&J shot given at least two months after the first results in 94 percent protection against mild to severe cases of Covid-19, according to Tara Parker-Pope for the New York Times. Medical experts note that the NIH study of booster shots used the original 100-microgram dose of the Moderna vaccine, instead of the half-size 50 microgram dose booster.

Because public health officials aren’t recommending a specific shot, individuals are left to make the often-confusing choice on their own—and have to balance specific health concerns associated with each vaccine. Rare cases of a type of heart inflammation called myocarditis have been linked to the mRNA-based shots, and the Johnson & Johnson vaccine carries a small risk of blood clots in young women. Experts recommend talking with a doctor about specific health concerns to decide which booster may be best. 

While evidence exists that older or immune-compromised individuals stand to benefit from a Covid-19 booster, experts emphasize that initial vaccine doses are still incredibly effective at protecting against serious illness and hospitalization.

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Corryn Wetzel is a freelance science journalist based in Brooklyn. Her work has also appeared in Audubon magazine, National Geographic and others.