The NIH study tested and compared every possible combination, and here’s the gist: If you need a booster, don’t take J&J. Two weeks after boosting, people who had followed a J&J → Moderna regimen registered average antibody levels that were 9.8 times higher than those who had gotten two J&J shots; antibody levels among J&J → Pfizer recipients hovered just behind. Overall, the highest antibody levels were found among people for whom all three doses were Moderna; Pfizer → Moderna produced the second-highest levels, then Moderna → Pfizer.
Moderna’s boosters seemed to be slightly more effective than Pfizer’s in general, but that doesn’t mean that Americans who have already gotten Pfizer boosters are missing out. The differences between those mRNA regimens were relatively small; more to the point, they’re absolutely dwarfed by the differences between either mRNA option and the J&J → J&J approach. Saad Omer, who directs Yale’s Institute for Global Health, told me that “we can’t be too precise” in interpreting these data, given the study’s small size. (There were only about 50 people in each of the nine trial groups.) But the apparent advantage of using the mRNA vaccines as boosters, compared with J&J’s, is so large, he said, that it’s unlikely to be an error.
Other factors might also limit the significance of the apparent Moderna-Pfizer divide. In a paper published earlier this week, a team including Omer and led by his colleague Akiko Iwasaki found that those who have recovered from a COVID-19 infection and been vaccinated might be approaching a plateau of immune protection after which “the juice [of a booster shot] is not worth the squeeze,” Omer said.