Researchers showed last year that the elite school inside of lymph nodes where the B cells train, called the germinal center, remains active for at least 15 weeks after the second dose of a Covid vaccine. In an updated study published in the journal Nature, the same team showed that six months after vaccination, memory B cells continue to mature, and the antibodies they produce keep gaining the ability to recognize new variants.
“Those antibodies at six months are better binders and more potent neutralizers than the ones that are produced one month after immunization,” said Ali Ellebedy, an immunologist at Washington University in St. Louis who led the study.
In the newest study, another team showed that a third shot creates an even richer pool of B cells than the second shot did, and the antibodies they produce recognize a broader range of variants. In laboratory experiments, these antibodies were able to fend off the Beta, Delta and Omicron variants. In fact, more than half of the antibodies seen one month after a third dose were able to neutralize Omicron, even though the vaccine was not designed for that variant, the study found.
“If you’ve had a third dose, you’re going to have a rapid response that’s going to have quite a bit of specificity for Omicron, which explains why people that have had a third dose do so much better,” said Michel Nussenzweig, an immunologist at Rockefeller University who led the study.
Memory cells produced after infection with the coronavirus, rather than by the vaccines, seem less potent against the Omicron variant, according to a study published last month in Nature Medicine. Immunity generated by infection “varies quite a lot, while the vaccine response is much more consistently good,” said Marcus Buggert, an immunologist at the Karolinska Institute in Sweden who led the study.
Although most people, vaccinated or not, show only a small drop in their T cell response against Omicron, about one in five had “significant reductions of their responses” of about 60 percent, Dr. Buggert said. The differences are most likely because of their underlying genetic makeup, he said.
Still, the recent studies suggest that in most people, the immunity gained from infection or vaccination will hold up for a long while. Even if mutations in new variants change some of the viral regions that T cells recognize, there would still be enough others to maintain a reasonably strong immune response, experts said.