Three shots might be all you need. T-cell immunity drops slowly. B-cell sophisticating and generalizability increases a lot after third shot
Specialized immune cells called T cells produced after immunization by four brands of Covid vaccine — Pfizer-BioNTech, Moderna, Johnson & Johnson and Novavax — are about 80 percent as powerful against Omicron as other variants, the research found. Given how different Omicron’s mutations are from previous variants, it’s very likely that T cells would mount a similarly robust attack on any future variant as well, researchers said.
This matches what scientists have found for the SARS coronavirus, which killed nearly 800 people in a 2003 epidemic in Asia. In people exposed to that virus, T cells have lasted more than 17 years. Evidence so far indicates that the immune cells for the new coronavirus — sometimes called memory cells — may also decline very slowly, experts said.
Antibodies recognize two or three key parts of the spike protein, a protrusion on the outside of the coronavirus that allows it to latch on to human cells. But T cells detect many more parts of the spike, and so are less likely to fail when the virus gains mutations in some of them.
Vaccines also encode a memory of the virus in B cells, which can churn out fresh batches of antibodies within four or five days after a new exposure to the virus.
This dual punch of T and B cells help explain why many people who received two or even three doses of vaccine could still be infected with the Omicron variant, but only a small percentage became seriously ill.
Memory B cells become increasingly sophisticated over time, and they learn to recognize a diverse set of viral genetic sequences. The longer they have to practice, the broader the range of virus variants they can thwart.
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.
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.