The vaccines made by Pfizer-BioNTech and Moderna set off a persistent immune reaction in the body that may protect against the coronavirus for years, scientists reported on Monday.

The findings add to growing evidence that most people immunized with the mRNA vaccines may not need boosters, so long as the virus and its variants do not evolve much beyond their current forms — which is not guaranteed. People who recovered from Covid-19 before being vaccinated may not need boosters even if the virus does make a significant transformation.

“It’s a good sign for how durable our immunity is from this vaccine,” said Ali Ellebedy, an immunologist at Washington University in St. Louis who led the study, which was published in the journal Nature.

The study did not consider the coronavirus vaccine made by Johnson & Johnson, but Dr. Ellebedy said he expected the immune response to be less durable than that produced by mRNA vaccines.

Based on those findings, researchers suggested that immunity might last for years, possibly a lifetime, in people who were infected with the coronavirus and later vaccinated. But it was unclear whether vaccination alone might have a similarly long-lasting effect.

Dr. Ellebedy’s team sought to address that question by looking at the source of memory cells: the lymph nodes, where immune cells train recognizing and fight the virus.

The broader the range and the longer these cells have to practice, the more likely they are to be able to thwart variants of the virus that may emerge.

After infection with the coronavirus, the germinal center forms in the lungs. But after vaccination, the cells’ education takes place in lymph nodes in the armpits, within reach of researchers.

That painstaking work makes this a “heroic study,” said Akiko Iwasaki, an immunologist at Yale. “This kind of careful time-course analysis in humans is very difficult to do.”

Dr. Ellebedy’s team found that 15 weeks after the first dose of vaccine, the germinal center was still highly active in all 14 of the participants, and that the number of memory cells that recognized the coronavirus had not declined.

“The fact that the reactions continued for almost four months after vaccination — that’s a very, very good sign,” Dr. Ellebedy said. Germinal centers typically peak one to two weeks after immunization, and then wane.

“Usually by four to six weeks, there’s not much left,” said Deepta Bhattacharya, an immunologist at the University of Arizona. But germinal centers stimulated by the mRNA vaccines are “still going, months into it, and not a lot of decline in most people.”

The results suggest that a vast majority of vaccinated people will be protected over the long term — at least, against the existing coronavirus variants. But older adults, people with weak immune systems and those who take drugs that suppress immunity may need boosters; people who survived Covid-19 and were later immunized may never need them at all.

Exactly how long the protection from mRNA vaccines will last is hard to predict. In the absence of variants that sidestep immunity, in theory immunity could last a lifetime, experts said. But the virus is clearly evolving.

“Anything that would actually require a booster would be variant-based, not based on waning of immunity,” Dr. Bhattacharya said. “I just don’t see that happening.”

People who were infected with the coronavirus and then immunized see a major boost in their antibody levels, most likely because their memory B cells — which produce antibodies — had many months to evolve before vaccination.

The good news: A booster vaccine will probably have the same effect as prior infection in immunized people, Dr. Ellebedy said. “If you give them another chance to engage, they will have a massive response,” he said, referring to memory B cells.

In terms of bolstering the immune system, vaccination is “probably better” than recovering from the actual infection, he said. Other studies have suggested that the repertoire of memory B cells produced after vaccination is more diverse than that generated by infection, suggesting that the vaccines will protect better against variants than natural immunity alone.

But that is an unfair comparison, because the Johnson & Johnson vaccine is given as a single dose, Dr. Iwasaki said: “If the J & J had a booster, maybe it will induce this same kind of response.”