Dr. Ellebedy and his colleagues recruited 41 people — including eight with a history of infection with the virus — who were immunized with two doses of the Pfizer-BioNTech vaccine. From 14 of these people, the team extracted samples from the lymph nodes at three, four, five, seven and 15 weeks after the first dose. 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.