“Sterilizing immunity doesn’t happen very often — that is not the norm,” said Alessandro Sette, an immunologist at the La Jolla Institute of Immunology and co-leader of the study.
More often, people become infected a second time with a particular pathogen, and the immune system recognizes the invader and quickly extinguishes the infection. The coronavirus in particular is slow to do harm, giving the immune system plenty of time to kick into gear.
“It may be terminated fast enough that not only are you not experiencing any symptoms but you are not infectious,” Dr. Sette said.
Dr. Sette and his colleagues recruited 185 men and women, aged 19 to 81, who had recovered from Covid-19. The majority had mild symptoms not requiring hospitalization; most provided just one blood sample, but 38 provided multiple samples over many months.
The team tracked four components of the immune system: antibodies, B cells that make more antibodies as needed; and two types of T cells that kill other infected cells. The idea was to build a picture of the immune response over time by looking at its constituents.
“If you just look at only one, you can really be missing the full picture,” Dr. Crotty said.
He and his colleagues found that antibodies were durable, with modest declines at six to eight months after infection, although there was a 200-fold difference in the levels among the participants. T cells showed only a slight, slow decay in the body, while B cells grew in number — an unexpected finding the researchers can’t quite explain.
The study is the first to chart the immune response to a virus in such granular detail, experts said. “For sure, we have no priors here,” Dr. Gommerman said. “We’re learning, I think for the first time, about some of the dynamics of these populations through time.”