Months after recovering from mild cases of COVID-19, people still have immune cells which produce antibodies against the novel coronavirus, according to a study.

The researchers from Washington University School of Medicine in St. Louis, US, noted that such cells could persist for a lifetime, churning out antibodies all the while.

The study, published on Monday in the journal Nature, suggests that mild cases of COVID-19 leave those infected with lasting antibody protection and that repeated bouts of illness are likely to be uncommon.

"Last fall, there were reports that antibodies wane quickly after infection with the virus that causes COVID-19, and mainstream media interpreted that to mean that immunity was not long-lived," said senior author of the study Ali Ellebedy, an associate professor at Washington University School of Medicine.

"But that's a misinterpretation of the data. It's normal for antibody levels to go down after acute infection, but they don't go down to zero; they plateau," Ellebedy said.

The researchers found antibody-producing cells in people 11 months after first symptoms.

These cells will live and produce antibodies for the rest of people's lives, and that's strong evidence for long-lasting immunity, they said.

During a viral infection, antibody-producing immune cells rapidly multiply and circulate in the blood, driving antibody levels sky-high.

Once the infection is resolved, most such cells die off, and blood antibody levels drop.

A small population of antibody-producing cells, called long-lived plasma cells, migrate to the bone marrow and settle in, the researchers said.

In the bone marrow, these cells continually secrete low levels of antibodies into the bloodstream to help guard against another encounter with the virus, they said.

Ellebedy and colleagues were already working on a project to track antibody levels in blood samples from COVID-19 survivors.

The team had enrolled 77 participants who were giving blood samples at three-month intervals starting about a month after initial infection.

Most participants had had mild cases of COVID-19, only six had been hospitalised.

The researchers obtained bone marrow from 18 of the participants seven or eight months after their initial infections.

Five of them came back four months later and provided a second bone marrow sample.

For comparison, the scientists also obtained bone marrow from 11 people who had never had COVID-19.

They found that antibody levels in the blood of the COVID-19 participants dropped quickly in the first few months after infection and then mostly levelled off, with some antibodies detectable even 11 months after infection.

Fifteen of the 19 bone marrow samples from people who had had COVID-19 contained antibody-producing cells specifically targeting the virus that causes COVID-19, the researchers said.

Such cells could still be found four months later in the five people who came back to provide a second bone-marrow sample, they said.

According to the researchers, none of the 11 people who had never had COVID-19 had such antibody-producing cells in their bone marrow.

"People with mild cases of COVID-19 clear the virus from their bodies two to three weeks after infection, so there would be no virus driving an active immune response seven or 11 months after infection," Ellebedy said.

"These cells are not dividing. They are quiescent, just sitting in the bone marrow and secreting antibodies. They have been doing that ever since the infection resolved, and they will continue doing that indefinitely," he added.