Washington, May 26 (IANS): US researchers have found that people who have had mild illness due to Covid-19, go on to develop antibody-producing immune cells that can last for a lifetime and give them protection against the virus.

Researchers at Washington University in St. Louis, US, found that 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.

However, a small population of antibody-producing cells, called long-lived plasma cells, migrate to the bone marrow and settle in, where they continually secrete low levels of antibodies into the bloodstream to help guard against another encounter with the virus.

"We found antibody-producing cells in people 11 months after the first symptoms. These cells will live and produce antibodies for the rest of people's lives. That's strong evidence for long-lasting immunity," said Ali Ellebedy, Associate Professor at the varsity's School of Medicine. The findings of the small study are published in the journal Nature.

For the study, the team involved 77 participants whose antibody levels in blood samples were assessed at three-month intervals starting about a month after initial infection. The team obtained bone marrow from 18 of the participants seven or eight months after their initial infections.

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

Of the bone marrow samples, 15 contained antibody-producing cells specifically targeting the virus that causes Covid-19. Such cells could still be found four months later in the five people who came back to provide a second bone-marrow sample.

None of the 11 people who had never had Covid-19 had such antibody-producing cells in their bone marrow.

"Mild infection by Covid-19 will also trigger the immune response as it will lead to immune stimulation both cell-mediated and antibody linked creating lasting immunity. In fact, this is the principle of vaccination that triggers the immune response by stimulating the antigen-induced reaction giving rise to immunity of the body.

"It has also been shown that the reinfection rate of people already having a mild infection is only two percent which is very less," Praveen Gupta, director and head, neurology, Fortis Memorial Research Institute, Gurugram told IANS.

"Based on this fact the principle of herd immunity was developed, if a large number of people will get a mild clinical infection that will lead to the development of herd immunity," he added.