Mild COVID-19 induces lasting antibody protection, study finds

Months after recovering from mild cases of the novel coronavirus (COVID-19), people still have immune cells in their body pumping out antibodies against SARS-CoV-2, the virus that causes COVID-19, according to a new study from researchers at Washington University School of Medicine in St. Louis published in the journal Nature.

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, where they continually secrete low levels of antibodies into the bloodstream to help guard against another encounter with the virus.

The key to figuring out whether COVID-19 leads to long-lasting antibody protection lies in the bone marrow, the researchers found. To find out whether those who have recovered from mild cases of COVID-19 harbor long-lived plasma cells that produce antibodies specifically targeted to SARS-CoV-2, researchers tracked antibody levels in blood samples from COVID-19 survivors.

The team already 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 hospitalized. The researchers then 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.

Antibody levels in the blood of the COVID-19 participants dropped quickly in the first few months after infection and then mostly leveled off, with some antibodies detectable even 11 months after infection. Further, 15 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. 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.

The findings suggest that mild cases of COVID-19 leave those infected with lasting antibody protection and that repeated bouts of illness are likely to be uncommon, the researchers said. People who were infected and never had symptoms also may be left with long-lasting immunity. It has yet to be investigated whether those who endured more severe infection would be protected against a future bout of disease, the researchers said.

"It could go either way," said Jackson Turner, PhD, first author of the study and an instructor in pathology and immunology, in a statement. "Inflammation plays a major role in severe COVID-19, and too much inflammation can lead to defective immune responses. But on the other hand, the reason why people get really sick is often because they have a lot of virus in their bodies and having a lot of virus around can lead to a good immune response. So, it's not clear. We need to replicate the study in people with moderate to severe infections to understand whether they are likely to be protected from reinfection."

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