Months after recovering from mild COVID-19 infection, people still have immune cells in their body generating antibodies against the virus that causes COVID-19.
The findings, which appeared in the journal Nature, on 24th May 2021, suggest that mild COVID-19 cases leave those infected people with lasting antibody protection ability and that reinfections of the virus are likely to be uncommon (Turner et al., 2021).
During a viral infection, immune cells rapidly multiply and circulate in the blood, generating antibody levels sky-high. However, once the condition is resolved, most such cells disappear, and blood antibody levels fall.
A small group of antibody-producing cells, also known as long-lived plasma cells, move to the bone marrow, where they continually secrete low levels of antibodies into the bloodstream to help guard against another encounter with the virus (Manz et al., 1997).
However, several recent reports indicated that anti-SARS-CoV-2 serum antibody levels rapidly falls within the first few months after infection, raising concerns that long-lived plasma cells may not be generated and immunity against the virus that causes COVID-19 may be short-lived (Edridge et al., 2020, Seow et al., 2020). Also, there were reports suggesting a COVID-19 vaccine may not provide long-lasting protection because of the rapid decline of the antibody.
Researchers at Washington University School of Medicine investigated if mildly infected COVID-19 patients had anti-SARS-CoV-2 antibodies in their plasma and if the long-lived plasma cells exist in the bone marrow.
For the investigation, the team recruited 77 participants who gave blood samples at three-month intervals starting about a month after the initial infection. Most participants were of mild COVID-19 cases; only six had been hospitalized.
The team found that in patients who experienced mild infections, serum anti-SARS-CoV-2 antibodies decline rapidly in the first four months after illness and then more gradually over the following seven months. However, even after 11 months, the level of the antibodies was at a detectable level.
Researchers then investigated whether the mild-infected patients had any long-lived plasma cells. For this, they collected bone marrow samples from 18 patients. Of the 18 given samples, 15 contained antibody-producing cells specifically targeting the virus.
None of the 11 volunteers, who never had COVID-19, had such antibody-producing cells in their bone marrow samples.
However, such cells were present in the second samples four months later in the five people who returned to provide bone-marrow samples again.
“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”, said senior author Ali Ellebedy, PhD, an associate professor of pathology & immunology, of medicine and molecular microbiology.