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Mild cases of COVID-19 leave people with long-term antibody protection against reinfection, according to a new study that challenges previous findings.
"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 study senior author Ali Ellebedy, an associate professor of pathology and immunology, medicine and molecular microbiology at Washington University School of Medicine in St. Louis.
"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 in a university news release. "Here, we 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. That's strong evidence for long-lasting immunity."
The study included 77 people who were giving blood samples at three-month intervals starting about a month after initial infection with COVID-19.
Most had had mild cases of COVID-19, and only six had been hospitalized.
Bone marrow samples were collected from 18 of the participants seven or eight months after their initial coronavirus infection. Five provided a second bone marrow sample four months later.
The researchers also analyzed bone marrow samples from 11 people who never had COVID-19.
As expected, antibody levels in the blood of the participants fell quickly in the first few months after infection and then mostly leveled off, with some antibodies still detectable 11 months after infection.
Fifteen bone marrow samples from participants who'd had COVID-19 contained antibody-producing cells that target the coronavirus seven to eight months after infection, and those cells were still present four months later in the five people who provided a second bone marrow sample.
None of the 11 people who never had COVID-19 had these antibody-producing cells in their bone marrow, according to the study published Monday in the journal Nature.
"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."
It's possible that people who were infected with the new coronavirus and never had symptoms also may have long-lasting immunity, according to the researchers.
It's not known whether people with severe COVID-19 would be protected against reinfection.
"It could go either way," said study first author Jackson Turner, an instructor in pathology and immunology at Washington University in St. Louis.
"Inflammation plays a major role in severe COVID-19, and too much inflammation can lead to defective immune responses," Turner explained in the release. "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."
The U.S. Centers for Disease Control and Prevention has more on COVID-19.
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