A recent study published in the journal Nature suggests that people who recovered from a mild case of COVID-19 may have developed lifelong immunity to the Wuhan coronavirus.
Researchers from the Washington University School of Medicine arrived at this finding after tracking people who have had mild COVID-19. They found that these individuals still had immune cells that produce antibodies specific to the virus nearly a year after infection. This led the researchers to conclude that these immune cells would never go away and would keep recovered patients protected for life.
"These cells will live and produce antibodies for the rest of people's lives," said Ali Ellebedy, an associate professor of pathology and immunology and a co-author of the study. "That's strong evidence for long-lasting immunity."
Mainstream media reported last year that immunity might be short-lived because COVID-19 antibodies wane quickly after infection. But Ellebedy said that that was 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," she explained. (Related: Infected patients develop long-term immunity to coronavirus.)
Immune cells that produce COVID-19 antibodies are in it for long the haul
For their study, Ellebedy and her colleagues recruited 77 people who recovered from a mild COVID-19 infection and collected samples of their blood three months after they tested positive. They also took bone marrow samples from eighteen participants seven to eight months after infection, five of which returned four months later to provide a second sample. For comparison, the researchers also collected bone marrow samples from 11 people who never contracted the virus.
The researchers found that COVID-19 antibodies decreased quickly a few months after recovery but did not disappear entirely. Instead, antibody levels stabilized and remained detectable within 11 months of infection.
Moreover, 15 of the participants who provided bone marrow samples still had bone marrow plasma cells that produce COVID-19 antibodies. The five volunteers that came back also had these immune cells four months later. Meanwhile, the researchers did not detect the cells in the bone marrow of all the volunteers who never contracted the virus.
In view of these findings, the researchers said that there was no reason to think that these immune cells would ever go away in those who recovered from mild infections even though their COVID-19 antibodies decreased. Ellebedy explained that the immune cells migrate to the bone marrow and start producing fewer antibodies after recovery, but that that doesn't mean they are gone for good.
"These cells are not dividing. They are quiescent, just sitting in the bone marrow and secreting antibodies," she said. "They have been doing that ever since the infection resolved, and they will continue doing that indefinitely."
But it remains unclear whether people who recovered from severe COVID-19 also developed long-lasting immunity. Jackson Turner, an instructor of pathology and immunology and the lead author of the study, said that things go could go either way.
"Inflammation plays a major role in severe COVID-19, and too much inflammation can lead to defective immune responses," he explained. "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."
That being said, Turner noted that the study needs to be replicated in people with moderate to severe infections to determine whether they would develop lifelong immunity to the virus. But overall, the researchers concluded that those with mild infections might be protected from the coronavirus for life.
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