This suggests that mild cases of COVID-19 may leave patients with lasting antibody protection, reducing the likelihood of reinfection.
"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," explained Ali Ellebedy, an immunologist at WUSTL and the senior author of the study.
"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. 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."
Long-time antibodies against COVID-19
The body makes antibodies during an infection to defend itself. As the body recovers, most of the immune cells that produce antibodies die off. This causes a decrease in antibody levels. However, immune cells called long-lived plasma cells move to the bone marrow, where they remain and produce antibodies, albeit at low levels, against future infections.
For their study, Ellebedy and his team looked at whether people who had recovered from mild cases of COVID-19 still had long-lived plasma cells in their system, in particular, those that specifically target the COVID-19 virus. A total of 77 participants were enrolled in the study. The team took bone marrow samples from the participants eight months after their infection; five returned four months after to provide a second bone marrow sample.
The researchers found that the levels of coronavirus-specific antibodies sharply decreased in the first few months after infection. These leveled off afterward, with some antibodies remaining detectable 11 months after infection.
Fifteen of the bone marrow samples from the COVID-19 participants contained antibody-producing cells that specifically target the coronavirus. This was seen in bone marrow samples taken 11 months after infection.
"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 explained.
"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."
According to the researchers, this long-lasting immunity may also be present in people who did not exhibit symptoms despite being infected with the COVID-19 virus. (Related: Immunity developed from previous infection with endemic coronaviruses reduces COVID-19 severity – study.)
However, this may not be the case for those who had severe COVID-19, explained co-author and immunologist Jackson Turner. For one, these people may have had an impaired immune response, overwhelmed with severe inflammation.
"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," said Turner.
Learn more about the Wuhan coronavirus (COVID-19) at Pandemic.news.
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