People who had mild cases of COVID-19 have develop antibody-producing cells that can last a lifetime, according to a new study from the Washington University School of Medicine. The findings suggest that, among people who experienced mild COVID-19 symptoms, repeated infections are likely to be uncommon.
“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 Dr. Ali Ellebedy.
“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.”
A viral infection causes antibody-producing immune cells to rapidly multiply and circulate in the blood. Most of these cells die once the infection subsides, but some of them migrate to the bone marrow. These antibody-producing cells are known as long-lived plasma cells, or LLPCs.
Once LLPCs have settled into the bone marrow, they continuously secrete low levels of antibodies into the bloodstream to help guard against another infection from the virus.
Dr. Ellebedy realized that the key to figuring out whether COVID-19 leads to long-lasting antibody protection lies in the bone marrow.
In addition to tracking antibody levels in blood samples from COVID-19 survivors, the team obtained bone marrow from 18 individuals seven or eight months after they had a mild infection. Four months later, some of the participants provided a second bone marrow sample. The scientists also analyzed bone marrow from 11 people who had never had COVID-19 for comparison.
The study revealed that, as expected, antibody levels in the blood of the COVID-19 participants dropped quickly in the first few months after infection before leveling off. Some antibodies were detectable even 11 months after infection.
Furthermore, most of the bone marrow samples from people who had had COVID-19 contained antibody-producing cells specifically targeting the virus that causes the disease. These LLPCs persisted, and were detectable months later in the second round of bone marrow samples.
By contrast, the participants who had never had COVID-19 possessed no such antibody-producing cells in their bone marrow.
“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,” said Dr. Ellebedy.
“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.”
The researchers speculate that people who were infected and never had symptoms may also be left with long-lasting immunity. However, they noted that it is yet to be determined whether those who endured more severe infection would be protected against a future bout of disease.
“It could go either way,” said study first author Dr. Jackson Turner. “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.”
The researchers are currently investigating whether vaccination may also induce long-lasting antibodies.
The study is published in the journal Nature.