Coronavirus test קורונה
Coronavirus test קורונהiStock

Patients who recovered from COVID-19 infections likely develop long-term immunity against the virus that could last for their entire lives, a new study shows – even if the infection was only mild.

According to the study entitled “SARS-CoV-2 infection induces long-lived bone marrow plasma cells in humans”, which was published in the British journal Nature last week, immunity to the SARS-COVID-2 virus remains robust in individuals who experienced natural infection, even when there was a dramatic decline in measured serum antibody levels.

The researchers, led by Washington University immunologist Ali Ellebedy, tracked antibody production in 77 recovered COVID-19 patients, including many who had experienced only mild cases of the disease.

Natural infection prompted a strong antibody response initially, the researchers found, though antibody levels declined rapidly starting roughly four-months after infection was reported.

This finding mirrored the results of other studies, which demonstrated a dramatic drop in serum antibody levels three to five months after natural infection.

However, the new study found that after the rapid decline in serum antibody levels which began at around four months from the time of infection, detectable antibody levels began to stabilize by the 11th month, with antibodies to the COVID-2 spike protein still at measurable levels eleven months after infection.

To understand the continued presence of antibodies in the blood stream, researchers tested other components of the subjects’ immune system, taking bone marrow samples and memory B cells.

Eighty-three percent of the 18 subjects who gave bone marrow samples seven to eight months after infection had detectable levels of bone marrow plasma cells which produced antibodies for SARS-COVID-2.

A follow up five of the subjects who gave bone marrow samples found that all five had stable levels of bone marrow plasma cells for COVID-2 months after the first sample was taken.

“This is a very important observation,” said Rafi Ahmed, an immunologist at Emory University who was part of the team which discovered BMPCs.

In contrast to natural infection, the long-term effects of Pfizer-BioNTech’s coronavirus vaccine on immunity remain unclear.

“My presumption is, we will need a booster,” said Ellebedy.

A study from Emory Vaccine Center in 2020 suggested while some vaccines against childhood illnesses promote long-lasting immunity, the immunity provided by influenza vaccines lasts only a matter of months.

“We could see that these new antibodies expanded in the bone marrow one month after vaccination and then contracted after one year. On the other hand, antibodies against influenza that were in the bone marrow before the vaccine was given stayed at a constant level over one year,” said Dr. Carl Davis, the lead author of the 2020 study.

“What this shows is that just getting to the bone marrow is not enough,” Ahmed says. “A plasma cell has to find a niche within the bone marrow and establish itself there, and undergo gene expression and metabolism changes that promote longevity.”

Previous studies on immunity to the COVID-SARS-2 virus developed by natural infection found extremely low rates of reinfection among recovered individuals even after the drop in antibody levels starting four months after infection.

A study in Qatar in 2020 found that of the 43,044 subjects who were tracked for up to 35 weeks, just 0.02% were reinfected. The study also found that those subjects who were reinfected usually experienced less severe cases of the disease.