New Delhi: Bone marrow from patients who recovered from Covid-19 revealed that the immune system’s ability to recognise and fend off the SARS-CoV-2 virus lasts at least a year.
In a study, published in the journal Nature Monday, researchers described how bone marrow plasma cells (BMPCs) — an essential source of protective antibodies that bind to the spike protein of the coronavirus — persist up to 11 months after Covid infections.
Conducted by a team from Washington University in the US, the study is the first direct evidence on the production of pathogen-specific bone marrow plasma cells after a viral infection in humans.
The researchers found that antibodies against the SARS-CoV-2 virus decline rapidly after the first four months of infection and then at a slower pace in the next seven months.
They explained that this is because in the first few months after infection, the antibodies are secreted by short-lived cells known as plasmablasts.
However, gradually there is a transition to antibodies secreted by a smaller but more persistent population of long-lived plasma cells, generated in the later stages of the immune response.
The majority of these plasma cells reside in bone marrow, once the threat of the pathogen subsides.
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Long-lasting protection
Several studies have shown that antibodies against SARS-CoV-2 rapidly decline after the first three months of infection. These findings gave rise to fears that people become vulnerable to reinfection within a few months of recovery.
However, the body’s immune system has many other components apart from the antibodies. These include T cells and B cells, which are types of white blood cells that are also known as natural killer cells for their role in the immune response. T cells are involved in a process that initiates the activation of B cells and the latter turns into plasmablasts or plasma cells.
These cells are responsible for retaining the memory encountering a pathogen, and launch a tailored attack when the body encounters the pathogen again.
The Washington University researchers studied bone marrow samples from recovered patients, up to 11 months after the Covid infection, and found plasma cells that can specifically recognise the spike protein of the virus.
In other words, even if antibody levels in the blood decline after the first few months, these plasma cells hide in the bone marrow and launch an attack if the body encounters Covid-19 for a second time.
The bone marrow was collected from 18 volunteers who had suffered from mild Covid after seven to eight months of their infection, then again from five of these volunteers, and a volunteer gave marrow 11 months after infection.
Eleven healthy volunteers who had no history of SARS-CoV-2 infection, and had not gotten a Covid vaccine also donated bone marrow for this study.
The data in the study indicated that mild SARS-CoV-2 infection elicits a long-lived bone marrow plasma cell response — something that is absent in people with no history of exposure to the virus.
The researchers said that the study also creates a new benchmark to evaluate the performance of vaccines in inducing long-lasting immune response.
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