This week in mBio, An open access journal from the American Society for Microbiology, a team of researchers from the Center de Recherche du CHUM in Montreal and the University of Montreal report that SARS-CoV-2 antibody levels in the blood of COVID-19 patients drop rapidly after the Symptoms have disappeared.
The study included 31 people who were recovering from COVID-19. Lead author Andrés Finzi and his colleagues collected blood samples six and ten weeks after their symptoms appeared. They assessed the level of antibodies (immunoglobulins G, A and M) and tested the ability of the antibodies to neutralize the virus. In a previous study, the researchers had reported that all antibody levels of IgG, IgA, and IgM decreased three to six weeks after symptoms appeared. In this new study, they reported that between six and ten weeks after symptoms began, all three of them went down, and IgA and IgM levels dropped more sharply. Simultaneously with the decline, the ability of the antibodies to neutralize the virus decreased.
These results have the potential to cause dismay in COVID-19 patients in many people who had high hopes for the prospect of long-term immunity. Nevertheless, according to Finzi, the lack of specific antibodies in the blood of recovered patients does not necessarily mean that these patients have lost their immune response to SARS-CoV-2 altogether.
The immune system has a memory
When antibodies disappear from the blood, the T cells and B cells that make up the antibodies remain. T cells “have a memory”. Several teams around the world see particular hope in a certain type of B-cell called “storage B-cell”.
By “remembering” viruses that defeated them, memory B cells play an important role in long-term immunity to many diseases. Once triggered by a disease, memory B cells divide rapidly and produce specific and potent antibodies. If they ever “see” the virus in the body they live in again, exactly the same thing will happen. Memory B cells can produce fresh antibodies years after the original batch has disappeared. They can remain in a person’s blood for years or even a lifetime, waiting to respond effectively and quickly to reinfections.
If memory B cells play a strong role in immunity to SARS-CoV-2, theoretically, COVID-19 patients would maintain the virus’s immune memory in their bloodstream and respond defensively and effectively to re-infection.
The potential of storage B cells to fight SARS-CoV-2 is being very actively investigated in several centers. In a still peer-reviewed article by researchers at Tel Aviv University, data is presented to support the idea that SARS-CoV-2 specific storage B cells remain in the blood of restored patients even after antibodies disappear.
In the absence of approved treatments for COVID-19, some hospitals have improvised convalescent plasma treatments.
Plasma is the liquid part of the blood; it contains antibodies. Convalescent plasma is spun from blood drawn from newly collected individuals. It is infused into sick people to boost an immune response.
Convalescent plasma treatments have been helpful against Ebola, SARS, MERS, and the H1N1 influenza virus.
In this pandemic, hospitals that make their own convalescent plasma will wait to draw blood from people who have recovered for at least two weeks after symptoms go away and tests show no active virus. Convalescent plasma for use against COVID-19 has not yet shown any benefit in controlled studies. Its usefulness may be further complicated by the Montreal Study’s discovery that antibodies decrease sharply after patients recover and the virus has eliminated the body. This is because the window for drawing blood from recovered patients is almost impossibly narrow. The antibodies peak two or three weeks after symptoms appear. For COVID-19, the mean recovery time is two weeks after symptoms appear. This means that medical personnel cannot draw blood and prepare plasma from a patient until four or five weeks after symptoms appear – and at the earliest. Six weeks is the mark up to which the Montreal study shows a decrease in antibody levels.
Contacted by email, Finzi simply replied with a concern set out in the mBio Paper: “Plasma from convalescent donors should be collected quickly after recovery from active infection.”
The message seems to be very quick and as soon as it is safe to do so.
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