This week in mBio, an open-access journal of the American Society for Microbiology, a team of researchers from the Centre de Recherche du CHUM in Montreal and from the University of Montreal report that SARS-CoV-2 antibody levels in the blood of COVID-19 patients plummet quickly after symptoms disappear.
The study involved 31 people recovering from COVID-19. Lead author Andrés Finzi and his colleagues collected blood samples at six and ten weeks after the onset of their symptoms. They assessed the level of antibodies (Immunoglobulins G, A, and M) and they tested the antibodies’ ability to neutralize the virus. In a previous study the researchers had reported that all antibody levels of IgG, IgA, and IgM all declined three to six weeks after symptom onset. In this new study they reported that all three declined between six and ten weeks after symptoms began, with IgA and IgM levels dropping more precipitously. Coincidentally with the decline, the ability of the antibodies to neutralize the virus fell.
These findings have the potential to create dismay in the minds of many people who had high hopes for the long-term immunity prospects recovered COVID-19 patients. Even so, according to Finzi, the absence of specific antibodies in the blood of recovered patients does not necessarily mean that those patients have lost their immune response to SARS-CoV-2 altogether.
The Immune System Has a Memory
As antibodies disappear from the blood, the T cells and B cells that make the antibodies remain. T cells “have a memory.” However, it is in a specific type of B cells called “memory B cells” that several teams around the world see particular hope.
By “remembering” viruses they’ve helped defeat, memory B cells play an important role in long-term immunity for many diseases. Once triggered by an illness, memory B cells divide briskly and produce specific and effective antibodies. If ever again they “see” the virus in the body they inhabit, the very same thing happens. Memory B cells can produce fresh antibodies years after the initial batch disappears. They can persist in a person’s blood for years or even a lifetime, waiting to potently and rapidly respond to reinfections.
In theory, anyway, if memory B cells play a strong role in immunity to SARS-CoV-2, COVID-19 patients would maintain in their bloodstreams the immune memory of the virus, and would react defensively and effectively to re-infections.
The potential of memory B cells in countering SARS-CoV-2 is under very active investigation in several centers. Meanwhile, an as-yet peer-reviewed paper by researchers at Tel Aviv University presents data supporting the idea that SARS-CoV-2-specific memory B cells persist in the blood of recovered patients even while antibodies disappear.
In the absence of approved treatments for COVID-19, some hospitals have improvised convalescent plasma treatments.
Plasma is the liquid portion of blood; it contains antibodies. Convalescent plasma is spun from blood harvested from newly recovered people. It is infused into ill people in order to boost an immune response.
Convalescent plasma treatments have been helpful against Ebola, SARS, MERS, and the H1N1 influenza virus.
In this pandemic, hospitals making their own convalescent plasma wait to draw the blood of people who have recovered until at least two weeks after symptoms have disappeared and tests show no active virus. Convalescent plasma for use against COVID-19 has yet to show a benefit in any controlled trials. Its utility may be further complicated by the Montreal study revelation that antibodies precipitously decline after patients recover and the virus has cleared their bodies. This is because the window for drawing blood from recovered patients is almost impossibly narrow. Antibodies peak two or three weeks after symptom onset. For COVID-19 the median recovery time is two weeks after onset of symptoms. This means that medical personnel can’t draw a patient’s blood and prepare plasma until four or five weeks after symptom onset — and that is at the earliest. Six weeks is the mark by which the Montreal study flags antibody levels as starting to decline.
Contacted by email, Finzi responded simply with a concern made clear in the mBio paper: “Plasma from convalescent donors should be collected rapidly after recovery from the active infection.”
Very rapidly, the message seems to be, and as soon as it’s safe to do so.