Researchers have now claimed that antibody levels in the blood of COVID-19 patients drop rapidly during the weeks after their bodies have cleared the virus and symptoms have subsided.

If convalescent plasma is ultimately shown to have a clear benefit, then it needs to be collected during a specific window of time after recovery, the study, published in the journal 'mBio', reported. "We don't want to transfuse the virus, just transfuse the antibodies," said study author Andres Finzi from the University of Montreal in Canada.

"But at the same time, our work shows that the capacity of the plasma to neutralize viral particles is going down during those first weeks," Finzi added. The spike protein of SARS-CoV-2 plays a crucial role in helping the virus grab and invade host cells.

Antibodies produced by the body's immune system bind to a part of this protein and block the capacity of this "key" to engage with the host's cellular "lock", said Finzi, preventing the viral particle from infecting a cell host. Previous studies suggest that antibodies against the SARS-CoV-2 spike protein peak two or three weeks after the onset of symptoms.

Findings from an earlier cross-sectional study by Finzi's group, involving more than 100 patients, suggested that the ability of plasma to neutralize the virus decreased significantly between three and six weeks after symptom onset. In the new longitudinal study, the research team analysed blood samples collected at one-month intervals from 31 individuals recovering from COVID-19. They measured levels of immunoglobulins that act against the coronavirus S protein and tested the ability of the antibodies to neutralize the virus.

The researchers observed variation on the level of individual patients but identified a consistent overall signal: The levels of Immunoglobulins G, A, and M that target the binding site decreased between six and 10 weeks after symptoms began. During the same time period, the ability of the antibodies to neutralize the virus similarly fell, the study noted.