During the study, scientists tested blood samples from 47 patients for the presence of different types of antibodies. It turned out that half of the participants who underwent SARS-CoV-2 without symptoms did not develop neutralizing IgG antibodies to the nucleocapsid (N) protein, one of the internal components of the virus. Usually, to confirm the fact of the transferred infection, patients are recommended to test for the determination of this specific type of antibodies.
But at the same time, almost all participants, regardless of how they transferred COVID-19, were found to have IgG antibodies to the receptor binding domain of the spike protein (RBD), which is located on the surface of the viral particle. This type of antibodies is also neutralizing, since they are able to prevent the binding of the S-protein of the virus to the receptor.
The researchers suggested that some asymptomatic carriers do not develop antibodies to the nucleocapsid protein due to the fact that their immune system successfully coped with the virus even before the stage of active reproduction in the cells of the body. In this scenario, after a while antibodies to RBD begin to be produced, since their “target” is on the outer surface of the virus.
Scientists made another interesting discovery: in all study participants, the antibody titer to the receptor-binding domain of the spike protein not only did not decrease, but even increased over time. These findings contradict the findings of earlier scientific work, but it is likely that the level of antibodies to RBD may be influenced by various factors, including lifestyle and behavior.
In those vaccinated, an increased level of antibodies to RBD may indicate that after vaccination there was a second encounter with the virus, which means there is no reason for revaccination, since the immune system has already reacted to a new contact with the virus, the researchers noted.
According to scientists, in order to confirm the fact of the transferred disease, it is most advisable to do an analysis to determine the name of this type of antibodies, and not to the nucleocapsid protein.