Mass testing for antibodies serves as an auxiliary method for diagnosing the current coronavirus infection, in particular, allowing epidemiologists to understand what proportion of the population is currently ill. However, such tests can only show whether a person has met with a coronavirus infection or, for example, has suffered from SARS. In addition, today, experts involved in the fight against the pandemic understand that even confirmed cases of COVID-19 and the presence of IgG — class antibodies in the results of blood tests do not guarantee complete protection against re-infection and immunity to SARS-Cov-2 may be completely unstable. That is why tests are becoming relevant now, allowing doctors to find out whether a particular patient has a protective immunity that protects him from re-infection after vaccination or after a disease.

The value of anti-RBD antibodies

SARS-Cov-2 enters the human cell due to the S-protein (spike), the receptor-binding domain (RBD) of which docks to the target on the host cell providing infection. The body reacts to this by producing antibodies, and as the immune response develops, IgG antibodies are formed to various proteins of the virus — to the nucleocapsid (N, to the shell consisting of nucleocapsid c-proteins) and to the spike (S-protein). Correctly selected laboratory tests of IgG-for a specific virus protein-allow doctors to judge the occurrence or absence of protective immunity that could have formed after the disease or vaccination.

The presence of IgG to the nucleocapsid protein shows a history of infection in the patient, but the detection of IgG to the receptor-binding domain (RDB) shows protective immunity. In numerous studies, as noted in the current tenth version of the temporary methodological recommendations of the Ministry of Health of the Russian Federation on the prevention, diagnosis and treatment of coronavirus infection, it has been shown: over 80% of virus-neutralizing antibodies are directed to the receptor-binding domain (RBD) of the SARS-CoV-2 S-protein. 

The human immune system, detecting the RBD of the virus, produces antibodies that have neutralizing properties – they do not allow the virus to bind to cells. Therefore, to assess the effectiveness of vaccination (post-vaccination testing) and the immune status of a person after COVID-19, only test systems that detect antibodies to S-protein RBD (anti-RBD antibodies) are needed.

The original article in Russian is posted here https://vademec.ru/news/2021/04/16/kakie-ekspress-testy-sposobny-otsenit-immunitet-k-covid-19/