The natural infection and vaccination against COVID-19 result in the production of antibodies against viral antigens, playing a vital part in the immune response monitoring [1]. Although it is expected that the naturally acquired immunity against SARS-CoV-2 would last long [2-3], serological equivalent of antiviral host defense has not yet been discovered. The lack of a standardized approach to laboratory testing is one of the obstacles to defining such correlates, which probably explains the conflicting literature data on the serological assessment of SARS-CoV-2 infection. The WHO have recently introduced the International Standard for anti-SARS-CoV-2 immunoglobulin (IS-SARS-CoV-2), which makes it possible to unify the results of measuring the levels of anti-SARS-CoV-2 antibodies using the IS-SARS-CoV-2 units, namely the binding antibody units (BAU) [1]. The study was aimed to measure the concentration of IgG against the SARS-CoV-2 receptor-binding domain (RBD) at different times over the 12-month period in a cohort of healthcare professionals, who have recovered from the SARS-CoV-2 infection, using the enzyme-linked immunosorbent assay (ELISA) kits, calibrated using the IS-SARS-CoV-2.

METHODS

The study, carried out from May 2020 to June 2021, involved the staff members of the Russian Children's Clinical Hospital, Pirogov Russian National Research Medical University. Inclusion criteria: positive PCR test result for COVID-19. There were no exclusion criteria.

In April and May 2020, after returning to work, individuals with positive PCR test results for COVID-19 gave the blood serum samples with an interval of a month for the anti-RBD-IgG measurement. The samples were collected on a monthly basis for 6 months, and the last sample was obtained 12 months after the positive PCR test result; a total of 4–7 samples was obtained from each subject. The samples collected were stored at a temperature of –80 °C.

In July 2021, all the samples were assayed in one batch using the ELISA kit for the anti-RBD-IgG quantification (XEMA; Russia) [4], calibrated using the IS-SARS-CoV-2. The analytical measuring range was 15–240 BAU/mL; samples with the antiRBD-IgG concentration exceeding 240 BAU/mL were further diluted 10–100 times and measured repeatedly in a separate batch. The samples were considered positive when the level of anti-RBD-IgG was 15 BAU/mL.

Statistical processing was performed using the IBM SPSS Statistics 27.0 software package (IBM Corp.; USA).

RESULTS

A total of 97 serum samples were obtained from 20 individuals during the study, including 14 women (70%) with the SARS-CoV-2 infection, confirmed by PCR. The median age was 50 years (Q₁–Q₃, 40–57 years). All the subjects had mild to moderate COVID-19, there were no severe cases of the disease.

In three individuals, blood serum samples were obtained a month before the SARS-CoV-2 infection; no anti-RBD-IgG were found in these samples. A month after the positive PCR test result for SARS-CoV-2, anti-RBD-IgG were found in all subjects in a concentration exceeding the threshold value of 15 BAU/mL, with the geometric mean titer (GMT) of 433 BAU/mL (95% CI 123–1,527 BAU/mL; range 36–25,900 BAU/mL) (see figure). At the time point 2 of the month the median GMT was similar, 456 BAU/mL (95% CI 154–1,353 BAU/mL). Later the concentration of anti-RBD-IgG gradually decreased and reached the median GMT value of 68 BAU/mL (95% CI 35–131 BAU/mL) by the 12^th month. All samples were anti-RBD-IgG-positive during the period between the first and the sixth months. Among 14 individuals assessed 12 months after the SARS-CoV-2 infection, 13 individuals (93%) were still seropositive for anti-RBD-IgG with the GMT values exceeding 15 BAU/mL. The median concentration of anti-RBD-IgG was 6.7 times lower  (95% CI 4.4–10.3 times) after 12 months compared with the highest median GMT value, defined during the second month after the SARS-CoV-2 infection.

DISCUSSION

To date, the data of only a few studies, involving the standardized values of the amount of antibodies against SARS-CoV-2 after the natural infection and/or vaccination, have been reported [5–7]. Thus, the live viral neutralization assay showed that anti-RBD-IgG in a concentration of ≥100 BAU/mL ensured the complete neutralization of three SARS-CoV-2 variants of concern a year after infection, which reduced the risk of reinfection with these virus strains [5]. Another report mentioned that on day 14 after vaccination, the anti-RBD-IgG GMT in individuals, vaccinated with mRNA vaccine, was 7,756 BAU/mL [6]. Thus, quantitative results provide the basis for seeking the common serological correlate of the protective immunity to SARS-CoV-2. Moreover, these data are important for monitoring the natural immunity and facilitate the comparison of immune responses to various vaccines [1]. Significant differences and systematic error in the numerical results, expressed in BAU/mL, associated with the use of different test systems [7], encourage further efforts to unify serological tests for detection of antibodies against SARS-CoV-2.

CONCLUSIONS

The study analyzes dynamic changes in the antibody response to SARS-CoV-2 infection during the natural immunity production. The use of IS-SARS-CoV-2 for calibration of the ELISA test system made in possible to demonstrate a change in the concentration of anti-RBD IgG over a wide range of 36–25, 900 BAU/mL with the widest disparity within two months after the infection. The levels of anti-RBD-IgG gradually decreased with time, however, positive values persisted throughout 12 months of follow-up in the majority of subjects.