Dynamics in maturation of SARS-CoV-2 RBD-specific IgG antibody avidity depending on immunization timeframe and type

Abstract

The aim is to examine dynamics of avidity maturation of IgG antibodies against SARS-CoV-2 RBD depending on the type of immunization (vaccination or infection), as well as on the duration and frequency of immunization. Materials and methods. The study was performed on two sample cohorts collected at two time points during COVID-19 pandemic. The first cohort (group No. 1) consisted of 87 samples of blood sera obtained from COVID-19 convalescents in the period from March to September 2020. The second cohort included 204 samples obtained in September 2021 from two patient groups. Group No. 2 (n = 64) patients immunized with a full course of Gam-Covid-Vac, group No. 3 (n = 140) COVID-19 convalescent patients and subjects vaccinated with Gam-Covid-Vac (hybrid immunity). Results and conclusion. The dynamics of avidity maturation for SARS-CoV-2 RBD IgG antibodies depending on the method and frequency of immunization, showed that the most effective immunity was formed in COVID-19 convalescent patients and subjects vaccinated with a full course of Gam-Covid-Vac. The hybrid immunity showed not only a significantly higher (compared with groups No. 1 and No. 2) level of IgG antibodies (median 228 BAU/ml vs 75 or 119 BAU/ml, p 0.001), but also a higher level of avidity (IA 90.5% vs 54.5 and 76.6, respectively, p 0.001, 4M urea). In the test for assessing the avidity index with the denaturing agent 8M urea in patients with hybrid immunity, the median level of IA was 25% versus 14.8% and 16% in COVID-19 convalescents and vaccinated subjects (p 0.001), only in 8 patients IA was higher than 50%. While comparing a single infection of COVID-19 with a full course of Gam-Covid-Vac, it was shown that vaccination leads to higher IgG levels (median values in groups 119 and 75 BAU/ml, p 0.001) and to a higher avidity index (median 76.6% vs 54.5%). Thus, the more rapid induction of high-avidity antibodies was in vaccinated individuals at early stages of immunization (up to 4 months), during the period when IgG avidity maturation has not yet been completed. Our results showed that during this period vaccination leads to production of antibodies with avidity index at median level of 82% versus 36% in COVID-19 convalescents at similar time point.