Total antibodies and neutralizing ability of convalescent sera against three different strains of SARS-CoV-2.

The aim of the study was to assess the level of humoral immunity to SARS-CoV-2 in COVID-19 convalescents.

Materials and methods: We used ELISA for antibody quantitation, microneutralization test using three SARS-CoV-2 strains for neutralizing activity measurement, Illumina MiSeq platform for NGS sequencing and NextClade resource for phylogenetic analysis.

Results: The mean concentration of antibody in convalescents was 133.42 ± 7.2 BAU/mL and the value depended on neither the gender and age of the patients, nor on the time elapsed since COVID-19 infection. The studied SARS-CoV-2 strains were sequenced and deposited in the international GISAID database. According to genetic analysis: EPI_ISL_19424272 phylogenetically belongs to the B1.1 clade, EPI_ISL_19424271 - to B.1.1.397 clade, EPI_ISL_19424270 - to Delta B.1.617.2.122. There were no significant differences in the neutralizing ability of convalescent sera (those who had been ill 2-3 months before the study) for the first two variants of SARS-CoV-2 and it was significantly reduced for the Delta variant, which appeared in the Novosibirsk region later.

Conclusions: The neutralizing activity of convalescent sera was the highest against those variants of the virus that the patient had recovered from, while was reduced or absent against the new variant. The antibody developed to the original variants of the SARS-CoV-2 may not be effective enough against newly emerging strains due to the emergence of mutations in the virus that allow it to evade previously developed humoral immune response.