New Variant of SARS-CoV-2 Shows Resistance to Convalescent Sera & MABs

Researchers studying the effect of convalescent sera and monoclonal antibodies from recovered patients found the UK and South African SARS-CoV-2 variants are more resistant to neutralization compared to the wild-type virus. The research team used previously obtained blood samples from 20 COVID-19 patients after their recovery, to collect eight receptor-binding domain (RBD) specific monoclonal antibodies. They expressed the wild type and the UK and South African mutant strains in a lentiviral pseudotype system.

The infectivity of the variant strains was assessed using the luciferase assay. The sera collected from patients about 25 days after symptom onset were less effective in neutralizing the mutants than the wild-type virus. Neutralizing antibody titers were 825 for the wild-type and 343 and 148 for the UK and South African variants, respectively. When tested against sera collected about eight months after symptom onset, 85% of the samples showed inhibitory dose (ID50) greater than 40 to the wild-type virus. About 40% and 90% of the samples had ID50 titers below the UK and South African mutant threshold, respectively.

Thus, some convalescent sera are not able to neutralize the variants. The effectiveness of the monoclonal antibodies collected from the convalescent sera was also tested. Of the eight types of antibodies collected, all had strong neutralizing activity on the wild-type virus. While the variants had almost no effect on two antibodies, the other six had diminished neutralizing activity against the variants. Of these six, three and five were less effective by 3-folds or more against the UK and South African strains, respectively. Two antibodies did not affect the South African variant, and it reduced the effect of the most potent antibody 26 folds compared to that of the wild-type virus.

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Reference: Hu, J. et al. (2021). Emerging SARS-CoV-2 variants reduce neutralization sensitivity to convalescent sera and monoclonal antibodies. bioRxiv,,