Its not the level of depletion, its the duration and persistent B cell depletion that is an issue
Normal antibody response after COVID-19 during treatment with cladribine.Celius EG.Mult Scler Relat Disord. 2020;46:102476. doi: 10.1016/j.msard.2020.102476. Online ahead of print.PMID: 32882501
Fernández-Díaz E, Gracia-Gil J, García-García JG, Palao M, Romero-Sánchez CM, Segura T. COVID-19 and multiple sclerosis: A description of two cases on alemtuzumab , 2020 Jul 17]. Mult Scler Relat Disord. 2020;45:102402. doi:10.1016/j.msard.2020.102402
These agents deplete T and B cells yet the individual stills can make an anti-SARS-CoV-2 antibosdy response.Persistently deplete B cells and the result can be different
COI multiple