CD19 and viral infectious disease: Concomitantly, we observed the similar dynamics of plasmablasts (CD19+CD20−CD27hiCD38hi) (Fig. 3b, d), which generally reflect the extrafollicular antibody responses during primary viral infection.12,24,25 In keeping with these observations, we also noticed the relatively higher S1- and S2- specific IgG titers at the middle stage (5–10 days post hospitalization) of hospitalization than those at both early (0–3 days post hospitalization) and convalescent stages (more than 10 days after COVID-19 nucleic acid test shown negative) (Fig. 3e).