CD19 and influenza: We quantified peripheral PBs by flow cytometry within 4 weeks after disease onset (median sampling at day 15, range 0–26; Figures S1A and S1B) in active COVID-19 patients (pt) as well as d14 ± 7 after influenza vaccination using a broad gating strategy (CD3−/CD19+/dim/CD20dim/−/SSCdim or high) to cover all stages of PB development as previously described (Jourdan et al., 2011; Sanz et al., 2019) (Figures 1A–1C).