S1 binding antibody concentration and neutralization. (A) IgG S1 concentration for each timepoint. Red and green: previously uninfected patients (U); blue and purple: previously infected patients (I). Dotted lines indicate seroconversion (S1 IgG > 10 BAUs/mL) and sufficient S1 IgG concentration (≥300 BAUs/mL). *P < .05. (B) Correlation of IgG S1 concentration and pseudovirus neutralization (r = 0.85; P < .001). ID₅₀, 50% inhibitory dose.