Huang Y, Zhang Y, Bailer R, et al. - Researchers identified HVTN 104 as a phase 1 trial of VRC01, a human IgG1 broadly neutralizing antibody (bnAb) that binds to the HIV-1 envelope glycoprotein. They here examined 1,079 longitudinal samples collected after passive administration of VRC01 in 84 HVTN 104 participants for measures of serum concentrations and serum neutralization of VRC01 and sought approaches for estimating VRC01 serum neutralization titers as assays for measuring such titers are resource-intensive. They used three approaches [observed serum concentration divided by IC50; pharmacokinetics model-predicted serum concentration divided by IC50; and joint modeling of the longitudinal serum concentrations and ID50 titers] to predict serum neutralization ID50 titers. Outcomes support the possibility of accurately predicting VRC01 serum neutralization, particularly when using pharmacokinetics models. Significant resources could be saved using the proposed prediction approaches for the characterization of serum neutralization of VRC01, including for other bnAbs and bnAb combinations.