Figure 1. Conceptual introduction to serum neutralization potency (NP).

(a) A hypothetical serum, which neutralizes tier 1A and some tier 1B viruses (red), but does not neutralize any tier 2 or 3 viruses (black), is assigned a neutralization potency (NP) of 1.1. (b) Another hypothetical serum may neutralize all tier 1A and B viruses and most tier 2 viruses, for NP of 2.8. In practice (c), the two outcomes do not segregate so clearly. Instead, positive and negative results among pseudoviruses are interspersed. Neutralization outcomes are scattered over the range of mean ID50s, and more sensitive viruses are enriched for positive neutralization. Logistic regression provides an objective way to distinguish neutralization outcomes. The neutralization outcome is treated as a probability (d). We use logistic regression to define the serum NP, which is the Env neutralization index (NI) value with 50% probability of neutralization that best separates neutralized and non-neutralized viruses.