Traditionally, cellular immunity is responsible for clearing an established viral infection, whereas humoral immune responses
play a more critical role in preventing future infection. Here, we
found that severely ill COVID-19 patients had the highest levels
of anti-RBD and anti-spike antibodies, which is in agreement
with previous studies (Shrock et al., 2020; Secchi et al., 2020).
To further characterize this antibody response, we measured
neutralization titers and developed a neutralization potency index derived from our quantitative readouts (NT50/IgG) to assess
the quality of anti-RBD IgG antibodies irrespective of the quantity
produced. Remarkably, anti-RBD IgG neutralization potency
was significantly diminished in severely ill patients, and survival
analysis demonstrated that an index of R100 was predictive of
100% 30-day survival, whereas <100 was associated with
87% 30-day survival in our limited cohort of 111 COVID-19 patients.

Further analyses using a total antibody composite variable (IgPC) revealed even more significant differences in neutralization potency among severe cases of COVID-19, highlighting
the importance of accounting for all antibody isotypes when assessing the neutralization response. Thus, this anti-RBD antibody neutralization potency index may be a useful metric for
physicians seeking to risk-stratify COVID-19 patients.