Commercial Serology Assays Predict Neutralization Activity against SARS-CoV-2

Abstract

Background: It is unknown whether a positive serology result correlates with protective immunity against SARS-CoV-2. There are also concerns regarding the low positive predictive value of SARS-CoV-2 serology tests, especially when testing populations with low disease prevalence.

Methods: A neutralization assay was validated in a set of PCR-confirmed positive specimens and in a negative cohort. In addition, 9530 specimens were screened using the Diazyme SARS-CoV-2 IgG serology assay and all positive results (N = 164 individuals) were reanalyzed using the neutralization assay, the Roche total immunoglobin assay, and the Abbott IgG assay. The relationship between the magnitude of a positive SARS-CoV-2 serology result and neutralizing activity was determined. Neutralizing antibody titers (50% inhibitory dilution, ID50) were also longitudinally monitored in patients confirmed to have SARS-CoV-2 by PCR.

Results: The SARS-CoV-2 neutralization assay had a positive percentage agreement (PPA) of 96.6% with a SARS-CoV-2 PCR test and a negative percentage agreement (NPA) of 98.0% across 100 negative control individuals. ID50 neutralization titers positively correlated with all 3 clinical serology platforms. Longitudinal monitoring of hospitalized PCR-confirmed patients with COVID-19 demonstrated they made high neutralization titers against SARS-CoV-2. PPA between the Diazyme IgG assay alone and the neutralization assay was 50.6%, while combining the Diazyme IgG assay with either the Roche or Abbott platforms increased the PPA to 79.2 and 78.4%, respectively.

Conclusions: These 3 clinical serology assays positively correlate with SARS-CoV-2 neutralization activity observed in patients with COVID-19. All patients confirmed SARS-CoV-2 positive by PCR develop neutralizing antibodies.

Keywords: COVID-19; SARS-CoV-2; immunity; neutralizing antibodies; serology.

Fig. 1.

Flowcharts of the cohorts used for A) Main cohort, B) SARS-CoV-2 PCR positive cohort, and C) SARS-CoV-2 negative cohort. SNF, skilled nursing facility, RPNA, respiratory pathogen nucleic acid.

Fig. 2.

COVID-19 patients produce neutralization activity against SARS-CoV-2 and SARS-CoV-1. Percentage of specimens (N=87) from SARS-CoV-2 PCR confirmed patients (N=31, dark grey) and a SARSCoV-2 negative patient cohort (N=100 specimens from 100 individuals, light grey) that had ID50 titers >50 against SARS-CoV-2 or SARS-CoV-1.

Fig. 3.

Longitudinal monitoring of SARS-CoV-2 neutralization titers in COVID-19 patients. The ID50 titersof 13 SARS-CoV-2 PCR confirmed patients are plotted on a semi-log scale (Y-axis) with the number of days following a positive PCR result indicated for each sample (X-axis). ID50 values of <50 are considered negative for neutralization activity and represented by the greyed-out area.

Fig. 4.

Interplatform agreement between serology and neutralization activity. A) The PPA between the Diazyme IgG and Roche assay and the Diazyme IgG and Abbott assays are shown for 4 different populations of individuals. B) The percentage of individuals with neutralization activity against SARS-CoV-2 or SARS-CoV-1 are shown for 4 different populations of individuals. C) The overall PPA between the Diazyme IgG assay alone and in combination with the Roche or Abbott serology platforms with SARS-CoV-2 neutralization activity are indicated for 164 seropositive individuals identified on the Diazyme IgG platform.

Fig. 5.

Distribution of commercial serology platform results in specimens negative and positive for SARS-CoV-2 neutralization activity that initially screened positive by the Diazyme IgG assay. Median AU/mL, COI, and S/C values (Y-axis) on the Diazyme IgG, Roche total Ig, and Abbott IgG platforms, respectively, are shown in boxplots (whiskers are up to but no greater than 1.5 times the IQR) for specimens without (negative) or with (positive) ID50 neutralization titers against SARS-CoV-2 (X-axis). **** indicates significant difference between positive and negative group at a p value of <0.00001 by Mann-Whitney.