Weak correlation between antibody titers and neutralizing activity in sera from SARS-CoV-2 infected subjects

Abstract

Plenty of serologic tests for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have been developed so far, thus documenting the importance of evaluating the relevant features of the immune response to this viral agent. The performance of these assays is currently under investigation. Amongst them, LIAISON® SARS-CoV-2 S1/S2 IgG by DiaSorin and Elecsys Anti-SARS-CoV-2 cobas® by Roche are currently used by laboratory medicine hospital departments in Italy and many other countries. In the present study, we firstly compared two serologic tests on serum samples collected at two different time points from 46 laboratory-confirmed coronavirus disease-2019 (COVID-19) subjects. Secondly, 85 negative serum samples collected before the SARS-CoV-2 pandemic were analyzed. Thirdly, possible correlations between antibody levels and the resulting neutralizing activity against a clinical isolate of SARS-CoV-2 were evaluated. Results revealed that both tests are endowed with low sensitivity on the day of hospital admission, which increased to 97.8% and 100% for samples collected after 15 days for DiaSorin and Roche tests, respectively. The specificity evaluated for the two tests ranges from 96.5% to 100%, respectively. Importantly, a poor direct correlation between antibody titers and neutralizing activity levels was evidenced in the present study. These data further shed light on both potentials and possible limitations related to SARS-CoV-2 serology. In this context, great efforts are still necessary for investigating antibody kinetics to develop novel diagnostic algorithms. Moreover, further investigations on the role of neutralizing antibodies and their correlate of protection will be of paramount importance for the development of effective vaccines.

Keywords: COVID-19 diagnostic assays; SARS-CoV-2 serology; neutralizing activity.

Figure 1

Sensitivity and specificity of the two commercial assays. A, “Pre‐pandemic” samples tested with both DiaSorin and Roche assays (●, black line). B, Antibody levels detected by DiaSorin (, T0 solid blue line, T15 dotted blue line) and Roche (, T0 solid red line, T15 dotted red line) tests of sera from subjects with positive nasopharyngeal swabs. C, Roc curves for DiaSorin (T0 solid blue line, T15 dotted blue line) and Roche (T0 solid red line, T15 dotted red line) tests are reported. *p < .001

Figure 2

Evaluation of serum neutralizing activity at different serum concentrations. A, Neutralizing activity of five patients’ serum, at T0 and T15. Sera were two‐fold serial diluted (starting from 1:10) and tested with 0.001 MOI of SARS‐CoV‐2. B, Antibody titers detected for each serum at both timepoints are also reported for DiaSorin and Roche, respectively. DiaSorin readings indicated as <3.8 AU/ml refers to values falling outside the instrument's linearity range. One out of five patients admitted to ICU is highlighted in the table

Figure 3

Characterization of anti‐SARS‐CoV‐2 serum antibodies and evaluation of neutralizing activity. A, Sera reported in the graphs ordered by their neutralizing capability at a dilution of 1:100 (, grey line). Levels of SARS‐CoV‐2 specific antibodies detected with the tests by Roche and DiaSorin are also indicated (●, green and blue lines). Red dots indicate patients admitted to ICU. B, Spearman correlation analyses between values obtained with the two diagnostic methods and neutralizing activity of sera at 1:100 dilution

Figure 4

Characterization of anti‐SARS‐CoV‐2 serum antibodies and evaluation of neutralizing activity at a lower dilution. A, Graphs rank the sera basing on their neutralizing capability. All sera were tested at a dilution of 1:200 (, grey line). Levels of SARS‐CoV‐2 specific antibodies detected with the tests by Roche and DiaSorin are also indicated (●, green and blue lines). Red dots indicate patients admitted to ICU. B, Spearman correlation analyses between values obtained with the two diagnostic methods and neutralizing activity of sera at 1:200 dilution