The results, in agreement with earlier observations from the screened group that seroconversion in mild COVID-19 cases might take longer time to mount, showed an increase in individuals who had an initial titer of 1:320, 1:160 or 1:80. 
The mean interval between the initial titer measurement and the second was 52 days (range 33-67 days), setting the second time point at a mean of 82 days post symptom onset (range 52-104 days). 
There was a slight drop from a geometric mean titer (GMT) of 670 to a GMT of 642 when comparing overall titers. Lastly, the higher titer range of 1:2880 and 1:960 also observed a slight drop in titer between the two time points. 

Results: Assessing the longevity of SARS-CoV-2 spike antibodies

After examining the correlation between seropositivity and antibody levels, and the neutralizing effects of spike antibodies, the longevity of spike antibodies will be assessed. Researchers evaluated the medium-range stability of spike antibodies by recalling 121 plasma donors at various titer levels that had initially been screened at approximately day 30 post symptom onset. 


University of South Florida

From performing the assay, neutralization titers significantly correlated with spike-binding titers.  Although there was some variability, sera with 1:320 ELISA titers had a geometric mean 50% inhibitory dilution (ID50) of approximately 1:30. For the 1:960 titer, it was approximately 1:75, and for the 1:2880 titer it was approximately 1:550. When considering any activity above background (noise), approximately 50% of sera in the 1:80-1:160 titer range had neutralizing activity, 90% in the 1:320 range had neutralizing activity, and all sera in the 1:960 - 1:2880 range had neutralizing activity. These results can potentially validate use of antibody titers to refer for plasma donation and further investigation into neutralizing effects at varying titer levels.

Results: Determining the neutralizing effects of SARS-CoV-2 spike antibodies 

Wajnberg, A., Amanat, F., Firpo, A., Altman, D., Bailey, M., Mansour, M., McMahon, M., Meade, P., Mendu, D. R., Muellers, K., Stadlbauer, D., Stone, K., Strohmeier, S., Aber, J., Reich, D., Krammer, F., and Carlos Cordon-Cardo. 
SARS-CoV-2 infection induces robust, neutralizing antibody responses that are stable for at least three months. (2020). 
https://doi.org/10.1101/2020.07.14.20151126

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