Development and validation of a multiplex microsphere immunoassay using dried blood spots for SARS-CoV-2 seroprevalence: application in first responders in Colorado, USA.

Schultz, Jonathan S; McCarthy, Mary K; Rester, Cody; Sabourin, Katherine R; Annen, Kyle; DomBourian, Melkon; Eisenmesser, Elan; Frazer-Abel, Ashely; Knight, Vijaya; Jaenisch, Thomas; Morrison, Thomas E; Rochford, Rosemary; Kedl, Ross M

Serological testing of large representative populations for antibodies to SARS-CoV-2 is needed to estimate seroprevalence, transmission dynamics, and the duration of antibody responses from natural infection and vaccination. In this study, a high-throughput SARS-CoV-2 multiplex microsphere immunoassay (MMIA) was developed for the receptor binding domain (RBD) and nucleocapsid (N) that was more sensitive than ELISA (98% vs 87%). The MMIA was then applied and validated in 264 first responders in Colorado using serum and dried blood spot (DBS) eluates, compared to ELISA and evaluated for neutralizing antibodies. Four percent (11/264) of first responders were seropositive in July-August 2020. Serum and DBS were highly correlated for anti-RBD and anti-N antibodies (R=0.83, p<0.0001 and R=0.87, p<0.0001, respectively) by MMIA. The MMIA accurately predicted SARS-CoV-2 neutralizing antibodies using DBS (R=0.76, p=0.037). On repeat antibody testing three months later, anti-RBD IgG decreased less rapidly than anti-N IgG measured by MMIA, with a median change in gMFI of 62% vs 79% (p<0.01), for anti-RBD and anti-N IgG respectively. This novel MMIA using DBS could be scalable for rapid and affordable SARS-CoV-2 serosurveillance in the U.S. and globally.