Quantitative serology for SARS-CoV-2 using self-collected saliva and finger-stick blood

Abstract

Convenient and widespread serology testing may alter the trajectory of the COVID-19 pandemic. This study seeks to leverage high-throughput, multiplexed serologic assays, which have been adopted as benchmarks for vaccine efficacy, to support large-scale surveys of SARS-CoV-2 immunity using finger-stick blood and/or saliva. Specifically, we optimized MSD’s serology assays, which were analytically validated for serum, to test self-collected finger-stick blood and saliva samples to identify prior infection. We show that these assays can be used with FDA-registered specimen collection devices to obtain quantitative measurements for self-collected samples. First, we show that salivary antibodies are stable without refrigeration or preservatives for at least 5 days. We selected classification thresholds for antibodies against SARS-CoV-2 N, RBD and Spike in finger-stick blood and saliva that provided 98% specificity in a set of individuals without known COVID-19 exposure. Using matched samples, we show that testing of saliva and finger-stick blood equivalently identified individuals with humoral responses to CoV-2 antigens. Moreover, we piloted a simple saliva collection kit that can be used to safely send samples through the mail using written instructions only. This work establishes key parameters to robustly assay self-collected finger-stick blood and saliva using quantitative immunoassays that could support large-scale serology testing.

Saliva-based SARS-CoV-2 serology using at-home collection kits returned via mail

“…The transit time of specimens from participants to the laboratory ranged from 1 to 31 days (1st quartile = 1.4 days, median = 1.9 days, 3rd quartile = 3.5 days) in the mail with 91% arriving in less than 5 days, which was our target window based on prior testing showing stability for 5 days 35 . Two specimens that were in the mail over 20 days were excluded from analysis of test performance, but were included in an analysis investigating potential indicators of sample degradation.…”

“…1 . Antibody positivity for SARS-CoV-2 antigens was determined based on pre-established thresholds set at the 98th percentile for saliva self-collected from presumed naive participants (no PCR confirmed diagnosis, no household exposure, and no symptoms of COVID-19) in a previous study 35 . Pre-established IgG thresholds for Spike, RBD, and N were 0.963, 0.244, and 3.18 AU/mL, respectively.…”

“…As an aggregate metric of reactivity to endemic coronaviruses, we computed the geometric mean of salivary IgG for HKU1, NL63, OC43, and 229E. We flagged eight outlier samples with geometric means below 0.17 AU/mL, which is the geometric mean of the 5th percentiles of the salivary IgG for these four antigens measured in a prior study 35 . These outliers appear to result from an issue with sample collection or sample deterioration, as opposed to the lack of immunity to an endemic coronavirus due to the absence of previous exposure or from general immunosuppression.…”

“…( b ) 91% of samples were in transit for five days or less, and transit time was not strongly correlated with the mean level of salivary IgG for the four endemic coronaviruses (CoVs). Dotted line indicates geometric mean for the 5th percentiles of endemic CoV salivary IgG measured in a prior study of presumed naïve controls without prior diagnosis, household exposure, or symptoms of COVID-19 35 . ( c ) Total levels of immunoglobulins (IgG, IgM, and IgA) were also measured in saliva.…”

“…Previously, we developed and pilot tested an at-home saliva-collection kit using the Salimetrics device 35 . Specimens were returned via mail without refrigeration or preservatives.…”