Quantifying post-vaccination protective anti-SARS-CoV-2 IgG antibodies in blood and saliva with a fully automated, high throughput digital immunoassay

BackgroundAntibodies induced by COVID-19 vaccination have been shown to wane over time. Current tests for assessing virus-neutralizing antibodies are complex and time-intensive. There is a need for a simple diagnostic test that measures levels of protective antibodies to help monitor immunity status. MethodUsing a commercially available FDA-authorized semi-quantitative SARS-CoV-2 IgG test, we monitored the duration of the immune response in dried blood microsamples (DBS) and saliva to vaccination by 3 different vaccines across prospective cohorts of 8 COVID-19 naive and 29 COVID-19 recovered individuals over a six-month period. We correlated the results to a binding blockade assay validated to a live virus neutralization assay to validate the test for measurement of protective antibodies. ResultsThe immune response characteristics between the two mRNA vaccines were similar over the 6-month period in both the COVID-19 naive and recovered cohorts. IgG titers in DBS were generally 3-4 orders of magnitude higher than in saliva, and longitudinal profiles were highly correlated between the two matrices (Rm = 0.80). Median IgG concentrations post-vaccination declined to <10% neutralization capacity with all vaccines by six months. ConclusionsThe potential of a simple, fully automated high throughput anti-SARS-CoV-2 IgG test to quantitatively measure protective antibodies in samples collected remotely or at the point of care was demonstrated. The IgG immune response and protective immunity was shown to decline significantly by six months. Plain Language SummaryIn response to infection the immune system produces proteins called antibodies that recognize and bind to foreign invaders. Vaccines train the immune system to recognize and produce antibodies against specific invaders, such as SAR-CoV-2. Measurement of antibody levels in blood help monitor a persons response to vaccination and have been shown to correlate with protection against disease, which wanes over time following vaccination. It is desirable to have an easy test that predicts protection against infection and measuring antibody levels may provide a solution, however different tests report results differently hindering the establishment of a cutoff for protected vs. not. We quantified antibody levels in saliva and dried blood microsamples (DBS) following vaccination using an automated semi-quantitative IgG test. By reporting concentration of antibodies, and if anchored to an international standard, this test could help establish a cutoff of protection that would be transferable across the multiple different test types. Furthermore, by measuring in saliva and DBS we demonstrate an easy path to at-home or point-of-care sample collection, which could allow wide-scale monitoring of immune protection against SARS-CoV-2.