Early, robust mucosal secretory IgA but not IgG response to SARS-CoV-2 spike in oral fluid is associated with faster viral clearance and COVID-19 symptom resolution (preprint)

High priority efforts are under way to support the development of novel mucosal COVID-19 vaccines, such as the US Governments Project NextGen and the Center for Epidemic Preparedness Innovations (CEPI) goal to respond to the next pandemic with a new vaccine in 100 days. However, there is limited consensus about the complementary role of mucosal immunity in disease progression and how the immunogenicity of mucosal vaccines will be evaluated. This study investigated the role of oral mucosal antibody responses in viral clearance and in COVID-19 symptom duration. Participants with PCR-confirmed SARS-CoV-2 infection provided oral fluid for testing with SARS-CoV-2 antibody multiplex assays, nasal swabs for RT-PCR and symptom information at up to eight follow-ups from April 2020 to February 2022. High and moderate oral fluid anti-spike (S) SIgA post infection was associated with significantly higher likelihood of viral clearance and of COVID-19 symptom resolution across age groups. Those with high and moderate anti-S SIgA cleared the virus and recovered 14 days (95% CI: 10-18 days) and 9-10 days (95% CI: 6-14 days) earlier, respectively. Delayed but higher oral fluid anti-S IgG was associated with significantly longer time to viral clearance and recovery. The effect size of moderate or high SIgA was equivalent to prior COVID-19 vaccine immunity, which was also associated with faster clearance and recovery. Unvaccinated adults with prolonged COVID-19 symptoms had significantly lower anti-RBD SIgA 15-30 days after infection onset (p<0.001). Robust mucosal SIgA early post infection appears to support faster clearance of SARS-CoV-2 and recovery from COVID-19 symptoms. This research underscores the importance of harmonizing mucosal immune response assays to evaluate new vaccines that can boost local mucosal immunity. DisclaimerThe findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention.

Methodological approaches to optimize multiplex oral fluid SARS-CoV-2 IgG assay performance and correlation with serologic and neutralizing antibody responses (preprint)

Background. Oral fluid (hereafter, saliva) is a non-invasive and attractive alternative to blood for SARS-CoV-2 IgG testing; however, the heterogeneity of saliva as a matrix poses challenges for immunoassay performance. Objectives. To optimize performance of a magnetic microparticle-based multiplex immunoassay (MIA) for SARS-CoV-2 IgG measurement in saliva, with consideration of: i) threshold setting and validation across different MIA bead batches; ii) sample qualification based on salivary total IgG concentration; iii) calibration to U.S. SARS-CoV-2 serological standard binding antibody units (BAU); and iv) correlations with blood-based SARS-CoV-2 serological and neutralizing antibody (nAb) assays. Methods. The salivary SARS-CoV-2 IgG MIA included 2 nucleocapsid (N), 3 receptor-binding domain (RBD), and 2 spike protein (S) antigens. Gingival crevicular fluid (GCF) swab saliva samples were collected before December, 2019 (n=555) and after molecular test-confirmed SARS-CoV-2 infection from 113 individuals (providing up to 5 repeated-measures; n=398) and used to optimize and validate MIA performance (total n=953). Combinations of IgG responses to N, RBD and S and total salivary IgG concentration (g/mL) as a qualifier of nonreactive samples were optimized and validated, calibrated to the U.S. SARS-CoV-2 serological standard, and correlated with blood-based SARS-CoV-2 IgG ELISA and nAb assays. Results. The sum of signal to cutoff (S/Co) to all seven MIA SARS-CoV-2 antigens and disqualification of nonreactive saliva samples with >15 g/mL total IgG led to correct classification of 62/62 positives (sensitivity [Se]=100.0%; 95% confidence interval [CI]=94.8%, 100.0%) and 108/109 negatives (specificity [Sp]=99.1%; 95% CI=97.3%, 100.0%) at 8-million beads coupling scale and 80/81 positives (Se=98.8%; 95% CI=93.3%, 100.0%] and 127/127 negatives (Sp=100%; 95% CI=97.1%, 100.0%) at 20-million beads coupling scale. Salivary SARS-CoV-2 IgG crossed the MIA cutoff of 0.1 BAU/mL on average 9 days post-COVID-19 symptom onset and peaked around day 30. Among n=30 matched saliva and plasma samples, salivary SARS-CoV-2 MIA IgG levels correlated with corresponding-antigen plasma ELISA IgG (N: {rho}=0.67, RBD: {rho}=0.76, S: {rho} =0.82; all p<0.0001). Correlations of plasma SARS-CoV-2 nAb assay area under the curve (AUC) with salivary MIA IgG (N: {rho}=0.68, RBD: {rho}=0.78, S: {rho} =0.79; all p<0.0001) and with plasma ELISA IgG (N: {rho}=0.76, RBD: {rho}=0.79, S: {rho} =0.76; p<0.0001) were similar. Conclusions. A salivary SARS-CoV-2 IgG MIA produced consistently high Se (>98.8%) and Sp (>99.1%) across two bead coupling scales and correlations with nAb responses that were similar to blood-based SARS-CoV-2 IgG ELISA data. This non-invasive salivary SARS-CoV-2 IgG MIA could increase engagement of vulnerable populations and improve broad understanding of humoral immunity (kinetics and gaps) within the evolving context of booster vaccination, viral variants and waning immunity.