Persisting Salivary IgG Against SARS-CoV-2 at 9 Months After Mild COVID-19: A Complementary Approach to Population Surveys

Abstract

Background: Declining humoral immunity in coronavirus disease 2019 (COVID-19) patients and possible reinfection have raised concern. Mucosal immunity, particularly salivary antibodies, may be short lived although long-term studies are lacking.

Methods: Using a multiplex bead-based array platform, we investigated antibodies specific to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) proteins in 256 saliva samples from convalescent patients 1-9 months after symptomatic COVID-19 (n = 74, cohort 1), undiagnosed individuals with self-reported questionnaires (n = 147, cohort 2), and individuals sampled prepandemic (n = 35, cohort 3).

Results: Salivary IgG antibody responses in cohort 1 (mainly mild COVID-19) were detectable up to 9 months postrecovery, with high correlations between spike and nucleocapsid specificity. At 9 months, IgG remained in blood and saliva in most patients. Salivary IgA was rarely detected at this time point. In cohort 2, salivary IgG and IgA responses were significantly associated with recent history of COVID-19-like symptoms. Salivary IgG tolerated temperature and detergent pretreatments.

Conclusions: Unlike SARS-CoV-2 salivary IgA that appeared short lived, specific saliva IgG appeared stable even after mild COVID-19, as for blood serology. This noninvasive saliva-based SARS-CoV-2 antibody test with home self-collection may be a complementary alternative to conventional blood serology.

Keywords: COVID-19; antibody; convalescence; immunoassay; saliva; serology.

Figure 1.

Measurement of IgG and IgA to spike-f (soluble trimeric form of the spike glycoprotein stabilized in the prefusion conformation) and NC-C (nucleocapsid C-terminal fragment) of SARS-CoV-2 in saliva of convalescent patients (cohort 1). A, Multiplex assay measured signal scores on indicated immunoglobulins to spike-f and NC-C in the pre–COVID-19 control samples (n = 35) and convalescent patient samples at indicated month postinfection (n = 74). The data are MFI and plotted using dot plots where each dot is 1 sample. Horizontal bars denote the mean and vertical lines represent standard error. Mann-Whitney U test for significance was performed. B, Spearman correlation analysis with coefficient indicated for respective antibody specificity pairs. Abbreviations: COVID-19, coronavirus disease 2019; Ctrl, control; IgG, immunoglobulin G; MFI, median fluorescence index; NS, not significant; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

Figure 2.

SARS-CoV-2–specific IgG and IgA in saliva of undiagnosed study participants (cohort 2) measured by the same method as in Figure 1. Samples were subgrouped by participant-reported COVID-19–like symptoms during 14 days to 3 months prior to the sampling. A, Multiplex assay measured signal scores on indicated immunoglobulins to spike-f and NC-C in pre–COVID-19 control samples (n = 35) and participants in cohort 2 reporting no or yes to recent COVID-19–like symptoms (n = 146). The data are MFI and plotted using dot plots where each dot is 1 sample. Horizontal bars denote the mean and vertical lines represent standard error. Mann-Whitney U test for significance was performed. B, Spearman correlation analysis with coefficient indicated for respective antibody specificity pairs. Abbreviations: COVID-19, coronavirus disease 2019; Ctrl, control; IgG, immunoglobulin G; MFI, median fluorescence index; NC-C, nucleocapsid C-terminal fragment; NS, not significant; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2; spike-f, spike glycoprotein.

Figure 3.

Stability tests of saliva samples subjected to HT, 1% Triton, and indicated time at room temperature. A, SARS-CoV-2–specific IgG and IgA reactivities in convalescent saliva samples (Pos) or prepandemic saliva samples (Neg) untreated, after HT at 56°C for 30 minutes, or after Triton inactivation for 60 minutes. Reactivities to spike-f and NC-C antigens are shown as box plots with each dot representing 1 sample. Each box represents the intensity signals interval included between the 25th and 75th interquartile, and the median is shown as a black line. Whiskers represent the interval including all intensity signals falling between the 5th and 95th percentile. For each condition (Untreated, HT and Triton), the cutoff for reactivity was calculated as the mean intensity of the negative samples included in the group plus 6x SD. The cutoff for each condition is represented by a colored line: grey = untreated, orange = HT, and purple = triton. B, Convalescent saliva samples (blue) or prepandemic saliva samples (gray) were aliquoted and placed at room temperature (22°C) for indicated times prior to freezing and subsequent measurement of SARS-CoV-2–specific IgG to spike or nucleocapsid. Abbreviations: HT, heat treatment; IgG, immunoglobulin G; MFI, median fluorescence index; NC-C, nucleocapsid C-terminal fragment; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2; spike-f, spike glycoprotein; Triton, Triton-X-100.