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

Background: Declining humoral immunity in COVID-19 patients and possibility of reinfections has raised concern. Mucosal immunity particularly salivary antibodies could be short-lived. However, long-term studies are sparse. 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 pre-pandemic time (n= 35, Cohort 3). Results: Salivary IgG antibody responses in Cohort 1 (mainly mild COVID-19) were detectable up to 9 month recovery, with high correlations between spike and nucleocapsid specificity. At 9 months, IgG remained in saliva in majority as seen in blood serology. Salivary IgA was rarely detected at this timepoint. In Cohort 2, salivary IgG and IgA responses were significantly associated with recent history of COVID-19 like symptoms. Salivary IgG also tolerated temperature and detergent pre-treatments. Conclusions: Unlike SARS-CoV-2 salivary IgA that appeared short-lived, the specific IgG in saliva appears stable even after mild COVID-19 as noted for blood serology. The non-invasive saliva-based SARS-Cov-2 antibody testing with self-collection at homes may thus serve as a complementary alternative to conventional blood serology.