Assessment of Salivary Glands as Reservoirs of SARS-CoV-2 and the Contagiousness of Saliva in Asymptomatic COVID-19 Patients: Is Delta Variant More Contagious?

Background: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection transmits when aerosols or droplets containing the virus are inhaled or come directly into contact, mainly in close contact with an infected person. Objectives: This study aimed to evaluate the role of the salivary glands in the secretion of SARS-CoV-2-infected saliva and determine the contagiousness of saliva in asymptomatic coronavirus disease 2019 (COVID-19) patients. Methods: In this cross-sectional analytical study between March 2021 and March 2022, 85 asymptomatic COVID-19 individuals with positive nasopharyngeal/oropharyngeal swabs were recruited. The SARS-CoV-2 cycle threshold (Ct) value was investigated in concomitant nasopharyngeal swabs (NPS), saliva, and pure saliva (collected directly from the salivary duct opening) using Real Time-PCR assay. Statistical analysis was performed using SPSS software (version 23), and a p-value of < 0.05 was considered significant. Results: The saliva Ct-value was the lowest (the highest viral load) for Delta (29.82 ± 4.66), Omicron (32.75 ± 4.82), and Alpha (36.83 ± 4.8) variants, respectively. Delta-infected saliva and pure saliva revealed the strongest correlation (correlation coefficient = 0.971, P < 0.001). Saliva Ct-value was significantly lower in Delta- (P < 0.001) and Omicron- (P = 0.012) infected patients than in Alpha-infected patients. The pure saliva Ct-value was significantly lower (P = 0.014) in Delta samples (30.13 ± 4.51). Asymptomatic Alpha- and Omicron-infected patients revealed significantly lower NPS Ct-value (30.52 ± 4.02 and 29.44 ± 3.34) than the saliva (36.83 ± 4.8 and 32.75 ± 4.82). Conclusions: The major salivary glands secrete SARS-CoV-2-infected saliva in nearly all Delta-infected and most Omicron-infected asymptomatic individuals. Although the transmission process is complex, saliva droplets and aerosols seem to have a higher contagiousness potential in individuals infected with the Delta variant.