Saliva: a diagnostic option and a transmission route for 2019-nCoV

Data sources This review included 13 clinical studies (observational or clinical trial) which reported results of studies of the 2019 novel coronavirus (2019-nCoV). The other 62 referenced papers were of different types (eg, reviews, WHO protocols, letter to editor etc). Study selection The study selected trials, reviews, and in-vitro research assessing the critical aspects of saliva as an easily accessible and early-stage diagnostic source, and also an entry route for 2019-nCoV. Most of the clinical studies were descriptive case series of patients who had contracted 2019-nCoV in China. These were mainly studies designed to compare saliva samples with throat swabs, with regard to the presence of 2019-nCoV RNA. Another aspect of the included studies was the susceptibility of oral tissues to 2019-nCoV due to the expression of angiotensin-converting enzyme II (ACE2) as a receptor for the 2019-nCoV. Some review studies and clinical infection control protocols were also included to discuss the transmission patterns of 2019-nCoV from the oral cavity. Studies were not restricted to English language and they were not all full-text papers. Data extraction and synthesis A narrative synthesis of the results was conducted using distinct headings and subheadings, defined by the authors based on relevancy to the consensus about the importance of saliva with reference to 2019-nCoV. Results There was an inherent heterogeneity among the included clinical studies concerning their designs, sampling techniques, and the results about the diagnostic value of saliva. The percentage of coronavirus disease of 2019 (COVID-19) patients with positive 2019-nCoV RNA varied from 12.9% to 91.67% among these studies. Regarding the possibility of direct virus invasion into the oral tissues, the results suggested that ACE2+ cells in salivary glands could possibly be the target cells of 2019-nCoV and theoretically could generate infectious saliva in a sustained way. Furin was suggested as another protein which makes the tongue more vulnerable to 2019-nCoV, especially in conditions inducing its upregulation (for example, squamous cell carcinoma). According to the comparisons between 2019-nCoV and SARS-CoV, saliva could be considered of diagnostic value via the early detection of viral RNA for both of the viruses. Whilst the viral peak was shown to be at onset of symptoms for 2019-nCoV, it can linger up to the tenth day after the appearance of symptoms for SARS-CoV. Finally, this paper warns about airborne transmission, particularly for close contacts. Conclusions Saliva can be proposed as an easily accessible diagnostic source although further clinical studies are required. Given the presence of viral RNA in saliva in the early stages of COVID-19, the recommendations to wear masks to prevent the rapid transmission of infectious droplets into the air, and keep a safe distance from other people are clearly based in evidence.