The world’s governments are under pressure to increase testing for the new coronavirus, officially named SARS-CoV-2, which has infected more than 2.6 million people worldwide, according to the Johns Hopkins Coronavirus Resource Centre.
To that end, six researchers sought to look into the potential diagnostic value of saliva, by looking closely at existing studies on saliva and Covid-19. They concluded that saliva swabs can be effective and yield quicker results – if done correctly.
Generation and transmission of saliva droplets
Saliva is a common medium for virus transmission. According to the World Health Organization (WHO), the new coronavirus virus spreads primarily through droplets of saliva or discharge from the nose – which can happen when an infected person coughs or sneezes.
Saliva, the authors of the paper note, is made up of 99% water, with the remaining 1% comprising components for, among others, the purpose of tasting and digesting.
Breathing, talking, coughing and sneezing are all ways saliva droplets are generated. The droplets are formed as particles and consist of a mixture of moisture and droplet nuclei of microorganisms.
The authors further explain: “The amount, distance, and size of saliva droplets vary among people, suggesting the infectious strength and transmission path of saliva droplets differ when [the] same pathogen was contracted.”
They further add that one cough can generate 3 000 saliva droplet nuclei, which is close to the amount generated during a five-minute talk. And, one sneeze that reaches several metres in air can contain up to 40 000 saliva droplets.
However, studies have shown that it is the smaller droplets that can “travel like a cloud over longer distance by air flow”, explain the authors. But unlike tuberculosis (TB), to date there is no solid evidence to suggest that the virus in saliva droplets can survive in airflow for any length of time.
Testing for SARS-CoV-2 via saliva
The official pathogen detection, the authors explain, occurs when there is a confirmation of SARS-CoV-2 nucleic acid from throat swabs. However, throat swabs, they note, have certain downfalls, including the fact that they are invasive, induce coughing and can cause bleeding. Should this occur, it can increase the risk of healthcare workers being infected.
With the nature of saliva specimen collection, however, it is more secure. The authors draw on three existing methods used to collect saliva: coughing, saliva swabs, and directly from the salivary gland duct.
Studies analysed by the authors included deep throat saliva, saliva in oral cavity, and salivary gland. Out of these three, they found that deep throat saliva had the highest diagnosis rate of SARS-CoV-2, making it the most reliable form of saliva testing for the virus.
One study they investigated involved twelve positive patients coughing out saliva a few days after hospitalisation, and 11 of their specimens tested positive for the virus. Another study (from the same group) used self-collected saliva from deep throat Covid-19 patients, and among 23 patients, 20 cases of their saliva showed detectable SARS-CoV-2 RNA.
Current evidence based on the second method, involving oral swabs, show that it is more applicable in early detection of the virus, but cannot be used as discharge criteria for patients. Saliva collected directly from the opening of the salivary gland can also detect SARS-CoV-2 nucleic acid, and, according to the authors, could possibly be a predictive and non-invasive test for patients with severe cases.
Despite mass testing campaigns, more and smarter testing with faster results is needed. In South Africa, currently, SARS-CoV-2 is detected via a polymerase chain reaction, or PCR molecular test, notes the National Institute for Communicable Diseases (NICD).
The test is done by health workers taking a swab of a person’s throat and/or nose, which gets placed into a fluid, after which it is sent to a laboratory to look for signs of the virus's genetic material, explains News24. If the test is positive, repeat testing is required – all of which can take up to four days.
CNBC recently reported on a new study by Yale University researchers that found that testing saliva for the new coronavirus yields results that are at least as accurate as nasal swabs. The Food and Drug Administration (FDA) also granted emergency use authorisation for a new saliva test which requires a person with coronavirus symptoms to spit into a cup, the Washington Times reports. Results are returned within 24 to 48 hours after arriving at labs.