Primary COVID-19 transmission is by close contact with infected people; nasopharyngeal swab-based testing provides the highest diagnostic accuracy
Respiratory droplets are considered the main mode of COVID-19 transmission, which can be prevented by protective measures, including physical distancing, good ventilation, and the use of face masks. In parallel, diagnostic testing to identify and isolate infected individuals and their close contacts is a key strategy to slow the spreading of the disease. While alternative methods are under study, nasopharyngeal swab-based RT-PCR testing remains the most accurate and the gold standard method for COVID-19 diagnosis.
Inaccurate: Although the mode of transmission of COVID-19 is not yet well understood, respiratory droplets are considered the main route. Thus, the higher risk of contagion is by close contact with infected people and can be efficiently prevented by physical distancing and face masks.
Misleading: Nasopharyngeal swab-based RT-PCR testing remains the most accurate and the gold standard method for COVID-19 diagnosis. The diagnostic potential of other specimens, such as saliva, is still under study.
Facebook posts published in July 2020, such as this meme, questioned the need for nasopharyngeal sample collection in which “they have to stick a Q-tip through your nose to the back of your head to swab a sample of COVID” when it would be much easier to collect saliva samples because “a droplet of saliva will infect a whole village at one time”. Similar posts have been circulating widely on Facebook since the beginning of July 2020 and have received more than 90,000 interactions.
The question raised in the post suggests two possible interpretations, both equally misleading. One implication is that COVID-19 is so contagious that it should be equally detectable in other specimens besides nasopharyngeal mucus and cells, and the other implication is that COVID-19 contagiousness has been overestimated. The first scenario would imply that nasopharyngeal swabs have been unnecessarily imposed on people. The second scenario suggests that public control measures have been excessive.
The contagiousness of a particular disease is defined as the number of people that can be infected by one infected person. Understanding the modes of transmission of the disease is essential for determining its contagiousness and the implementation of appropriate control measures. Current epidemiological data suggest that respiratory droplets are the main mode of COVID-19 transmission, which means that the higher risk of contagion is by close contact with infected people or by sharing a confined space for long periods of time.
As previously explained in a Health Feedback review, respiratory droplets are small particles of mucus and/or saliva that a person releases when coughing, sneezing, or talking. Most of these particles tend to fall from the air quickly and are capable of infecting only those individuals located within a few feet of an infected person. However, particles smaller than 5 micrometers in diameter can form aerosols, which are able to float farther away compared to respiratory droplets. Increasing evidence suggests that aerosol transmission may play a role in infection by COVID-19, but the extent of its contribution is not yet clear.
Given that COVID-19 transmission occurs primarily in close proximity to infected individuals, the claim that “a droplet of saliva will infect a whole village at one time” is inaccurate and misleading. Two recent studies published in Emerging Infectious Diseases comprehensively analyzed COVID-19 transmission in a restaurant in China and at a call center in South Korea. Detailed evaluations of similar outbreaks among members of a choir in Washington, and among passengers on a bus and conference attendees in China3, are available as preprints but have not yet been peer-reviewed. In all cases, ventilation systems in recirculation mode promoted COVID-19 transmission in the direction of the airflow, spreading the infection to people who were downwind from an infected individual. But the risk of infection correlated with four key factors: proximity to an infected individual, the amount of time spent near the infected individual, the size and ventilation of the space shared with the infected individual, and the number of people sharing the space. Therefore, the transmission of COVID-19 is not random but follows a clear pattern in which physical distancing, good ventilation, and the use of face masks are effective protection measures.
Another crucial aspect of diseases that is used to determine their contagiousness is the amount of virus present in different specimens at different stages of the infection—also called viral load. Assessing viral load at different stages of a disease enables health care providers to determine when a person is contagious. SARS-CoV-2—the virus that causes COVID-19—resides primarily along the respiratory tract, but is also detectable in saliva, stool, blood, ocular secretions, and semen[4-9]. However, the role of these specimens in disease transmission is still unclear because the mere presence of viral genetic material is not necessarily enough to make them infectious.
Finally, accurate diagnosis of COVID-19 is essential for preventing its transmission. The detection of SARS-CoV-2 genetic material by reverse transcription polymerase chain reaction (RT-PCR) is considered the gold standard method for diagnosing respiratory infections, including COVID-19. The post misleadingly questions the use of nasopharyngeal swabs for sample collection for COVID-19 testing. However, nasopharyngeal swab-based testing is the recommended method as its sensitivity is superior to testing that uses other specimens such as saliva.
However, nasopharyngeal swabs are an uncomfortable procedure that is not well-tolerated in some cases, like children or people with disabilities. Thus, alternative collection methods which are less invasive are being explored, as reported in this previous review. Saliva is one of the specimens which has drawn more attention, mainly because it is very easy to obtain, although its diagnostic potential is still being tested in large experimental studies10.
One of these studies, conducted at the Peter Doherty Institute for Infection and Immunity, compared the sensitivity of saliva and nasopharyngeal specimen testing in 600 people with COVID-19 symptoms11. The director of the institution, Sharon Lewin, stated in a press conference that the saliva test “has a sensitivity of around 87%” compared to nasopharyngeal swab-based tests. “The gold standard, of course, is a throat swab”. According to Lewin, the saliva test is not intended to replace the nasopharyngeal swab, but to “[bolster] testing reach across the state, particularly in vulnerable populations or in people who have trouble with the throat swab such as children or other individuals who find it more acceptable. Much better to have this test than no test at all”.
In summary, these posts employ the tactic of “just asking questions” to propose ideas about COVID-19 without providing any evidence to back them up. The result is that the posts misleadingly suggest that either the contagiousness of COVID-19 has been overestimated or the nasopharyngeal swab test has been imposed unnecessarily on the public. The question contains the incorrect assumption that if a disease is very contagious, the virus should be detectable in any specimen tested. But successful viral detection depends on many factors, including the type of virus, viral load, and clinical course. While alternative methods are under study, nasopharyngeal swab-based RT-PCR testing remains the gold standard method for COVID-19 diagnosis.
- 1 – Lu et al. (2020) COVID-19 Outbreak Associated with Air Conditioning in Restaurant, Guangzhou, China, 2020. Emerging Infectious Diseases.
- 2 – Park et al. (2020) Coronavirus Disease Outbreak in Call Center, South Korea. Emerging Infectious Diseases.
- 3 – Shen et al. (2020) Airborne Transmission of COVID-19: Epidemiologic Evidence from Two Outbreak Investigations. SSRN Preprint. [Note: This is a pre-print that has not yet been peer reviewed or been published in a journal at the time of this review’s publication.]
- 4 – Chen et al. (2020) Detectable 2019-nCoV Viral RNA in Blood is a Strong Indicator for the Further Clinical Severity. Emerging Microbes and Infections.
- 5 – Wang et al. (2020). Detection of SARS-CoV-2 in Different Types of Clinical Specimens. Journal of American Medical Association (JAMA).
- 6 – Colavita et al. (2020) SARS-CoV-2 Isolation From Ocular Secretions of a Patient With COVID-19 in Italy With Prolonged Viral RNA Detection. Annals of Internal Medicine.
- 7 – Cheung et al. (2020) Gastrointestinal Manifestations of SARS-CoV-2 Infection and Virus Load in Fecal Samples From a Hong Kong Cohort: Systematic Review and Meta-analysis. Gastroenterology
- 8 – Zheng et al. (2020) Viral Load Dynamics and Disease Severity in Patients Infected with SARS-CoV-2 in Zhejiang province, China, January-March 2020: Retrospective Cohort Study. BMJ
- 9 – Lin et al. (2020) Clinical Characteristics and Results of Semen Tests Among Men With Coronavirus Disease 2019. JAMA Network Open.
- 10 – Czumbel et al. (2020) Saliva as a Candidate for COVID-19 Diagnostic Testing: A Meta-Analysis. medRxiv [Note: This is a pre-print that has not yet been peer reviewed or been published in a journal at the time of this review’s publication.]
- 11 – Williams et al. (2020) Saliva as a non-invasive specimen for detection of SARS-CoV-2. Journal of Clinical Microbiology