Claim that mouthwash could reduce the spread of COVID-19 has not yet been confirmed by clinical trials in humans
Studies showing that oral rinses can inactivate certain coronaviruses in a laboratory setting may not apply to humans. There is currently no scientific evidence demonstrating that these products can reduce person-to-person transmission of COVID-19. Clinical trials demonstrating that oral rinses can effectively reduce the level of infectious virus in the mouth have not yet been completed. Therefore, people should continue to practice effective measures for reducing COVID-19 spread, such as physical distancing, good hand hygiene, and the use of face masks.
Overstates the scientific impact of a finding: A few studies show that some oral rinses can effectively inactivate coronaviruses by direct contact in a test tube, but no clinical data validate their efficacy in the human mouth. At this point in time, the claim that oral rinses can reduce the transmission of the virus that causes COVID-19 is unsupported.
Articles such as this one claiming that “mouthwashes could help curb COVID-19” circulated on social media in October 2020, receiving more than 27,000 interactions on Instagram and Facebook in one week according to the social media analytics tool CrowdTangle. These articles are based on a September 2020 study published in the Journal of Medical Virology that investigated whether over-the-counter oral and nasopharyngeal rinses could reduce the infectivity of certain coronaviruses by breaking their outer membrane through direct contact in a test tube.
Coronaviruses are surrounded by a lipid bilayer, or a membrane composed of fats, that can be disrupted by detergents, such as soaps. Detergents cause structural damages to the lipid bilayer that can physically destroy viral particles and/or remove their ability to cause infection, depending on the type of virus and the detergent used. Therefore, handwashing is an effective measure for reducing the spread of COVID-19 and other infectious diseases. Other common chemicals, such as ethanol, iodine, and hydrogen peroxide, also break the outer membranes of viruses and are highly effective at decontaminating surfaces. Since most oral rinses contain one or more of these chemicals, researchers proposed that mouthwashes could be a potentially useful strategy to reduce the number of infectious viral particles in the mouth of infected individuals and consequently the risk of transmitting different types of viruses, including SARS-CoV-2, the virus that causes COVID-19.
In the study published by the Journal of Medical Virology, the researchers tested the efficacy of 11 different nasal and oral rinsing products by mixing each product with a suspension of the human coronavirus type 299E, which causes the common cold, for 30 seconds, one minute, or two minutes. To calculate the percentage of the virus inactivated by each oral rinse, they added each mixture of virus and oral rinse to human cell cultures and counted the number of surviving cells after several days. The results from the study showed that most oral rinses inactivated 90% of infectious viral particles after one minute.
Based on these results, some articles claimed that mouthwash could reduce the amount of infectious SARS-CoV-2 virus in mouths, thereby reducing the spread of COVID-19. However, this claim is unsupported because the study authors did not specifically test the efficacy of oral rinses on SARS-CoV-2, nor did the authors test the efficacy of these oral rinses in humans.
A previous study published in The Journal of Infectious Diseases also found that some oral rinses could efficiently inactivate several SARS-CoV-2 strains, although these experiments were also conducted in test tubes, not in people. Scientific findings observed in such controlled laboratory environments may not be replicated in humans, as the laboratory does not accurately reproduce the complex environment of the human body. Therefore, researchers still need to conduct clinical trials in humans to determine if the effects observed in the laboratory can be reproduced in people.
Several factors specific to the human body can influence the efficacy of mouthwash in viral inactivation. For example, as Health Feedback explained in this previous review, the constant production of saliva dilutes the effect of an oral rinse in the mouth. The chemical composition of saliva also differs from the liquid medium used in the laboratory and might influence the effect of oral rinses on infectious viral particles. In addition, SARS-CoV-2 is not only present in the mouth of infected people, but it can also infect different types of cells along the respiratory tract. Even if oral rinses could efficiently inactivate SARS-CoV-2 in the mouth, infected cells from the nasal cavity, bronchia, and lungs would continually shed new infectious viral particles.
While there are several studies demonstrating the efficacy of mouthwash in inactivating viruses, clinical studies of the virucidal action of oral rinses in humans are scarce. In 2005, a clinical trial involving 20 subjects with recurrent oral herpes outbreaks reported that a 30-second rinse with a solution containing ethanol and essential oils reduced the amount of virus in their saliva for at least one hour.
However, these results cannot be extrapolated to SARS-CoV-2. A small study on herpesviruses showed that viral shedding in the mouths of infected people occurred mostly periodically and during a limited period of time. In contrast, cells infected with coronaviruses shed new viral particles almost constantly when the host is infected. Therefore, a greater and more frequent use of oral rinses would likely be needed to maintain low levels of infectious coronavirus as compared to herpesviruses.
In the absence of a vaccine or effective treatments for COVID-19, researchers are exploring oral rinses as a potentially simple and inexpensive measure to reduce the viral load and transmission of SARS-CoV-2 in infected individuals. At the time of this review’s publication, ten clinical trials to investigate whether oral rinses can reduce the levels of SARS-CoV-2 in the mouth of infected people are scheduled or already recruiting volunteers in the U.S. and other countries. These clinical trials may provide further insights about a potential use of oral rinses to reduce the spread of COVID-19.
In summary, the claim that mouthwash could reduce COVID-19 spread is currently unsupported. Clinical trials examining whether oral rinses reduce the amount of infectious SARS-CoV-2 in the human mouth are necessary to further determine whether oral rinses have the potential to reduce person-to-person transmission of SARS-CoV-2. Given the lack of scientific evidence supporting the use of mouthwash for reducing the spread of COVID-19, mouthwash should not be used as a substitute for effective public health measures that reduce the spread of COVID-19, such as physical distancing, good hand hygiene, the use of face masks.
This article from The New York Times also analyzes the results from the study published in the Journal of Medical Virology, including comments from several scientists.
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