Rinse and Repeat? Mouthwash Might Mitigate COVID-19 Spread

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Multiple mouthwash and oral rinse products wiped out a human coronavirus closely related to the SARS-CoV-2 virus in a laboratory comparison study.

Listerine Antiseptic led the list of most effective mouthwashes for inactivating the coronavirus. Interestingly, a 1% nasal rinse solution of Johnson's Baby Shampoo also worked, eliminating up to 99.9% of the viral load in the in vitro experiments.

In contrast, use of a neti pot nasal solution yielded no decrease in virus levels.

The study was published in the Journal of Medical Virology.

Dr Craig Meyers

Because the mouthwash and hydrogen peroxide oral rinses in the study are widely available and easy to use, "I would recommend the use of the rinses on top of wearing mask and social distancing. This could add a layer of protection for yourself and others," lead study author Craig Meyers, PhD, professor of microbiology and immunology and obstetrics and gynecology, Penn State College of Medicine in Hershey, Pennsylvania, told Medscape Medical News.

Meyers and colleagues found that efficacy aligned with duration of time the cell cultures were exposed to each mouthwash or rinse product. Although it varied, the products required at least 30 seconds to kill most of the virus. Waiting 1 or 2 minutes tended to fortify results.

"This study adds to and further confirms the recently published evidence from virologists in Germany that mouthwashes can inactivate the virus that causes COVID-19 in a test tube," Valerie O'Donnell, PhD, co-director of the Systems Immunity Research Institute of Cardiff University, Cardiff, Wales, said when asked to comment on the study.

"While this is great to see, what is still lacking is in vivo evidence, since we know the virus will be continually shed in the mouth," O'Donnell said. "So, the question now becomes, by how much could mouthwashes reduce viral load in the oropharynx of infected people, and if so, then for how long?"

Meyers noted that studies of people positive for COVID-19 using each product would be informative. It remains unknown, for example, if swishing, gargling, and/or spitting out mouthwash would add or decrease the efficacy demonstrated in the lab.

The investigators used the human coronavirus HCoV‐229e as a surrogate for SARS-CoV-2. They noted HCoV-229e is analogous, and SARS-CoV-2 would have been more expensive, less available, and would have required biosafety level 3 laboratory conditions.

Listerine Antiseptic Leads the Way

"Surprisingly, we found that several of these common products had strong virucidal properties, inactivating from 2 log10 [or 99%] to greater than 4 log10 [or 99.99%] of infectious human coronavirus," the researchers note.

Table. Efficacy of Mouthwash and Oral Rinse Products
	30 Seconds	1 Minute	2 Minutes
Listerine Antiseptic	> 99.99%	> 99.99%	> 99.99%
Listerine Ultra	≥ 99.99% to < 99.99%	≥ 99.99%	≥ 99.99%
Equate Antiseptic	> 99% to < 99.99%	> 99% to < 99.99%	> 99.9% to ≥ 99.99%
CVS Antiseptic Mouthwash	> 99.9% to < 99.99%	≥ 99.9% to ≥ 99.99%	≥ 99.9% to ≥ 99.99%
Peroxide Sore Mouth	< 90% to 99%	> 90% to < 99.9%	> 90% to < 99%
Orajel Antiseptic Rinse	> 90% to < 99%	≥ 90% to < 99.9%	< 90% to < 99%
1.5% H₂O₂	< 90% to < 99%	< 90% to < 99.9%	< 90%
Crest Pro Health	≥ 99.9% to < 99.99%	> 99.99%	≥ 99.9% to > 99.99%
Betadine 5%	> 99.9% to < 99.99%	> 99.9% to > 99.99%	> 99.99%

The researchers added a small amount of organic material (extra protein) to each product to more closely mimic physiologic conditions in the nasopharynx.

Listerine Antiseptic "historically has claimed numerous antimicrobial properties," the researchers note. Although the label currently only claims to kill germs that cause bad breath, "our tests show that it is highly effective at inactivating human coronavirus in solution. Even at the lowest contact time of 30 seconds, it inactivated greater than 99.99% of human coronavirus."

Interestingly, the mouthwashes that contained the same active ingredients as Listerine Antiseptic — Listerine Ultra, Equate Antiseptic, and CVS Antiseptic Mouth Wash — were less efficacious. Meyers said the reason remains unclear, but he and colleagues found the same result when they repeated the comparisons.

Timing of the Essence?

Meyers and colleagues also tested a nasal rinse solution of 1% baby shampoo because it is sometimes used to treat people with chronic rhinosinusitis. They found 30 seconds led to < 90% to < 99.99% effectiveness, but that, by 2 minutes, efficacy climbed to > 99.9% to > 99.99%.

"Thirty seconds for some products just was not enough time for the efficacy to be observed," Meyers said. "Whereas, after a minute or two the active ingredient had enough time to work. Thirty seconds may be at the border to see full efficacy." More research is needed to confirm the timing and determine which active ingredients are driving the findings.

A future trial could test the efficacy of mouthwash products to reduce the viral load in people with COVID-19. "If we are able to get funding to continue, I would like to see a small clinical trial as the next step," Meyers said.

Meyers and O'Donnell disclosed no relevant financial relationships.

J Med Virol. Published online September 17, 2020. Full text

Damian McNamara is a staff journalist based in Miami, Florida. He covers a wide range of medical specialties, including infectious diseases, gastroenterology, and neurology. Follow Damian on Twitter @MedReporter. For more news, follow Medscape on Facebook, Twitter, Instagram, and YouTube.

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