A rash of provocative headlines recently offered a tantalizing idea: that mouthwash can “inactivate” coronaviruses and help curb their spread. The stories sprang from a new study that found that a coronavirus that causes common colds — not the one that causes COVID-19 — could be incapacitated in a laboratory when doused with mouthwash. The study’s authors concluded that the products they tested “may provide an additional level of protection against” the new coronavirus.
But outside experts warned against over-interpreting the study’s results, which might not have practical relevance to the new coronavirus. Not only did the study not investigate this deadly new virus, but it also did not test whether mouthwash affects how viruses spread from person to person.
“I don’t have a problem with using Listerine,” said Angela Rasmussen, a virologist at Columbia University. “But it’s not an antiviral”.
The study, which was published last month in The Journal of Medical Virology, looked only at a coronavirus called 229E that causes common colds — not the new coronavirus, which goes by the formal name of SARS-CoV-2 and causes far more serious disease. Researchers can study SARS-CoV-2 only in high-security labs after undergoing rigorous training.
The two viruses are in the same family, and, in broad strokes, look anatomically similar, which can make 229E a good proxy for SARS-CoV-2 in certain experiments. But the two viruses shouldn’t be thought of as interchangeable, Rasmussen said.
The researchers tested the virus-destroying effects of several products, including a watered-down mixture of Johnson’s baby shampoo — which is sometimes used to flush out the inside of the nose — and mouthwashes made by Listerine, Crest, Orajel, Equate and CVS. They flooded 229E coronaviruses, which had been grown in human liver cells in the lab, with these chemicals for 30 seconds, one minute or two minutes — longer than the typical swig or spritz into a nose or mouth. Around 90% to 99% of the viruses could no longer infect cells after this exposure, the study found.
“It’s clear that if these compounds come into contact with the virus, they can inactivate it,” said Craig Meyers, a researcher at Pennsylvania State University, who is an author of the study.
But because the study didn’t recruit any human volunteers to gargle the products in question, the findings have limited value for the real world, other experts said. The human mouth, full of nooks and crannies and a slurry of chemicals secreted by a diverse cadre of cells, is far more complicated than the inside of a laboratory dish.
Nothing should be considered conclusive “unless human studies are performed,” said Maricar Malinis, an infectious disease expert at Yale University.
Meyers said he hoped his team would undertake such studies soon, and noted that a handful of clinical trials had already begun investigating these questions.
The study’s findings aren’t necessarily surprising, or even unprecedented. Other researchers have conducted similar experiments, including one that looked at the effects of mouthwash on the new coronavirus, with comparable results.
And since the early days of the pandemic, scientists have stressed the effectiveness of hand-washing and disinfection with soap, alcohol and other similar chemicals that can bust through the new coronavirus’s fragile outer layer, or envelope. (Not all mouthwashes or nasal rinses contain such potent ingredients, however.)
Valerie Fitzhugh, a pathologist at Rutgers University, pointed to a study from the 1990s, in which researchers in Texas inactivated a strain of flu virus by swirling it in Listerine for 30 seconds. But that study “was never shown to be clinically relevant,” she said.
Even if people did a very thorough job coating the inside of their mouths or noses with a coronavirus-killing chemical, a substantial amount of the virus would still remain in the body. The new coronavirus infiltrates not only the mouth and nose, but also the deep throat and lungs, where mouthwash and nasal washes hopefully never enter.
Viruses that have already hidden away inside cells will also be shielded from the fast-acting chemicals found in these products. “It’s not like your cells get infected and then they secrete a bunch of virus and they’re done,” Rasmussen said. “Infected cells are constantly making more virus. It’s a timing issue.”
Relying on mouthwash or a nasal rinse to rid the body of infectious virus would be about as futile as trimming the top of a cluster of weeds, paying the roots little mind, and expecting the garden pests to disappear.
Meyers acknowledged this limitation. After a quick swizzle of mouthwash, “How long do you have? I don’t know,” he said. “All we’re saying right now is, this could add an extra layer of protection,” he explained, on top of proven protective measures like mask-wearing and physical distancing.
Rasmussen said she remained cautious. In the context of the pandemic, a false sense of security could be dangerous, she noted, adding that people shouldn’t rely on bathroom products to cleanse their bodies of the coronavirus.
“You can use mouthwash to reduce your own chance of getting gingivitis,” she said. “I don’t think it’s going to have a meaningful impact on your ability to transmit this virus.”
Katherine J. Wu c.2020 The New York Times Company
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