Vigilant mask wearing might have spared nearly 140 people from catching the coronavirus at a hair salon in Missouri, according to a report published Tuesday by the Centers for Disease Control and Prevention. In May, the people interacted with two hair stylists with confirmed coronavirus infections, but none ended up showing symptoms of COVID-19.

The team behind the study, which includes members of Missouri’s Springfield-Greene County Health Department, cannot be certain of all the factors that helped avert what might otherwise have been a disastrous outbreak. But policies instructing locals to cover their mouths and noses, put in place by the city of Springfield and by the salon where the stylists worked, Great Clips, appear to have played a substantial role in curbing the spread of disease.

“This really shows the power of face coverings, especially in indoor settings,” said Nadia Abuelezam, an infectious disease epidemiologist at Boston College who wasn’t involved in the study.

The findings reiterate what scientists have been saying for months: Face masks are an essential part of the disease-prevention tool kit, said Juan Gutiérrez, a mathematical biologist at the University of Texas at San Antonio who models coronavirus transmission but wasn’t involved in the study. “If we get that message out to people consistently, we might be able to contain this.”

Both stylists fell ill in mid-May. But they continued to work with clients for about a week after they started to feel symptoms, said Kendra Findley, a researcher at the Springfield-Greene County Health Department and an author of the study.

At the time, Springfield’s businesses had just begun to reopen, and confirmed coronavirus case numbers in the region were extremely low, said Dr. Robin Trotman, an infectious disease physician and an author of the study.

“There were days when we had one or two cases, max,” he said. Such low local prevalence may have been part of the reason that the first stylist chalked her cough and fever up to allergies and kept returning to work, Findley said. Against recommendations, she returned to her job even while awaiting her coronavirus test results, which she got two days after taking her test.

The second Great Clips employee fell ill within days of her colleague, although none of the other four stylists on staff ended up feeling unwell. The two sick stylists both eventually tested positive for the coronavirus, after which they were told by the salon to isolate at home.

By that point, however, the pair had come into close contact with 139 clients seeking haircuts, facial hair trimmings and perms — appointments that bring people within inches of each other for 15 to 45 minutes at a time, more than enough time for the virus to travel through the air from person to person.

And perhaps it would have, had it not been for the masks.

In the days after, health officials contact-traced all 139 people exposed to the stylists and asked them to self-quarantine for two weeks. None reported feeling sick during the 14 days that followed their salon appointments. The researchers also offered the clients free diagnostic tests for the coronavirus. Sixty-seven of them accepted; the rest declined. Of those tested, all turned up negative.

“I was shocked,” Findley said.

Follow-up interviews with 104 of the clients revealed that, in accordance with guidelines, patrons and stylists alike had worn masks for the duration of almost all the encounters documented by the study.

A small handful of clients had donned N95 respirators — devices designed to filter out 95% of airborne particles and one of the best forms of protection for health workers. Since the start of the pandemic, N95s have been in dire short supply.

But a majority of people in the study, including the two stylists, opted for cloth coverings or surgical masks — loosefitting accessories that don’t form an airtight seal around the face.

These products are imperfect. But several studies, including some initiated long before the pandemic’s start, have pointed to their usefulness in stymieing the spread of viruses from the wearer’s airway, Julian Tang, a virologist at the University of Leicester who wasn’t involved in the study, said in an email. To a lesser extent, they may also protect a user from incoming spray.

In this case, even homemade masks that can’t reliably snuff out every virus-laden particle seemed to do the trick — an incredibly encouraging finding, Gutiérrez said. “Had they not been using those masks, we would have expected a totally different situation,” he said.

Of course, masks alone can’t be considered a foolproof “safety net,” said Saskia Popescu, a hospital epidemiologist and infectious disease expert in Arizona who wasn’t involved in the study. “This is not an excuse to let you go about and do whatever you want, especially if you’re symptomatic.”

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