TORONTO -- The case of two hairstylists who developed symptoms of COVID-19 and then saw 139 clients before they stopped working, seemingly without passing on the virus to any of them, demonstrates the effectiveness of mandatory face mask policies, according to new research.

The story of the salon in Springfield, Mo., has already been well-publicized, but a report released Tuesday sheds new light on how the stylists were able to work while having COVID-19 for about a week each and not transmit the novel coronavirus.

It all started May 12, when one stylist at the salon first noticed respiratory symptoms. Three days later, a second stylist developed similar symptoms. Three days after that, the first stylist took a test for SARS-CoV-2, and kept working despite a recommendation to self-isolate.

Two days later – May 20 – the first stylist tested positive for the virus. The second stylist took a test on that day too, and heeded the suggestion to self-isolate. Her test also came back positive.

Springfield's public health unit then got to work with contact tracing, getting in touch with all 139 clients the two stylists had seen between when they first noticed their symptoms and when they stopped working. Some of them had been in close contact with the stylists for as long as 45 minutes; the average was closer to 20 minutes.

The contact tracers found that none of the clients showed any COVID-19 symptoms over the two weeks following their appointments, and neither did any of their close contacts. About half of the clients agreed to be tested for the virus; every single test came back negative.

It wasn't that the stylists were incapable of transmitting the virus. The first stylist's husband developed symptoms and tested positive, as did her daughter and son-in-law, and their roommate.

This is why the researchers believe the masks helped prevent the stylists from passing on the virus to their clients. Face masks were mandatory in salons in Springfield at the time, and both stylists wore cloth or surgical masks when working, as did all 139 clients. But the stylists did not wear masks when at home or when at the salon with no clients around – likely making it easier for the first stylist to transmit the virus to her family and her colleague.

"Both company and city policies were likely important factors in preventing the spread of SARS-CoV-2 during these interactions between clients and stylists," the researchers wrote.

"These results support the use of face coverings in places open to the public, especially when social distancing is not possible."

Although the researchers concluded that mask use is the most likely explanation for none of the 139 clients catching the virus from the stylists, they said it wasn't the only possible factor. Glove use and hand hygiene could have also been factors – there was no data collected on this topic – and there may have been low potential for any exposure to the virus because the salon was only performing low-risk activities such as haircuts, facial hair trimmings and perms.

Additionally, the researchers said, it is possible that the stylists could have transmitted the virus to clients they saw in the two or three days before their symptoms began, as these clients were not part of the contact-tracing effort.

Their work was published in the U.S. Centers for Disease Control's Morbidity and Mortality Weekly Report.