“This really shows the power of face coverings, especially in indoor settings”

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 on 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 on 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, Ms. 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.