The omicron variant has been identified in two coronavirus cases in a Hong Kong quarantine hotel where scientists believe the virus spread through the air in a hallway.

Two travelers, one who had arrived from South Africa and one from Canada, were staying across the hall from each other in separate rooms. Neither person left his room during the quarantine, and no items were shared between the travelers, according to the study by scientists at the University of Hong Kong published Friday in the journal Emerging Infectious Diseases.

The lack of contact between the travelers, confirmed by a review of surveillance camera footage, suggested that “airborne transmission” in the hallway was the most likely way the virus spread between the two people, the study said.

The traveler from South Africa, a 36-year-old man, had arrived on Nov. 11 and tested positive for the coronavirus two days later. A 62-year-old man had arrived from Canada on Nov. 10, and on Nov. 18 tested positive for the virus. Both men were vaccinated.

When opening his door to retrieve delivered meals and deposit trash, the traveler who came from South Africa sometimes wore a mask with a valve or wore no mask at all, researchers said last month.

The Hong Kong health department said last month that the hallway had “unsatisfactory” air flow. According to the study, none of the 12 other travelers staying on the same floor tested positive, nor did any of the hotel staff.

Yuen Kwok-yung, a leading microbiologist at the University of Hong Kong who advises the Hong Kong government on covid-19 policies, called valve-equipped face coverings “selfish,” explaining that these vented designs “filter air when inhaling, but when the air is breathed out through the air valve, it is not filtered, which is not good,” the South China Morning Post reported.

Most travelers have to undergo two to three weeks of quarantine at a government-appointed facility upon their arrival in the international financial hub. As countries across Southeast Asia begin to reopen their borders, Hong Kong authorities have steadfastly stuck to strict quarantine measures and prioritized efforts to establish a quarantine-free travel agreement with mainland China.

The Regal Airport Hotel did not respond to questions about how it enforces mask-wearing requirements for guests in mandatory quarantine, or what measures it plans to take in the future to prevent cross-infection.

The hotel has promised that staff would wear masks at all times. The Hong Kong government also asks that all hotel quarantine guests wear masks when they open their room doors, but hasn’t publicly specified any punishment for violations.

The company 3M, which designs valve-equipped masks for construction work, says these face coverings allow for easier breathing as wearers take part in physically demanding tasks in “hot or humid” conditions.

The U.S. Centers for Disease Control and Prevention has warned against wearing face masks with exhalation valves or vents, saying they can allow virus particles to escape, and some major airlines have banned them.