Closed-circuit television camera footage showed neither person left their room nor had any contact, leaving airborne transmission when respective doors were opened for food collection or COVID testing the most probable mode of spread, researchers at the University of Hong Kong said in a study published Friday in the journal Emerging Infectious Diseases.

Omicron, with an “unprecedented” number of mutations in the spike protein, has raised concern that it could evade vaccine-induced protection, worsen a surge in COVID cases and frustrate efforts to reopen economies. Some 450 researchers around the world have begun urgent studies to understand the extent to which omicron’s mutations may affect vaccine effectiveness and increase its transmissibility in a global effort that may yield answers in a few days, a World Health Organization scientist said last week.

“Detection of Omicron variant transmission between two fully vaccinated persons across the corridor of a quarantine hotel has highlighted this potential concern,” Haogao Gu, Leo Poon and colleagues wrote in the study.

COVID-19 cases have risen exponentially in parts of South Africa over the past month, heralding an omicron-fueled fourth wave of the pandemic disease. Dozens of countries have detected omicron infections. In Singapore, cases have “mostly displayed mild symptoms, and no omicron-related deaths have been reported so far,” the Ministry of Health said Sunday. Still, it’s too early to draw conclusions about the strain’s severity and omicron remains an “unknown threat,” it said.