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That same day, a 36-year-old man left South Africa for Hong Kong, a city with some of the strictest quarantine rules on earth. On Nov. 13, while staying in a designated quarantine hotel, he took a test that confirmed him as one of the earliest cases of Omicron.

Five days later, a 62-year old man staying in the room across the corridor also tested positive. He, too, had the variant, and the genomes of the two samples were so close that one had clearly caught the virus from the other, said Yuen Kwok-yung, an infectious diseases professor at the University of Hong Kong who advises the city’s government on their pandemic response.

CCTV monitors, however, showed that the two men had never met, nor opened their doors at the same time, nor shared any items, and had only contacted hotel personnel in full protective gear, said Prof. Yuen. Most likely, air from one of their hotel rooms spread into the hallway and through a door opening, where the other breathed it in, said Prof. Yuen, who inspected the site using a smoke test.