According to the Hong Kong government, the Omicron variant of the coronavirus was introduced by a traveler from South Africa who had been in a quarantine hotel since arriving on November 11. Then on November 13th he tested positive.
It is assumed that the man infected a 60-year-old in the opposite room of the quarantine hotel, despite strict isolation. This tested positive on November 18th during its fourth PCR test. In both cases, later genome sequencing made it clear that they had been infected with the omicron variant of the coronavirus.
According to a statement by the Hong Kong government, the person entering from South Africa may not have worn an adequate face mask when he opened his hotel door to take food orders. This may be the reason for the infection despite quarantine isolation.