Two women, 27 and 48, and a man, 57, arrived in Hong Kong last Wednesday from the Vietnamese city, The Standard newspaper reported quoting a source from the island's Center for Health Protection.
The women had received two doses of the AstraZeneca vaccine in Vietnam, and the man had received two doses of BioNTech in Hong Kong.
They had the L452R mutant strain, found in the Delta variant, but no symptoms, the newspaper said.
Vietnam is now listed as a medium-risk area under Hong Kong's five-tier system for overseas Covid threats. This means fully vaccinated travelers are quarantined for seven days on arrival at an approved hotel and must furnish a vaccination certificate and negative PCR test.
Hong Kong has recorded at least five Omicron cases while Vietnam has not detected any so far. But it is seeing a steady rise in Delta infections.