Health officials on Wednesday reported a preliminary positive local Covid-19 case, after Hong Kong lived through almost 60 days with no local infections.
The Centre for Health Protection said the man is a 43-year-old construction worker who lives in Tung Lo Court, Sham Shui Po, works in Wan Chai, and has no recent travel history.
The centre's Chuang Shuk-kwan said the patient hasn't been vaccinated.
He has been regularly tested for the coronavirus in recent months, and the latest one done earlier in the week returned preliminary positive.
But Chuang said the viral load in his samples were too low for them to tell if he was infected with any mutant strains, adding antibodies were found in the samples as well.
Chuang said the man was probably a "re-positive" case, but authorities were not able to pinpoint the exact time of his infection yet.
"The preliminary positive case has been undergoing regular testing, his regular testing started from May – although he had one or two tests at the end of March – so one of the possibilities is that he had an infection quite early on, maybe before May or March," she explained.
"At that time we did have quite a number of cases in the community, so it's possible he got infected but he was not detected," said Chuang.
But she said if the man has indeed been infected recently via an unknown source, it means there is silent transmission in the community.
She said, for precaution's sake, people who have been to places the man visited, including some restaurants in Kwai Fong, Shek Kip Mip and Wan Chai, will have to be tested, while his family will be quarantined. The construction sites he worked at also had to be closed temporarily.
Separately, Chuang said a 13-year-old girl and her family are being sent to a quarantine camp, as the girl was a close contact of a patient in Macau who came down with the Delta variant.
She said the girl and the Macau patient went on a dancing trip in Xi'an together last month. The girl returned to Hong Kong on Monday and tested negative on Wednesday.
There were two imported cases from the US and the United Arab Emirates.