In this file photo dated Jan 26, 2021, police officers and health officials set up a "restricted area" in Yau Ma Tei covering several buildings and required all residents there to stay put and be tested for COVID-19. (PHOTO PROVIDED TO CHINA DAILY)

HONG KONG – The Hong Kong government on Thursday night cordoned off a building in Yau Ma Tei for targeted testing after a preliminary positive COVID-19 case involving the Omicron variant was detected in the area.

In a statement, the government said it set up a restricted area at Cheung Hing Building, 44-48 Pitt Street, Yau Ma Tei after a 36-year-old male cargo crew member tested positive for the mutant variant.

The government set up a restricted area at Cheung Hing Building, 44-48 Pitt Street, Yau Ma Tei after a 36-year-old male cargo crew member tested positive for the Omicron variant

Residents of the building were required to stay in their premises and undergo compulsory testing. The government said it expected to finish the operation by 7 am Friday.

In a separate statement, the Centre for Health Protection said the patient tested negative before he left for the United States on Dec 11. He returned to Hong Kong on Dec 13 and also tested negative at the airport.

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The patient underwent medical surveillance and regular compulsory testing and, except for the time he bought food Dec 15 and went to a mobile specimen collection station, he remained at home.

The patient tested preliminary positive for COVID-19 on Wednesday and developed symptoms the same day, the CHP said, adding that he had received two Comirnaty shots on March 22 and April 15 in Hong Kong.

Testing by the Department of Health's Public Health Laboratory Services Branch revealed that the spike protein gene of the patient's sample involved T478K mutation, the CHP said.

“Both N501Y and T478K are two of the targets to detect the Omicron variant, hence the CHP suspects at this stage that the patient carries the Omicron variant, but a whole genome sequencing still needs to be conducted to confirm whether the patient carries the Omicron variant,” it added.

The CHP said the places that the patient visited in Hong Kong during the incubation period will also be included in a compulsory testing notice.

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"The government has remained vigilant and has been closely monitoring the latest scientific data on mutant strains as well as the epidemic situation of various places. The most stringent anti-epidemic measures will be implemented to prevent the mutant strain from entering the local community," a spokesman for the CHP said.

At the moment, the adverse impact on the pandemic caused by the newly emerged mutant strains is not fully known yet, but vaccination is still essential to prevent severe cases and deaths from COVID-19 infection, the CHP added.