This photo taken on Feb 6, 2020, shows buildings in Tuen Mun, Hong Kong. (ANTHONY WALLACE / AFP)

HONG KONG – The Hong Kong government cordoned off a building in Tuen Mum in the New Territories on Thursday evening for targeted testing after a preliminary positive imported case of COVID-19 involving the L452R mutant strain was detected.

The government set up a “restricted area” at Block 6, Handsome Court, Tuen Mun at around 7:30 pm, requiring residents there to stay in their homes to get tested by midnight. The government said it expected to wrap up the operation by 7 am the next day.

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The government set up a “restricted area” at Block 6, Handsome Court, Tuen Mun at around 7:30 pm, requiring residents there to stay in their homes to get tested by midnight

In a statement, the Centre for Health Protection said that a 36-year-old female resident of the building had tested positive for the L452R mutant strain.

During the past 21 days, the patient stayed in Hong Kong from June 30 until July 12, when she traveled to Dubai. She returned to Hong Kong on Monday via Bangkok, Thailand, by flight EK384, the CHP said.

It added that the patient, who had no symptoms, gave a specimen upon arrival at the Hong Kong airport and tested negative for COVID-19. But her samples on day 3 after her arrival tested preliminarily positive.

“The test result by the Department of Health's Public Health Laboratory Services Branch revealed that the patient carries the L452R mutant strain but does not carry N501Y or E484K mutant genes,” the CHP said, adding her case has been classified as an imported one.

It also said that the patient received two doses of the BioNTech vaccine on May 9 and June 5 in Hong Kong and her blood specimen tested positive for anti-spike protein antibody on July 11.

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The woman works in Eton Tower, 8 Hysan Avenue in Causeway Bay, and last went to work on July 11. Her workplace and the places she had visited during the incubation period will be included in a compulsory testing notice, the CHP said.