Harbour Pinnacle on 8 Minden Avenue has been locked down from 7pm Tuesday. Authorities aim to finish the exercise by 7am Wednesday.

The two male crew members, aged 29 and 57, lived in Harbour Pinnacle and a house in Headland Village, Discovery Bay respectively.

They are cargo flight crew members of the same airline. They left Hong Kong on October 31 for Germany and returned to Hong Kong on November 6 by the same flight (CX2066) from Germany.

Their specimens collected upon arrival at Hong Kong International Airport tested negative. However, samples collected on Monday at Henry G Leong Yaumatei Community Centre and a mobile specimen collection station at Tung Chung North Park tested preliminary positive for Covid-19.

Their specimens carried the L452R mutant strain, and the 29-year-old's sample had a low Ct value – a warning sign that there is a high risk for the patient to infect the others.

Due to presence of a more transmissible mutant strain and risk of infection, the government has locked down the Tsim Sha Tsui building and ordered residents at the house where the 57-year-old patient resided to undergo quarantine.

Authorities also announced two more preliminary positive cases, and said the places they visited will be named in a compulsory testing notice.

One case was a 30-year-old female patient who is a foreign domestic helper. She arrived in Hong Kong on October 17 from the Philippines by flight CX906. The results of the six tests conducted during the compulsory quarantine period at Silka Tsuen Wan Hotel were all negative.

Upon completion of the compulsory quarantine on November 7, she stayed at Rosedale Hotel in Causeway Bay. She underwent another test on November 8 and the test result was positive but with low viral load.

The other case was a 15-year-old female patient who arrived in Hong Kong on October 14 from Italy via Qatar by flight QR818. She tested negative during the compulsory quarantine period.

Upon completion of the compulsory quarantine on November 4, she stayed at Gateway Hotel, Hong Kong in Tsim Sha Tsui. She underwent test on November 8 at Henry G Leong Yaumatei Community Centre and the test result was positive.

The possibilities of these two cases being re-positive cases cannot be excluded at the moment.

Meanwhile, existing social distancing measures will be extended for 14 days till November 24.

A spokesman for the Food and Health Bureau said social distancing measures have to be maintained, as the government strives to protect public health and at the same time foster favorable conditions for resuming cross-border travel with the mainland.

"In addition, in order to enhance the efficiency of contact tracing upon confirmation of an infected case as a way to protect the safety of members of the public and employees, the government is exploring extending the use of the 'LeaveHomeSafe' mobile application in a comprehensive manner in catering premises and suitable scheduled premises. Details will be announced later,” he said.

The spokesman also said that the current vaccination rate for the elderly aged 80 or above in Hong Kong is only 15 percent, meaning that around 85 percent of the elderly in that age group remain in danger.

"The BioNTech and Sinovac vaccines are highly effective in preventing severe cases and deaths from Covid-19. They can provide effective protection to those vaccinated in preventing serious complications and even deaths after infection," he said.

The government called upon senior citizens, chronic patients and other immunocompromised persons who face a much higher chance of death upon Covid-19 infection to get vaccinated as early as possible, before the fifth wave strikes Hong Kong.