The lock down started at 7pm Tuesday and is expected to last till 7am Wednesday.

The preliminary positive case is a 22-year-old man who lived in the building. He left Hong Kong in mid-April, and returned on September 12 from Serbia on flight QR818.

He tested negative at the airport, and again tested negative in four subsequent tests during hotel quarantine.

He finished quarantine on September 26 and returned home. But he tested preliminary positive in a test on September 27 at the community testing center at Ngau Tau Kok Road Sports Centre, Kwun Tong.

As he was confirmed with Covid-19 in India back in April, he is suspected to be a re-positive case.

As he previously lived in the North Point building and the case is very likely to involve a mutant strain, authorities locked down the building for tests.

The center is also investigating two other preliminary positive cases.

One is a 23-year-old woman who lives in Tung Yik Building in Yuen Long. Another is a 24-year-old woman who lives in Fortune Garden, Tam Mei.

They left Hong Kong for Jordan on September 12 on flight QR817, and returned to Hong Kong on QR818 on September 27. They tested positive at the airport, and were found to carrying the L452R mutant strain.

They were fully vaccinated and were asymptomatic.

The places they have visited will be included in mandatory testing notices.