Hundreds of Tung Chung residents at Caribbean Coast were sent to quarantine last night after a domestic helper and a 10-month-old baby of her employer tested preliminarily positive for the more infectious mutant Covid-19 strain.

The Centre for Health Protection believes the helper, 39, who has been working here since 2019, is the SAR's first infection from the community involving the N501Y and E484K strains.

She developed symptoms on Friday and the baby girl had diarrhoea yesterday.

Her employers are a female teacher at Ling Liang Church Sau Tak Primary School and a male physiologist at a Omni Mind Gym Group's Mong Kok branch.

Authorities have traced her whereabouts during incubation and issued mandatory test notices for St Thomas the Apostle Church, Fu Tung Plaza, Man Tung Road Park, Citygate mall, and Fusion supermarket at Caribbean Coast.

At 7pm yesterday, the 65-story tower 11 of Carmel Cove, Caribbean Coast, was cordoned off to test all residents from some 400 units by 2am, with the target of ending the lockdown by 7am.

Diploma of Secondary Education candidates living in the tower can leave earlier as long as they present the relevant documents and have tested negative.

Authorities said the unknown-source case carries a "risk of infection that is assessed to be likely higher."

This is the fourth time the strain was found in the community in two weeks, with the first three being an Indian who infected his friend and a Filipina helper. He was believed to have contracted the virus in his quarantine hotel.

Their infections triggered lockdowns in their places of residence in Parkes Building in Jordan and Kennedy Terrace at Mid-Levels and evacuations of all residents in the blocks for 21-day quarantine.

Residents from four Kennedy Terrace floors have issued a legal letter to the Department of Health for unreasonably quarantining them.

This came as Hong Kong yesterday saw 15 new cases: 13 from Nepal, one from India and one from Indonesia.

It also comes as authorities said the first two batches of over one million Comirnaty vaccine doses with packaging defects would be returned to BioNTech.

The German firm has found the loose vial caps and leakages to be caused by an imperfect crimping process but said no safety issues were involved. It and Fosun Pharma have arranged for replacements from another plant.

The packaging defects caused the SAR to suspend inoculations with BioNTech's Comirnaty vaccine from March 26 to April 4.

The Caribbean Coast lockdown at tower 11.

New mutant case sparks lockdown