The 48-year-old helper with the confirmed infection carried the N501Y mutation, which has been linked to more transmissive variants.

The woman, who arrived in Hong Kong from the Philippines on March 31, completed her quarantine period in the Dorsett Wanchai hotel on April 21. She visited the eighth floor of the Immigration Department building in Wan Chai and a clinic in Causeway Bay for a body check on the day she left the hotel. She moved in with her employers the next day.

“As she carried the N501Y mutation, and also had a travel history, and her antibodies also tested positive upon hospital admission, therefore we cannot rule out this could be an imported case,” said Dr Chuang Shuk-kwan, head of the communicable disease branch of the Centre for Health Protection.

Antibodies for the virus would suggest she was infected earlier.

But Chuang said the helper could also have contracted the infection locally as she had spent some time in the community.

“We will see later whether to classify her as a local case. At the moment we consider her as a possibly local case,” Chuang said.

In a statement on Tuesday evening, the centre said the Tsuen Wan case had been classified as a possibly local one with unknown sources.

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The helper was among four new cases confirmed on Tuesday. Also among the latest cases was a housewife who returned from India about a month ago and whose infection with the N501Y mutation triggered the quarantining of all residents in her Tsim Sha Tsui block on Monday night.

It took a month for her infection to be revealed. The Department of Health said the firm BGI, which has been embroiled in several performance controversies, was responsible for the tests during the woman’s quarantine in the Regal Oriental Hotel. The department said it was highly concerned about the quality of specimen collection and testing of its contractors.

The city’s tally of confirmed infections stood at 11,790, with 210 related deaths.

Fewer than 10 people tested preliminary-positive, including two other helpers whose infections were also discovered through the compulsory testing exercise covering the city’s 370,000 domestic workers.

The two cases involved a helper in Kornhill Block N and her friend in the Royalton at Pok Fu Lam. Both cases carried a variant.

The pair visited a McDonald’s outlet on Connaught Road in Central on April 25, and a branch of the Saizeriya restaurant chain in the Dragon Centre shopping mall in Sham Shui Po on Sunday. They also visited a small park near Victoria Park on Saturday.

The worker living in Kornhill shopped at Quarry Bay market twice a week. Her friend visited a Fairwood restaurant and a market in Kennedy Town, but authorities were still tracing her movements.

As the Kornhill helper had been in the city since 2019, both cases were classed as local infections, Chuang said.

Also among the preliminary-positive cases was the female employer of the helper in Tung Chung with the first untraceable locally transmitted infection of a variant.

The woman’s 10-month-old daughter had earlier been confirmed with the variant and Chuang said the mother tested positive for the mutated strain.

She teaches in Tung Chung at Ling Liang Church Sau Tak Primary School, which has been ordered to stop classes as a precaution. All staff, pupils and visitors to the school will have to undergo mandatory testing.

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Anyone who visited the Citygate Outlets mall in Tung Chung between April 12 and 24, and on April 26 will also have to undergo mandatory testing as the woman had shopped there.

The centre earlier had issued a mandatory testing order for visitors to the mall on April 11 after confirming three other cases carrying the N501Y variant had been there on that day.

Citygate said the mall had been disinfected on Tuesday.

The N501Y mutation has health officials especially worried as it has been linked to more transmissive variants, including those first identified in Britain, South Africa and Brazil.

03:26

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So far, 10 variant cases have been detected in the community, of which at least seven were classified as local infections.

Infectious disease specialist Dr Joseph Tsang Kay-yan warned that the recent infections among domestic workers suggested the virus had spread among their community.

“It is pretty certain to say there is a silent transmission chain going on among foreign helpers … and there could be more than one chain,” Tsang said.

He said it was important to identify whether the virus found in imported cases and locally infected workers had identical genomes. If so, it suggested quarantine measures had loopholes.

But respiratory medicine expert Dr Leung Chi-chiu said it would be more worrying if the two preliminary-positive helpers were also confirmed with variants.

“If they are not infected with variants, it will just be local transmission … it is not surprising to see if one among 370,000 workers got infected,” Leung said.

Dr Gilman Siu Kit-man of Polytechnic University’s department of health technology and informatics said the city was at a “very critical moment”.

“Learning from the fourth wave, we know one single imported case can spark a large wave of an outbreak,” Siu said, adding that any invisible carriers had to be traced.

Because of the threat of variants and to ease pressure on quarantine hotels, Siu said, the government could consider capping the number of incoming travellers from other Asian countries, where most imported cases with mutated strains come from.

Health officials also reclassified the cases of two women who returned from mainland China on April 20 as locally transmitted infections. Chuang said genetic sequencing found the virus they carried was similar to those circulating during the city’s fourth wave.