“We can’t let down our guard on N501Y,” said Dr Chuang Shuk-kwan, head of the Centre for Health Protection’s communicable disease branch. “Not every single case will cause a big community outbreak, but one case can still lead to that. We know N501Y is highly transmissive. Will a small amount of viral load cause a spread? I also don’t know.”

Officials confirmed another nine Covid-19 cases on Friday, all but one of which were imported from the Philippines, India, Indonesia and the Netherlands. The sole local case, which was untraceable, involved a 23-year-old woman living in Block 10 at South Horizons Phase 2 in Ap Lei Chau, triggering what was expected to be an overnight lockdown while testing is carried out.

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. The city has recorded more than 230 cases involving the mutant strain, but almost all of them were imported.

The domestic worker was isolated at the Ramada Hong Kong Harbour View from March 27 to April 16, according to Chuang. For four of those days, a family of three stayed in the room next to the woman, two of whom were later found to be infected with the virus carrying the N501Y mutation and deemed imported cases.

Anyone who stayed on the same floor during the four days would be tracked and sent into quarantine.

“At the moment we cannot prove whether the transmission was from the room next door, and could only know that if we did a genetic analysis,” she said. “We will consult experts to see whether we need to do a more detailed inspection.”

In considering other possible ways the virus spread, Chuang earlier noted the ventilation pushed air first into the room from the corridor and then out from the bathroom, which would make the spread of the virus from room to room difficult.

The S-shaped hooks affixed to door handles for food deliveries and which were the suspected route of transmission in the first local case of the mutant strain had tested negative for traces of the virus, she added.

“Of course, we can’t rule them out,” she said. “But we can’t prove it was the medium for transmission either. We will look at whether there are other common facilities or objects.”

What you need to know about the mutant coronavirus strains in Hong Kong

Inspecting the hotel at night, University of Hong Kong microbiologist Professor Yuen Kwok-yung suspected the air from the contaminated room could enter the corridor when opening the door while the windows remained open.

“When the adjacent room opens the door, the air infused with the virus can therefore enter the room,” he said, adding the hotel’s ventilation met standards.

Yuen urged hotel occupants to wear a mask and close the window when opening the door, and for management to install air purifiers in the corridor and reduce the occupancy rate. He could not conclude whether the woman was already infected when she arrived in the city or if she was contracted the virus at the hotel.

The domestic worker moved to her employer’s residence at No 8 Kennedy Road after completing her isolation, and had visited Melbourne Plaza in Central, the eighth floor of the Immigration Department building in Wan Chai and an employment agency in the Kam On Building on Queen’s Road Central, according to Chuang.

The worker, who was asymptomatic, was screened again at a community testing centre in Wan Chai on Thursday and the result came back as preliminary-positive before her case was confirmed by the centre late on Friday night.

She is the third locally detected case of the N501Y strain. The first one, confirmed last Saturday, involved a 29-year-old engineer arriving from Dubai, while the second concerned his friend, a 31-year-old woman who did not have any recent travel history.

Government pandemic adviser Professor David Hui Shu-cheong said that genome sequencing would be important in determining the source of the woman’s infection. 

“We need to see whether the genome sequencing of the woman’s viral samples was the same as the people staying next door [during her quarantine],” Hui said. “If they are the same, we then can not rule out environmental factors.” 

The woman was highly unlikely to have been infected in the Philippines and had a long incubation period, as the time between her dates of arrival and testing positive was too far apart from each other.

Experts on Friday had also almost completed retesting an “unusual” batch of preliminary-positive results returned by the Chinese testing firm BGI two days prior.

The 30 preliminary-positives, revealed to the public on Thursday, had raised eyebrows among health officials as the cases were scattered across different districts and had low viral loads. Chuang said 78 close contacts of the cases who had been sent to quarantine would be released once the cases were confirmed to be virus-free.

Officials said the suspension covering the mobile testing stations would remain in effect until an investigative report on the mistakes was submitted to the government, which did not rule out reducing other screening services provided by BGI.

Anthony Wu Ting-yuk, a director of Sunrise Diagnostic Centre, a joint venture company of BGI in Hong Kong, apologised for the incident, saying the firm had stepped up its monitoring and training for staff.

Contamination may be behind 30 positive Covid-19 results at Hong Kong lab

Authorities also revealed that a 43-year-old man with a history of hyperlipidaemia and obesity had a heart attack and died on Thursday. He had received a BioNTech shot on April 18. A woman, 66, also died about 10 days after she received the German-made vaccine. The two cases would be assessed by experts.

As of Friday, 1,222,814 vaccines have been administered in Hong Kong. Some 806,626 people received their first dose, while 416,188 people have taken their second jab, representing a full vaccination rate of 5.5 per cent. A total of 558,974 people took the BioNTech vaccine and 663,840 took the Chinese-made Sinovac.

The city’s tally of confirmed infections stood at 11,727 with 209 related deaths.