“We cannot be rest assured because of a few days of single-digit [increase],” Lam said. “Now it’s not a time to let one’s guard down and not a time to relax restrictive measures implemented earlier.”

She said the government would closely monitor the situation and could extend the curbs if necessary.

Lam’s message came after a leading health official called the daily figure – the lowest since March 12 when two cases were recorded – “a good sign”. Dr Chuang Shuk-kwan, head of the communicable disease branch at the Centre for Health Protection, also noted all new infections since Friday had been traceable.

“But still we should not be complacent,” Chuang said, stressing the need for people to practise good hygiene and physical distancing.

Sweeping curbs on public gatherings and social venues have transformed the city in recent weeks, put pressure on the economy and tested residents’ tolerance for confinement. Officials expressed alarm over scenes of people flocking to beaches and country parks over the long Easter holiday.

Dr Wong Ka-hing, the centre’s controller, said the government would review protection measures but

any policy change must account for the situation on mainland China and overseas, as the decision was more “dynamic” than it might appear.

Thirteen patients remained in critical condition and 37 had been discharged, according to Dr Linda Yu Wai-ling, the Hospital Authority’s chief manager of clinical effectiveness and technology management.

Yu gave further details about a malfunctioning of a system that helps to contain airborne contaminants at Princess Margaret Hospital in Kwai Tsing on Monday. A maintenance contractor

was repairing the system, which pushes air out of isolation wards, when a router failed, affecting seven floors and 69 patients – including 44 with the coronavirus. Staff were not told about the repair ahead of time and the contractor was reminded to notify hospitals before carrying out such work, Yu said.

But she said staff had worn protective gear, with patients remaining in their wards, so the chance of the virus spreading was low.

The Electrical and Mechanical Services Department called the repair routine but said it would investigate the “rare” incident.

Detailing the latest cases, Chuang said a 61-year-old Hong Kong man was treated in Thailand after visiting the United States and Germany. “There was a report of a negative test,” she said. “He also had a health certificate to show that he was fit for travelling.”

The man did not show any symptoms upon arrival on Sunday but his deep-throat saliva sample later tested positive. She doubted the man was reinfected and said the city’s “rather sensitive” testing might have accounted for the confirmation. Chuang said the centre was in touch with Thai authorities to see whether the patient was counted as a confirmed case there. If so, he would not be among Hong Kong’s tally.

Two sources earlier said the patient was Warren Mok, one of “China’s Three Tenors”, who fell ill in Thailand where he was admitted to hospital on March 17. He was discharged on Saturday and sent to Pamela Youde Nethersole Eastern Hospital in Chai Wan on Monday night.

Chuang said the student, 22, who returned from Britain tested positive through the private system.

Given there were tens of thousands of people under home quarantine, it would be impossible to test them all, she warned. “We will see whether we can offer testing to those with high-risk or returning from specific areas,” she said.

Additional reporting Alvin Lum