The two other cases with unknown sources of infection were two housewives living in Tai Po and Sham Shui Po, both of whom went to hospital on Tuesday after developing symptoms.

Explainer: Hong Kong is opening up again – here’s what you can and cannot

The latest infections marked a drop after cases bounced back from two straight days of single digits to 16 on Wednesday. The city’s overall case tally stood at 10,820, with 197 related deaths.

The new cases came as police were investigating the assault of a McDonald’s worker at the fast-food chain’s Choi Wan Estate branch in Ngau Chi Wan.

Police said a man, aged 20 to 30, was suspected of assaulting the employee, 26, with his mobile phone as the worker tried to help a customer operate the government’s “Leave Home Safe” app, the use of which is now mandatory for many businesses.

“The male employee was conscious after the attack and sent to United Christian Hospital for treatment. Police are still searching for the man,” a police spokeswoman said.

The suspect was said to be around 1.75 metres tall and wearing dark clothing. Officers received the report at about 7am and said the case was classified as assault occasioning bodily harm.

Hong Kong lifted tough restrictions in place since the outbreak of the city’s fourth wave of infections in November. Other stringent measures have included stepped-up mandatory testing and “ambush-style” lockdowns.

Under the relaxed rules, dine-in service at restaurants has been extended from 6pm to 10pm, while the number of people allowed per table has increased from two to four. Other premises, such as gyms, cinemas, theme parks, and beauty and massage parlours, have also reopened.

But patrons will need to either download the government app – which records the places they visit and stores them on their phone for 31 days – or leave their personal details for contact tracing via other methods.

Users will receive an alert if a confirmed case is identified at any of the venues they have visited.

A staff member at the McDonald’s branch where the attack took place said customers there had also been allowed to write down their personal information if they preferred not to use the app.

McDonald’s Hong Kong said it cared about the safety of employees very much and expressed concerns over the incident. It hoped similar incidents would not happen again.

Restaurant industry leaders had previously expressed concern over the implementation of the new contact-tracing rule, fearing it could spark disputes between customers and frontline staff.

Meanwhile, a staff member at a beauty salon in Central said bookings were filled immediately they reopened, as some customers who had already bought packages had been waiting a long time for their services to resume.

Hong Kong’s gyms and beauty parlours ready for post-lockdown rush

The employee, who declined to be named, said more customers preferred to leave their personal information rather than use the app. There had been some confusion over use of the app, she added, prompting a few customers to complain about the wait.

Health authorities, meanwhile, apologised over 16 false positive cases found among virus tests conducted between January 29 and February 1 by private lab BGI. An investigation report submitted by the Shenzhen-headquartered company to the government cited human error, reagent residue or contamination and environmental contamination as possible causes.

No further false positive cases were discovered after the firm took corrective measures from February 4, health officials said.

Separately, government officers conducting an enforcement operation on Thursday morning found 65 people who violated compulsory testing notices, issuing them HK$5,000 fines and ordering them to get screened within a specified period.

The original notices had been among those issued at eight To Kwa Wan buildings last week, and the test records of some 790 residents were checked by officers during Thursday’s operation.