“After reviewing all data, we believe the fourth wave has come to an end and we have maintained zero local infections for 28 days,” said Yuen, who concluded local cases in the past few weeks were not recent or community infections.

The time frame, which covers two 14-day virus incubation periods, is generally seen as the benchmark for defining whether a place has reached zero local infections. Hong Kong last recorded a local case with an untraceable source on April 23.

Health officials confirmed one new imported case involving a 42-year-old woman who arrived on Friday last week from Indonesia. She tested positive on her seventh day of hotel quarantine.

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Fewer than five preliminary-positive cases were reported. The latest confirmed infection pushed the official tally to 11,837, with 210 related deaths.

Sources familiar with government policy said zero infections could ease Beijing’s concerns and help pave the way for quarantine-free travel to mainland China, with a scheme expected to kick off as early as next month for business travellers to neighbouring Guangdong province first.

Despite the end of the fourth wave, which hit the city last November, Yuen warned against easing Covid-19 rules, as another surge could occur, involving variant strains of the virus.

“Now is not the time to relax social-distancing rules, rather we need to prepare for the impending fifth wave of infections,” he said.

The most important infection control measure was vaccination, he stressed, and if the inoculation rate reached 80 per cent of the city’s 7.5 million population, perhaps masks would no longer be necessary.

Yuen said society should be able to carry on with normal life as much as possible, with those who had been vaccinated able to enjoy some exemptions. But the movement of unvaccinated people in the community should be limited.

He suggested opening hours of restaurants and entertainment premises should not be restricted for inoculated patrons and floated the idea that unvaccinated people should only be allowed to dine in for lunch but not dinner.

Yuen also agreed quarantine rules should be relaxed for fully vaccinated arrivals on condition tests showed they had virus antibodies and underwent repeated screening. He proposed making all arrivals take antibody tests eventually.

Dr Chuang Shuk-kwan, head of the communicable disease branch of the Centre for Health Protection, said local authorities would have to assess their testing capacity and methods if the government decided to screen immunised travellers for antibodies.

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Addressing vaccine hesitancy among residents, Yuen said it was “puzzling” why authorities did not do a better job to rectify disinformation.

“There are misconceptions even among medical workers and nurses that if they get the jab, they will die,” Yuen said, stressing such claims had no scientific basis.

He urged authorities to take a scientific approach to explain vaccines were safe and that adverse reactions happened to a minority.

Fully vaccinated secondary and tertiary students could continue to attend in-person classes even if another wave occurred, Yuen suggested. Health officials previously said they were looking into expanding vaccination coverage to the 12-15 age group.

“Those vaccinated will learn in school, but those not vaccinated will learn online [if a fifth wave arrives],” Yuen said.

Secretary for Education Kevin Yeung Yun-hung said his bureau would consider ways to make it easier for teachers and pupils to get jabs on campus if authorities lowered the age.

“We do hope schools can return to a normal class resumption when the new academic year begins in September,” Yeung said.

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Bookings for jabs surged on the back of news of the lottery and lucky draws. Online Sinovac bookings hit a two-month high of 10,500 on Friday, with the same figure again on Saturday. The last peak was on March 17 with 15,000, but by April and May, that figure gradually fell to around 2,000 to 4,000 daily. The lowest was 1,800 bookings on April 11.

The mark for BioNTech on Saturday was about 17,700 – one of the highest in the past month. Daily bookings for April had hovered beyond 10,000, but the figure dropped by mid-May, with around 6,000 on some days.

Officials hailed the incentives dangled by the business sector, including free flight tickets and the lottery grand prize of a Kwun Tong home, with the health minister warning that those who adopted a wait-and-see attitude would not be eligible for a potential booster shot if they had not taken their first two doses.

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Secretary for Food and Health Sophia Chan Siu-chee also warned about a possible fifth wave, and urged the public to get vaccinated.

“Our main focus is to prevent the fifth wave of infections from happening, as you can see the situation could suddenly turn bad in other jurisdictions,” she said.

Chan said the slow inoculation rate would affect the purchase of the next batch of vaccines.

Around 998,000 Sinovac doses and 744,000 BioNTech shots are still in storage.

Hong Kong is far below its target rate of vaccinating 70 per cent of the population to achieve herd immunity. As of Saturday, the rate for a first shot was 19.1 per cent, and 13.2 per cent for both doses.

Yuen, meanwhile, reviewing several locally detected Covid-19 cases reported in the past month, said the data suggested they were either not infected recently or the infections were not real.

Of five cases, involving domestic helpers and a construction worker, who had a recent travel history, Yuen said all carried a type of antibody that indicated an infection a few months ago, and at least three did not carry a strain linked to the fourth wave.

In the case of a four-year-old boy who was confirmed two weeks ago, Yuen maintained his belief the child was likely to have caught the virus in January, when a family of four living in the same building in Pak Tin Estate in Shek Kip Mei were infected.

“He could have a mild infection without any symptoms, but tested positive occasionally and shed virus bit by bit,” Yuen said.

He said for a 61-year-old nurse who worked at Kwun Chung Sports Centre community vaccination centre and was initially suspected to be infected but was given the all-clear, it was probable she was contaminated by inactivated virus in the vaccine.

Eight of 43 environmental samples collected at the inoculation site were found with the virus.

Additional reporting by Chan Ho-him