According to the Centre for Health Protection (CHP), the 35-year-old policeman from the force’s Wan Chai division was now a confirmed case despite negative test results earlier upon his admission to hospital. He had not been found with any antibodies yet.

Dr Chuang Shuk-kwan, head of the CHP’s communicable diseases branch, said the policeman’s case suggested there were still hidden transmissions in the community.

“We do not currently have any other explanation for his infection, so we have categorised it as a local case with an untraceable source,” she said, adding that his samples had been sent to the University of Hong Kong for genetic sequencing.

“If the results show the samples are similar to those from the fourth wave of infections, then that means there are remnants of that wave which have yet to be thoroughly cleared from the community,” she said.

“Of course, we haven’t had a local case in a while, so even if there is a transmission chain, we hope it’s a low-level, controllable spread.”

Chuang said private contractor Prenetics had tested the officer’s samples by pooling them together with those taken from three others, a standard practice to save time.

As Hong Kong had not seen any local infections in a while, Chuang said, the CHP had asked Prenetics for all four samples to study them further.

Repeated tests by the CHP’s laboratory returned positive results for the police officer, but he tested negative in hospital.

She noted there had been previous cases of patients testing negative upon admission to hospital, and it was possible that samples taken at different times would return different results.

“Of course, we need to wait a few more days to see what the situation is,” Chuang said.

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But infectious disease expert Dr Leung Chi-chiu said it was rare for a patient carrying such a high viral load in early tests to later show negative results repeatedly, and he was concerned that human error in the laboratory could have led to a mix-up of samples.

“That means there could be someone out there in the community with a high viral load that we have missed,” he said.

If the policeman’s infection was finally confirmed, Leung added, it could be a remnant of the fourth wave.

“It’s unlikely this single infection could trigger a fifth wave, as our current control measures are stringent enough,” he said.

01:20

Malaysia goes into total lockdown amid worsening Covid-19 outbreak

Malaysia goes into total lockdown amid worsening Covid-19 outbreak

David Hui Shu-cheong, a pandemic adviser to the government, said it would take a few more days to see if the officer developed antibodies to truly confirm his infection.

“Do not assume that the city has achieved complete zero infections just because we have seen no cases in more than 30 days,” Hui said.

He also suggested authorities in mainland China might need to observe the city’s situation for another 28 days, covering two 14-day incubation periods for the coronavirus, to discuss the possibility of allowing cross-border travel.

The CHP said around 40 to 50 of the infected officer’s close contacts had been identified, including about 30 of his colleagues. His two children attended the Spastic Association of Hong Kong’s Child Development Centre in Lok Fu and Lutheran Church Kwun Tong Kindergarten, both of which would have to close for two to three days while awaiting final test results.

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Although both his children had displayed symptoms, their tests so far had returned negative results, officials said. The officer mostly ate at home, but had also visited several restaurants in Yau Tong, Lok Fu, Wong Tai Sin, Tin Shui Wai and a Chinese restaurant at police headquarters in Wan Chai.

In another development, authorities announced that passenger flights between Jakarta and Hong Kong operated by Garuda Indonesia would be banned for 14 days from Wednesday until June 15. The action was taken after three passengers from flight GA876 which arrived on Sunday were confirmed to be infected, a threshold that triggered the flight suspension mechanism.

As the city’s first untraceable case in weeks emerged, financial authorities’ latest quarantine waivers for potentially thousands of vaccinated senior business executives a month also came under scrutiny.

Professor Gabriel Leung, dean of the University of Hong Kong’s faculty of medicine, said the exempted persons must also pass an antibody test on arrival if the city was to avoid a fifth wave.

“Up to 20 per cent of recipients of the BioNTech vaccine could still catch Covid-19. So only those who test positive for antibodies – meaning their vaccines are working – should come in without quarantine,” said the epidemiologist, who advises the government on the pandemic.

Referring to the government’s argument that senior business executives deserved quarantine exemption because of their contribution to Hong Kong’s economy, Leung suggested others such as foreign domestic workers should enjoy the same privilege using the same argument.

“We should believe in vaccine science, and science does not discriminate,” he said.

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Chief Executive Carrie Lam Cheng Yuet-ngor said earlier on Tuesday that it would not be a “free-for-all” and business executives exempted from quarantine would still be subject to restrictions against mingling with the community.

In another vaccine-related move, the government said the announcement of individual death cases following inoculation would only be made if a “potential association” with the jab was detected. Starting from July, a monthly report on safety monitoring of jabs would be published on the government’s vaccination website.

The Food and Health Bureau, meanwhile, said it had received phase three clinical trial data from Fosun Pharma, which distributes BioNTech Covid-19 shots, along with an application to lower the age limit for receiving the vaccine to 12 years old. The information will be submitted to the government’s vaccine advisers for consideration and their suggestions will be passed on to the secretary for food and health.

A business sector push to encourage employees and the public to get vaccinated continued on Tuesday with more incentives on offer. The Hong Kong General Chamber of Commerce and Henderson Land Development announced vaccination leave for their staff.

Swire Properties will splash out HK$8 million (US$1 million) on perks, including a lucky draw for 500 HK$10,000 (US$1,300) shopping vouchers to use at its malls, for fully vaccinated residents. Great Eagle Group will offer discounts on room rates and dining at its hotels.

The Ng Teng Fong Charitable Foundation, Sino Group and Chinese Estates Holdings got the ball rolling last Friday, offering a HK$10.8 million (US$1.4 million) one-bedroom flat as the grand prize of a lucky draw for people who had received two vaccine doses.

Additional reporting by Lilian Cheng