On June 5, the teen had tested positive for the variant, first found in Britain. Her mother and sister were later also found to have contracted the virus.

On Monday, government adviser Professor Yuen Kwok-yung said authorities and scientists were investigating three possible sources of transmission for the girl – people, rats or other animals, and items such as frozen food.

Officials took more than 60 environmental samples from places such as the family’s home in Tin Shui Wai, as well as from a pet shop and a unit at Tak Wing Industrial Building where the 17-year-old had been. Three samples were taken from the freezer compartment of the patients’ fridge. 

Results came back on Tuesday and showed one sample taken from the surface of a pre-packaged frozen food item tested positive. According to the Centre for Food Safety (CFS), the item was a pack of frozen crocodile spare ribs from Thailand. 

The Centre for Health Protection said it was “vigorously following up on the epidemiological investigations and relevant contact tracing”. 

The CFS said it had asked the relevant vendor to stop sales of the item for the time being, and to provide samples of products for testing, with the results pending.

By the end of last month, more than 10,000 samples of food and packaging were collected for tests. The results, including those for more than 40 related crocodile products imported from Thailand, were all negative, a government statement said.

“The CFS will continue the surveillance on imported frozen foods and their packaging to closely monitor the risk of the spread of the Covid-19 virus through imported frozen foods,” the statement said.

Hong Kong may relax quarantine restrictions for fully vaccinated: commerce chief

Concerns over Covid-19 transmission via objects surfaced last year when several cities in mainland China detected the virus on the surface of frozen food, such as shrimps and chicken wings, as well as their packaging and in one case, a salmon chopping board. Yuen on Monday said samples would be taken from frozen food markets in Tin Shui Wai where the patients lived.

But Professor David Hui Shu-cheong, also a government pandemic adviser, said he believed there was a simpler explanation for the positive result for the fridge sample. “Self-contamination through a sneeze or a cough is the most likely explanation at this stage,” he said. 

Hui cautioned against undue alarm in the girl’s case, saying the virus discovered could already be dead and remain on a surface for weeks or months, while only a viral culture test could demonstrate whether the pathogen was still alive and infectious. 

“There have been almost no community cases recently, and none reported in frozen food markets, which means they are very unlikely to be the origin [of the patients’ infections],” Hui added. 

With two more infections from Indonesia, meanwhile, health authorities announced that boarding and quarantine requirements for arrivals from the country would be tightened from Monday because of the “persistent unstable epidemic situation in the region” and the prevalence of more transmissible mutant virus strains.

Arrivals will be required to test negative for Covid-19 and present a 21-day booking at a designated quarantine hotel. They must undergo four tests during quarantine, followed by seven days of self-monitoring and compulsory screening on the 26th day. The shortened quarantine period for those who are fully vaccinated will not apply to people who stayed in or travelled from Indonesia in the previous three weeks.

Indonesia on Sunday recorded nearly 10,000 new Covid-19 cases, the highest figure since February 22. The country has officially confirmed more than 1.91 million infections, with 53,116 deaths.

Professor Lo Chung-mau, chief executive at the University of Hong Kong-Shenzhen Hospital, said its Covid-19 vaccination centre, one of eight venues in the city serving residents of Hong Kong, Macau and Taiwan as well as foreigners, had administered 7,318 doses to Hongkongers since April.

Lo said the hospital’s booking slots, which last week stood at 800 per day, would be increased to 1,000 as part of a plan to expand its vaccination programme, though appointments could only be booked after 5.30pm the day before.

The number of available slots at this hospital would also rise again soon, he said.

“I estimate our new vaccination clinic [under expansion] can be up and running around the end of July … then we can inoculate 2,000 people a day,” he told a local radio show.

Lo also said the mainland had more than enough vaccines, though some might find it difficult to book their preferred dose, as the hospital could only provide one brand per day. The hospital currently supplies three mainland-produced shots – one each from Sinovac, Sinopharm and Shenzhen Kangtai Biological Products.

He also conceded the facility’s policy of reserving 90 per cent of slots for those getting their second jab could have been a factor in causing booking difficulties.

“The public needs to understand it is impossible to help all 600,000 Hongkongers staying in Guangdong province get vaccinated within one day,” he said.

While the mainland has made massive strides in its vaccination efforts, Hong Kong’s inoculation drive, which began in late February, has remained sluggish. As of Monday, about 3 million doses had been administered in the city.

Some 1.79 million residents, or 23.9 per cent of the city’s 7.5 million population, had received at least one dose while 16.2 per cent had taken their second jab.

On Sunday, lawmaker Alice Mak Mei-kuen of the Federation of Trade Unions suggested the Hong Kong government consider sending a portion of the city’s vaccines to Lo’s hospital to help those who had struggled to secure a jab.

But while acknowledging the proposal was well-intentioned, Lo said it was unnecessary.

“Hong Kong’s vaccine stock should be used to take care of people in Hong Kong,” he said. “[Sending the doses to our hospital] is not appropriate.”

Additional reporting by Christy Leung