After visiting the building in the evening, Professor Yuen Kwok-yung from the University of Hong Kong and an adviser to the government on its pandemic response, suggested residents who lived in “flat 09” on all floors be evacuated. About 50 residents were affected, according to sources.

Yuen suspected the coronavirus had spread through pipes running from the flat on the 23rd floor where an infected woman lived down to the flat of the 66-year-old living directly below her, and then to the young boy who resided on the 21st floor. Health authorities subsequently endorsed Yuen’s recommendation.

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The new infections push Hong Kong’s Covid-19 case tally to 11,562, while the number of related fatalities is now at 207 following the deaths on Friday of a 69-year-old man and an 83-year-old woman.

The city had previously ordered 7.5 million doses from AstraZeneca, which was originally expected to ship the first batch to Hong Kong in the second half of this year. 

But the British-Swedish firm, which co-developed the vaccine with Oxford University, has been engulfed in a crisis of confidence, after the European Medicines Agency said unusual blood clots should be listed as a very rare side effect of the AstraZeneca vaccine.

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The revelation prompted local experts, including government adviser Professor David Hui Shu-cheong, to call for the city to reconsider its deal with the company in favour of other options such as the Johnson & Johnson vaccine.

Hui also pointed to data showing that the AstraZeneca vaccine, while 70 per cent effective against the non-mutated coronavirus, offered only 10 per cent protection against a new South African variant.

Ending days of public speculation and administration silence, Secretary for Food and Health Professor Sophia Chan Siu-chee on Friday told the Legislative Council’s panel on health services that the government had told the manufacturer not to deliver its doses as planned this year. 

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SCMP Explains: What’s in a Covid-19 vaccine?

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A government source said the shipment would be postponed until the company could tweak its vaccine and improve its efficacy rate against the rapidly spreading South African variant. 

“The main reason is because we already have enough supplies,” Chan said, referring to the two batches of 7.5 million shots already ordered from China’s Sinovac and German firm BioNTech. 

But Chan said the Health Authority was also concerned about the recent safety reports on the jabs.

“It is not necessary to deliver it to the city this year, to avoid any waste,” she said. “We have also begun looking at next-generation vaccines that can offer better protection against new virus variants.”

Chan declined to comment when asked by lawmakers how much the government had paid the drug giant for the jabs.

The news follows the multinational firm telling the media only a day ago it planned to deliver its shipments to Hong Kong and Macau according to the original agreement. But it is understood that AstraZeneca representatives were told the deal was off in a Thursday meeting.

The company said that while Hong Kong currently had sufficient supply of vaccines, any wastage should be minimised.

“Therefore, AstraZeneca will continue to communicate closely with the … government on the supply and shipment of vaccines to reach a consensus in due course,” it said.

Since Hong Kong’s vaccination programme began in late February, about 743,600 doses have been administered to the public.

At Friday’s coronavirus briefing, the Centre for Health Protection’s Dr Chuang Shuk-kwan said the sole untraceable patient from the latest infections was a 42-year-old woman who lived in Yuen Long’s Pak Nai and worked at a Mannings store in Tai Hang Street.

Of the 12 new imported cases, six were domestic workers from the Philippines, and the others were from Canada, India, Egypt and Indonesia.