Tycoon Li Ka-shing’s companies and charity foundation joined the list on Tuesday, saying they would give out HK$20 million worth of shopping coupons through raffles.

In a continuing reflection of Hong Kong’s progress in tackling the pandemic, authorities confirmed three new Covid-19 cases – two brought in from Colombia and one from Indonesia. The city’s total number of infections now stands at 11,868, with 210 related deaths.

On Monday Guangdong confirmed 19 new infections, 14 of them from the provincial capital of Guangzhou.

“[Experts have said that] apart from requiring them to be tested for the coronavirus and have two doses of vaccine, they should also be tested for antibodies. I agree with this in principle. It is always good to add more protective nets,” Lam said on Tuesday.

“I know that the Centre for Health Protection is studying at which point this should be introduced, and which testing method should be used.

“We need to consider how to do this so it matches our requirements for health protection without causing greater inconvenience to those affected. We will study this seriously.”

The testing methods under “close study” involve drawing blood with a syringe or a finger prick.

David Hui Shu-cheong, an expert from the Chinese University of Hong Kong who advises the government on the pandemic, said a finger-prick test could deliver results in just 20 minutes, but he suggested running a comparative study of the technique’s sensitivity against the blood-drawing method before any roll-out of the airport testing scheme.

Another antibody test could be performed at the end of the 21-day quarantine period for arrivals before releasing them into the community, he added.

Fears over loopholes as Covid-19 variant case breaks Hong Kong’s 42-day run

“The upper respiratory tract is a large area, and sometimes, owing to the timing or sampling technique, you may not be able to swab the virus in the PCR test, therefore producing false negative results,” he said.

Serology tests that screen for the existence of antibodies in blood would have less of a sampling problem.

Leung suggested placing self-isolating individuals who had tested positive for antibodies under more active surveillance, including monitoring of symptoms and even more frequent PCR testing, as they might finally yield a positive result upon more accurate sampling.

“Now that daily caseloads are low, we can do more at the border,” Leung said.

Neighbouring Macau has from Saturday required arrivals from Brazil, India, Nepal, Pakistan and the Philippines to provide proof of either vaccination or a negative antibody test, on top of three negative PCR ones taken in the week before departure.

Hong Kong bans the entry of travellers who have stayed in those five countries plus South Africa for more than two hours in the preceding 21 days.

Boston Logan International, Istanbul, Istanbul Sabiha Gokcen, John F. Kennedy International, London Gatwick, London Heathrow, London Luton, London City, Munich and Newark Liberty International are among the airports providing antibody tests along with other screening options as an entry requirement.

Calls for Hong Kong to tighten rulesp on residents returning from mainland China

Lam conceded the emergence of coronavirus variants indicated cracks in the city’s pandemic-control measures.

“But these cases being missed does not mean that there are big loopholes in our policies,” she said.

“For the coronavirus, there is no definite answer as to exactly how long the incubation period can be, and Hong Kong’s measures may be the strictest in the world, as we have 21 days [of quarantine].”

The chief executive added that recent developments had made it even more crucial for Hong Kong residents to get vaccinated.

Additional reporting by Chris Lau