The SAR recorded its first XBB1 case involving a 50-year-old woman who arrived from the United States on September 25.

And a 32-year-old woman returning from Singapore on September 23 was carrying the BA2752 sublineage. Both cases were asymptomatic, health authorities said.

This came as Hong Kong yesterday recorded 4,874 cases - 4,478 local infections and 396 imported cases as well as 10 deaths.

Government adviser David Hui Shu-cheong said XBB1 is a sublineage of the Omicron variant.

Hui said XBB1 cases were detected in Singapore and India last month and caused a Covid rebound in Singapore recently.

But he believes the XBB1 strain will not have a huge impact on the local pandemic situation because Hongkongers have the protection of vaccines.

Still, a research team from Peking University has found that the XBB strain is one of the most antibody-evasive strains. Its ability to evade immunity from vaccination and previous infections is 28 times higher than the BA2 subvariant and 19 times higher than the BA5 strain.

But respiratory specialist Leung Chi-chiu told The Standard that such a laboratory test result does not mean the new strain will definitely trigger an outbreak in the real world, where people have been protected by hybrid immunity.

Several other experts yesterday sought to allay fears over a rebound in the number of Covid infections in Hong Kong by suggesting the SAR buy BioNTech's second-generation jab targeting the BA4 and BA5 subvariants after the administration said it has received recognition application of the new jab.

University of Hong Kong health expert Ivan Hung Fan-ngai said on radio the new booster shot is "well worth introducing and administering to Hongkongers."

Although the new vaccine is based on Omicron BA1 it provides good protection against the BA2, BA4 and BA5 subvariants.

Hung noted too that the Omicron variant is similar to ones now spreading in the world. "Therefore, it can bring better protection compared to first-generation Covid vaccines."

Hung said the new booster jab can protect people from the latest Omicron BA2752 subvariant, which appeared overseas.

But the new booster jab can mainly reduce hospitalizations and deaths, Hung added. It cannot effectively prevent people from being infected. On that, nasal spray Covid vaccines may work better.

Hung also said the Omicron BA275 or BA2752 strains may replace the current BA5 subvariant as a dominant strain in Hong Kong, noting that the new subvariant has a stronger ability to evade immunity from vaccination and previous infections.

But he said people do not need to worry about it as most BA2752 cases are mild or asymptomatic.

Infectious disease expert Ho Pak-leung also urged authorities to procure the new BioNTech vaccine as soon as possible to boost vaccination.

Priority should be given to seniors who have received two doses more than six months ago, Ho added.

But Lau Yu-lung, chairman of the Scientific Committee on Vaccine Preventable Diseases, said it would be "a waste of resources" and "inappropriate" if authorities purchase the new shot targeting BA4 and BA5 cases as it may be "outdated" again when the new BA275 strain hits Hong Kong.