This comes as Hong Kong yesterday reported one new case from an unknown local source involving a 50-year-old logistics worker at the airport, breaking the SAR's streak of zero local cases for 33 days.

The man tested positive on a preliminary basis on Saturday for the L452R mutation, which is commonly present in the highly contagious Delta variant first discovered in India. And he had a high viral load.

It was on June 24 that a 27-year-old male ground crew staffer was confirmed to have contracted the Delta variant.

Contact tracing led to a belief he contracted the variant from three Indonesian arrivals while at the airport's specimen collection center.

Although genome sequencing to show whether the logistics worker's infection involved the Delta variant had not been completed, Chuang Shuk-kwan, head of the Centre for Health Protection's head communicable disease unit said yesterday having two airport workers contract the Delta variant within a short period set off alarm bells.

"We can't see any direct link between the two cases so we're concerned there could be silent transmission," she said. But "airport workers are considered to be at high risk as they can have direct or indirect contact with imported cases."

The tens of thousands of personnel who worked at the airport from June 20 to Saturday must be tested immediately, even if vaccinated, Chuang said..

A testing station set on the sixth floor of terminal one will operate from 10am to 8pm until tomorrow.

Chuang said authorities are also trying to identify loopholes in the procedures of testing, identification and transporting of imported cases from the airport to hospitals.

"Now that we are exposed to new variants the chances of transmission are inevitably higher," she said. "The best strategy is to vaccinate all who can be vaccinated while improving controls."

Chuang also said the logistics worker, employed by SATS HK, was responsible for moving cargo and luggage on and off planes, and he did not have contact with cabin crews or passengers.

"He mostly worked for cargo flights and occasionally passenger flights," she said. "We're looking at flights he had been responsible for and to see if any crew members or passengers were confirmed cases."

Health authorities will compare his Covid variant form with the ground crew staffer and other imported cases carrying the L452R strain in trying to trace the source of his infection.

The man lives on Kam Fung Street in Wong Tai Sin. The building was under overnight lockdown from Friday evening to test 28 residents, but none had tested positive by last night.

He lives with a friend in a partitioned flat. And his friend, others in the flat, workmates and relatives who had dined with him were among 90 sent to quarantine centers.

The man had also been to various restaurants in Sha Tin and Wong Tai Sin as well as Ocean Park on June 16, and all went on the immediate testing list.