As a result, anyone who had worked at the airport between June 20 and Saturday, estimated to number in the tens of thousands, had been ordered to take a test, the health official added.

According to Chuang, the 50-year-old patient had received one jab of the BioNTech vaccine, so the protective effect might not have emerged yet. He is asymptomatic, despite having a high viral load.

He lives with a friend and other co-tenants at a subdivided flat at 1-3 Kam Fung Street in Wong Tai Sin. His work at the airport mainly involved loading and unloading cargo goods from flights at restricted area 60.

Chuang said the patient’s work did not normally bring him into contact with flight crew and passengers, and he did not know the 27-year-old airport employee infected earlier.

“Airports, even in other places, may be the weakest link,” she said, citing high passenger flow.

“We think human-to-human transmission is still more likely, as a spread via cargo goods would have infected many people. But we don’t rule anything out.”

During the infectious and incubation period of the disease, the patient had been to Ocean Park on June 26, dined at various restaurants in Sha Tin and Wong Tai Sin, and received a haircut in Sham Shui Po.

Sticking with strict Covid-19 travel curbs may hurt Hong Kong: business chiefs

Officials will quarantine some 90 close contacts, including flatmates and his hairdresser.

Experts will also conduct genome sequencing for his virus sample to determine if he indeed carried the more infectious Delta variant, and will compare it to the specimen of the earlier infected airport worker as well as other recent imported cases to see if there is a match indicating transmission.

Preliminary analyses have found he carried the L452R strain, but without the N501Y and E484K mutations, meaning it was possibly the Delta variant.

Infectious diseases expert Dr Leung Chi-chiu said it would take a few more days to see if the man’s case posed a risk to the city.

“We will need to see if any of his close contacts test positive in the coming days,” Leung said. “Additionally, his high viral load could be attributed to early detection, which means the risk of a spread within the community would be low as well.”

Despite the case’s current classification, Leung said he personally believed chances were low the man had caught the infection locally, as there had been no such cases in the past few weeks.

Authorities have previously reclassified infections once genetic analysis of the virus has established links to other imported cases.

The airport, Leung noted, was the only way more transmissive variants could enter the city, something that dictated stronger measures among workers at the entry point to prevent infections.

He added that any workers coming into contact with passengers or their belongings should be considered high-risk, and that steps should be taken to ensure hand hygiene and reduce the chances of cross-contamination.

“It is also not an unreasonable request to expect these workers to be vaccinated,” he said. “This is a workplace health and safety issue.”

But even vaccinated workers would still need to be tested regularly for Covid-19, he said, as a chance of breakthrough infections involving variants remained, even among those already jabbed.

“Right now, there is still not a strong enough mechanism in place to prevent any variants from spreading into Hong Kong. If we happen to catch any now [as in the most recent case], it’s just luck. If the variants end up seeping into the community, we could have a situation similar to Taiwan on our hands,” he said, referring to a recent surge in cases on the island, which experienced its worst outbreak since the pandemic began.

Twenty-eight residents at 1-3 Kam Fung Street in Wong Tai Sin, where the airport employee lives, tested negative for the virus in a lockdown operation overnight Saturday.

Households in the block will also be tested four more times over the next 19 days because of the connection to the mutant strain.

The last time Hong Kong recorded a community case was on June 7, when a 20-year-old woman, whose younger sister contracted the Alpha variant locally, was confirmed as infected.

As of Sunday, the city had confirmed 11,951 infections, with 212 related deaths.

Additional reporting by Sammy Heung