A cleaner at a Hong Kong quarantine hotel has tested preliminary-positive for a more infectious mutant strain of Covid-19, potentially dashing officials’ hopes that a previous Delta transmission scare had been brought under control.

The infection prompted an overnight lockdown of the Port Centre in Aberdeen, where the patient lives, for compulsory screening, but no new infections were detected following the testing of 530 people.

If confirmed, the case would bring the city’s 24-day run of zero local infections to an end.

The Centre for Health Protection (CHP) announced on Thursday evening that a 41-year-old woman tested positive for the L452R mutated Covid-19 strain, and was currently being classified as a local case with an unknown source. The patient had no recent travel history.

Officials said the woman, who was asymptomatic, worked part time as a cleaner at the Bridal Tea House Hotel in Yau Ma Tei, a designated hotel for incoming travellers to undergo quarantine.

The infection was detected during regular testing for staff on Wednesday. She last went to work on Thursday.

The patient also worked part-time to provide cleaning service at Bluejay Residences in Ap Lei Chau and House 2, 12A South Bay Road in Repulse Bay, where she last went to work on Sunday and Tuesday, respectively, the CHP added.

Test results from the centre revealed she carried the more infectious L452R strain, but not the N501Y and E484K strains, making it likely it was a Delta variant. That will have to be confirmed via full genome sequencing conducted by local university experts.

“As mutant strains carry higher transmissibility, the CHP has to carry out prudent infection control measures to stop the potential risk of spreading the virus,” the centre said, adding that those who had been to the patients’ workplaces would also have to take a mandatory coronavirus test.

The development came just as the city breathed a sigh of relief after two recent, locally detected Delta infections appeared to have been contained without causing a wider outbreak.

One of them, a 24-year-old woman, had been confirmed as carrying the Delta variant on Thursday by a research team at a local university, after her colleague was also found to be infected with the same strain.

The two Covid-19 cases, which emerged late last month, first put Hong Kong on high alert because the infection of the woman’s co-worker was untraceable, raising fears that community transmission had resurfaced.

A 27-year-old man who works at Hong Kong International Airport as a member of the ground staff was the first Delta variant carrier confirmed in the city.

But the concern was eased on Tuesday when the government officially reclassified the two infections as “epidemiologically linked with imported case”. The woman’s colleague, a 27-year-old airport employee who also worked with her part time at Uptown Plaza in Tai Po, was found to have identical genome sequences with three previous cases from Indonesia.

Dr Gilman Siu Kit-hang, an associate professor from Polytechnic University who studies local Covid-19 cases, told the Post the woman was confirmed to be carrying the Delta variant as well upon genome sequencing run by his team.

He said the sequencing result, which uncovered three genetic differences between the two cases, showed that the 24-year-old was likely to have been infected by her colleague.

“We did observe three mutation differences between cases with epidemiological linkage, although it is not very common,” Siu said.

He said it was also possible the woman had been infected by someone else, but aside from the latest preliminary-positive case, Hong Kong so far had not identified another patient with a locally transmitted Delta variant.

On Thursday, the city confirmed four imported infections, involving a domestic helper from Indonesia, two arrivals from Britain, and a traveller from Russia. Around 10 people also tested preliminary-positive for the coronavirus.

The total number of confirmed cases stood at 11,927, with 211 related deaths.