Hong Kong health authorities have admitted a domestic helper undergoing treatment for cancer and who has tested preliminary positive for Covid-19 was kept in general wards for a month after her employer tested positive for the virus.

The revelation came as the city confirmed 25 new cases on Sunday, including a worrying new cluster at a care home for people with mental disabilities.

The city is already battling outbreaks at facilities for older residents, with the Elderly Services Association informing care homes they can refuse admission to anyone discharged from hospital who lacks a negative Covid-19 test result.

Six of the latest cases were imported, while eight of the local transmitted ones were untraced. The city’s tally now stands at 4,682, with 77 related deaths.

Among the newly infected were three staff members at Hong Chi Lei Muk Shue Hostel in Kwai Chung.

The facility has about 45 residents, with five people sharing each of its nine rooms, and roughly 50 staff. All would undergo quarantine, according to Dr Chuang Shuk-kwan, head of the communicable disease branch at the Centre for Health Protection.

“We visited the home today and as it is difficult for the residents to wear masks properly. We will quarantine all of them,” Chuang said.

The mix-up at the hospital involved a 44-year-old Indonesian domestic helper who was admitted for leukaemia treatment on July 27, according to Dr Lau Ka-hin, a chief manager at the Hospital Authority. She had a slight fever but her initial coronavirus test was negative while a lung scan also showed no sign of infection.

However, her fever persisted and another lung scan on August 9 showed some ground glass patterns commonly found in Covid-19 patients. Her fever worsened and she was retested on Saturday, which returned a preliminary positive result, Lau said. The employer was confirmed infected late last month and had since been released, but health authorities should have followed up on the result and screened the maid at the time, Chuang admitted.

“When [the helper] was admitted to hospital, her employer had not yet been confirmed as ill, so she did not know,” she said. “We may have missed her quarantine order after passing it to the other team that handles them.”

The helper stayed in two wards and the authority has listed up to 52 people as close contacts. Among them, 17 remain in hospital, while the rest have been discharged.

Another new case involved a 72-year-old man who drove a cargo truck at Kwai Chung Container Terminals, where dozens of dock workers have been confirmed as infected. He was an employee of a company at the port with three previous cases, but Chuang did not identify the company.

“He mostly spent time in his truck and rarely stayed in any of the break rooms,” she said.

One of the imported cases was a crew member of a Malaysian cargo ship previously docked in Hong Kong waters after other crew were confirmed ill. The ship will not be able to leave for another 14 days.

The remaining imported cases were four arrivals from India and one from Bangladesh.

Labour and welfare minister Law Chi-kwong appealed to families of elderly people to avoid taking their loved ones out of care homes even for short reunions. His plea came after a resident at a nursing home in Tai Kok Tsui who was said to have met family members for meals was found to be infected.

“Elderly homes have no choice but to ban visits or even limit outings of residents, which could be seen as an ‘evil rule’ and having a negative psychological impact on them,” Law said. “A way to alleviate the problem is to make the best use of information technology, which puts elderly people in touch with their families through online gatherings.”

Law also explained why the government did not test all 60,000 residents at nursing homes across the city, saying the logistics would be too complicated, requiring tests to be conducted in negative-pressure wards in hospitals. Minimising infection risks would be a more effective strategy, he said.

The Elderly Services Association of Hong Kong, which represents more than 400 care homes, said it wanted public hospitals to test elderly patients before discharging them.

Chairman Kenneth Chan Chi-yuk said it recently issued new guidelines to members advising them they could reject anyone released from hospital who had not tested negative for the coronavirus within 72 hours of discharge.

“We are not kicking away our residents,” Chan said. “The proposal is to avoid cluster infections at elderly care homes. The discharged resident concerned may not be a confirmed case. But sometimes he or she can become a close contact when another patient in the ward where he or she stayed becomes a confirmed case. This would pose a great threat to the elderly home.”

Chan said he would write to the Hospital Authority on Monday to express the association’s concerns.

Lawmaker Fernando Cheung Chiu-hung, who sits on the welfare services panel in the Legislative Council, urged the government to issue official guidelines for the industry, rather than allowing it to establish its own. “Otherwise, senior citizens could risk having nowhere to stay after leaving the hospital,” Cheung said.

The Labour Department also announced foreign domestic workers awaiting employment could book an appointment for a test beginning on Monday.