The government should make mask wearing mandatory and step up enforcement of social-distancing laws to prevent Covid-19 transmission, the University of Hong Kong’s top microbiologist has said.

Dr Ho Pak-leung’s comments came a day after HKU infectious disease expert Yuen Kwok-yung warned of a third wave of infections as mainland Chinese gradually return to Hong Kong, and senior government adviser Bernard Chan suggested the possibility of a strict lockdown.

Ho on Monday urged the Hong Kong government to set up emergency laws making mask wearing mandatory in all public places.

“Mask wearing works. It can block the spread of the virus from people who may not show symptoms yet,” he said on an RTHK radio programme. “There are already regulations in place for masks in restaurants other than when customers are eating. The rules should be extended to hospitals and other public places.”

Hong Kong has since late March banned public gatherings of more than four people, while cinemas, fitness centres, bars, pubs and other leisure venues have been ordered to close for two weeks. Restaurants also are required to keep tables 1.5 metres apart. As of Sunday, the city has 890 confirmed cases of Covid-19.

Executive Council convenor Bernard Chan on Sunday said preventive measures could be extended to limit restaurants to takeaway service, or even see non-essential businesses closed entirely, leaving most residents little choice but to stay home for two weeks.

Ho echoed Chan’s call for stronger measures, suggesting existing measures should also be extended beyond the two-week period, while tough enforcement of social-distancing regulations were needed to stop people from flouting the rules.

“If we drag our feet and do not take action to break the transmission chain now, the effect on small businesses will be even worse in the future,” Ho said.

Speaking on the same programme, Dr David Hui Shu-cheong, a respiratory disease expert at Chinese University, said tougher measures would be needed if the number of infections continues to surge this week.

“If the cases only increase by single digits, we may not need stronger rules, but if not, then it should be considered,” he said.

Hui also said Hong Kong’s first possible case of hospital transmission of the coronavirus could have been caused by contaminated medical equipment.

More than 200 patients and health workers at Pok Oi Hospital in Yuen Long have undergone testing for Covid-19 after a 93-year-old stroke patient was infected during his stay. It was later found he had contracted the disease from a 33-year-old man in the same ward who had first tested negative but was later confirmed sick.

Of the 174 people who received testing, 171 have returned negative results, the hospital’s chief executive Dr Chong Yee-hung said on Monday.

Speaking about the case, Dr Hui said medical equipment could have been contaminated, as it was unclear whether the two men had personal contact.

“We would need to confirm with hospital staff whether the [33-year-old] man may have come in contact with the older patient. Ideally, he would be placed in a transition ward after the first negative test, but with isolation wards reaching full capacity, it is difficult to do so,” Hui said.

The suspected hospital transmission raised fears of a repeat of the large-scale outbreak in ward 8A of Prince of Wales Hospital in Sha Tin during 2003’s severe acute respiratory syndrome crisis. That case saw a patient infect 11 health care staff before eventually spreading to a total of 137 people.

But Dr Arisina Ma Chung-yee, president of the Hong Kong Public Doctors’ Association, said the ventilation systems had improved since then, making a repeat of that scenario unlikely.