The nature of activities at KTV lounges that are supposed to operate as food and beverage outlets makes it more difficult for the authorities to test and trace those who are linked to a cluster involving several outlets, while making it easier for Covid-19 to spread, experts say.

They expect the number of cases from the cluster, which rose to 54 yesterday after 42 new cases were recorded, to remain high over the next few days.

Professor Teo Yik Ying, dean of the Saw Swee Hock School of Public Health at the National University of Singapore, told The Straits Times: "Contact tracing is much harder in such settings as patrons will be unwilling to come forward to admit they have been to such locations, even if it is purportedly for meals.

"The alleged hostesses are also unlikely to volunteer information about the people they have been in contact with."

The infections also show that operators of the KTV outlets linked to the cluster have been unable to properly enforce the safe management measures they are supposed to adhere to, he added.

On Tuesday, the Ministry of Health said there was likely ongoing Covid-19 transmission at Club Dolce in Balestier Point, Wu Bistro in Golden Mile Complex and Club De Zara in the Textile Centre. Last night, it added Terminal 10 at Clarke Quay; and One Exclusive, Level 9, Club M, all in Middle Road, to the list of KTV outlets where there is likely ongoing transmission.

Prof Teo compared the cluster to similar outbreaks in other countries that also arose from nightlife establishments.

In May, an outbreak of infections among hostesses and patrons of bars, nightclubs and karaoke lounges in Taiwan contributed to record jumps in infection numbers. Last year, South Korea saw more than 200 cases linked to a nightclub in Seoul's Itaewon district.

"Even though these lounges were mainly serving food, I suspect a lot of the activities would be similar to those of nightclubs."

KTV hostesses are known to move from one table to another over the course of a night, mingling with different groups.

This rotation is colloquially known as "butterfly", and often involves the hostess splitting her time between multiple guests, who each pay her a sum of about $50 to $100 at the end of the night.

The hostesses have also been known to move between multiple KTVs in one night, going to lounges that have more guests if their usual haunt sees lower patronage on certain nights.

Dr Leong Hoe Nam, an infectious diseases specialist from Rophi Clinic, told ST: "Such behaviour greatly increases the ease of spreading the virus and it is why I wouldn't be surprised if the cluster blows up even bigger."

He said that the KTV cluster could be a weak link in Singapore's fight against Covid-19, especially since the hostesses and patrons may be trying to lie low, making it difficult for the authorities to ring-fence those who are infected. Some hostesses, he said, could be working illegally.

"My concern is whether there are any overstayers who have been working under the radar. They are difficult to detect and they will not come into our hospitals for fear of being discovered by the authorities," he said, adding that some patrons, such as married men, may also be reluctant to come forward.

Health Minister Ong Ye Kung said yesterday that testing at screening centres would be confidential for any patron of KTV lounges or those who have interacted with hostesses since June 29. Testing is also free.

Dr Leong noted that while 60 per cent to 70 per cent of the population have received the first dose of a Covid-19 vaccine, the vaccination rate would be lower among certain groups of people, such as those who are working illegally here. These groups pose a risk and could cause other clusters to arise, he said.

"Even for (the hostesses) who are here legitimately, some of them may not be vaccinated yet because we only recently started vaccination for foreigners. Some may also have misconceptions about the vaccines that prevent them from getting their jab."

Noting that some individuals may have already infected their family members, Prof Teo urged those who have visited a KTV lounge or come into contact with hostesses to get tested.

"Coming forward for testing provides peace of mind not just to the individuals, but to all others that they may be in contact with at home or at their workplaces."

• Additional reporting by Clara Chong and David Sun