Foreign workers exercise on the balcony of their dormitory rooms on April 21, 2020 in Singapore. Singapore is now battling to control a huge outbreak in the coronavirus local transmission cases among the migrant workers. Suhaimi Abdullah | Getty Images

Previously a poster child for its efficient handling of the coronavirus spread, Singapore is now back in the spotlight as it struggles to contain a new outbreak among a section of its population — migrant workers. Singapore had stepped up measures in the past three weeks, including closing schools and non-essential workplaces, in what has been termed a "circuit breaker." Those measures were further extended to June 1 with more non-essential services being suspended and tighter mobility restrictions imposed, the government announced Tuesday. The number of Covid-19 cases in the city-state has spiked in the past month — from about 1,000 cumulative cases on April 1 to more than 10,000 today. Most of the newly infected patients are foreign migrant workers residing in dormitories, who hail from countries including India and Bangladesh. As of April 22, Singapore reported an additional 1,016 new infections, bringing the total number of cases to 10,141. There have been at least 12 fatalities so far, according to data tabulated by its Ministry of Health. More than 8,000 cases so far have been linked to migrant workers living in dormitories.

Singapore has close to 300,000 workers who live in either purpose-built or factory-converted dormitories. The government has acknowledged that the large number of cases at the dorms is a "serious problem." Prime Minister Lee Hsien Loong said in a national address on Tuesday that Singapore has embarked on aggressive testing. "Not only those who reported sick, or showed fever or flu symptoms. But also those who were well and asymptomatic" were being tested, he said. "That's why you see high numbers popping up everyday, because it's a very aggressive sweep of the workers inside the dorms even when they are not sick, even when they have no symptoms," Singapore's minister for national development, Lawrence Wong, said at a virtual press conference after the address. "What this suggests is that, in fact, the infections have been occurring for some time starting very early and it has been going on and circulating," Wong explained.

Trojan horses

Dr. Leong Hoe Nam, an infectious disease specialist from Mount Elizabeth Novena Hospital in Singapore, noted on CNBC's "Squawk Box" last week that cases outside the dormitories have been much more contained than cases within them. "It is a bit embarrassing; we've controlled the inbound (cases) from the overseas patients returning, within the local community, the numbers have controlled somewhat — we're talking about 40 cases a day," the doctor said. "But for the foreign workers who work in a dormitory, we really have a big problem controlling them."

"Foreign workers are housed in dorms that can go up to thousands in numbers, each of them housed in rooms to about 10 to 12 with a communal toilet that serves 10 people also ... you can understand that they would be living in a situation that allows easy transmission of the virus," Leong told CNBC's "Squawk Box Asia" last week. "And they will be mixing among each other across different rooms. And hence ... one person transmits to easily 10, 20, 30 people at a snap of a finger," added Leong, who contracted SARS himself while treating Singapore's first SARS patient in 2003.

I call them the Trojan horse ... because you have no idea the person is sick, the person can spread the infection all around. Dr. Leong Hoe Nam infectious disease specialist, Mount Elizabeth Novena Hospital

One of the things scientists have learned in the last week or two is that there are asymptomatic or pre-symptomatic transmitters, said Leong. Asymptomatic transmitters are people who remain infectious even though they do not develop any symptoms, while pre-symptomatic transmitters are those who end up developing symptoms later, he later explained in an email to CNBC. "I call them the Trojan horse ... because you have no idea the person is sick, the person can spread the infection all around," Leong said. "Remember — the virus mingles with us, spreads through social contacts so you have to cut off all social contacts in order for the virus transmission to stop." Leong also warned countries currently under lockdown that are contemplating easing restrictions, saying that it could be premature. "You must understand that this virus thrives with any form of interaction whatsoever with people. And when you open up, you can actually spread it," he said.

No easy solution

In a widely shared Facebook post, veteran diplomat Tommy Koh, Singapore's former permanent representative to the United Nations, said that overcrowded dormitories — which were "not clean or sanitary" — were like a "time bomb waiting to explode." However, Leong pointed out that it would have been hard to find two- or three-bedroom apartments quickly to house close to hundreds of thousands of workers who are currently living in the dormitories. It is "practically impossible" for the government to implement a solution within a short span of a few weeks or a month, he said.

Migrant workers can be seen in the Cochrane Lodge II, a purpose-built migrant workers dormitory that has been gazetted as an isolation area on April 18, 2020 in Singapore. Ore Huiying | Getty Images