When infections started to spike in foreign worker dormitories in April, the priority was to isolate residents most at risk, including older ones and those who had other conditions.

Attention then turned to building up capacity for hundreds of thousands of others, said infectious diseases expert Dale Fisher during The Straits Times Covid-19 webinar yesterday.

The focus was on preventing deaths, rather than transmission. Such a strategy in taking care of the migrant workers was "very novel", he said.

It helped keep deaths low, despite the high living density in the dormitories. This was despite there being no capacity to isolate all the migrant workers as was done for the community cases, said Professor Fisher, a senior infectious diseases consultant at the National University Hospital.

"Part of this is why we've seen over 48,000 cases now and 27 deaths. The (low mortality) is absolutely staggering. The rest of the world is going, 'How did you do that?'"

There were 341 new Covid-19 cases yesterday among migrant workers living in dorms.

New cases in dorms fell from a high of 1,369 on April 20 to between 100 and 200 a day since the end of last month. But there have been over 300 new cases a day in dorms since Tuesday, when numbers hit 399 cases that day.

"There is still some transmission obviously happening, but a lot of the dorms... are being cleared. But it's been slow," said Prof Fisher.

A combination of isolation, serology and polymerase chain reaction tests is used to clear dorms.

As of Wednesday, 891 dorms, including purpose-built ones and temporary quarters, have been cleared of Covid-19. According to the Ministry of Manpower's website, over 1,200 dorms are slated to be cleared in the coming weeks.

Prof Fisher said all migrant worker dorms should be cleared within weeks or a month.

Minister for National Development Lawrence Wong also said last Friday that as the country nears the end of clearing the dorms, the number of daily infections among migrant workers can be expected to rise. For the final phase of testing, many of the workers come from dorms with higher prevalence of coronavirus cases.

Prof Fisher said the surge of infections among migrant workers was "a marathon that (the country) didn't want to have".

"Every effort is being taken to ensure that we're not sending infected men out, and creating new protocols to make sure that for those who didn't get infected, there are no new clusters," he said.