Case numbers in foreign worker dormitories have not come down as fast as expected, a clear sign that there are hidden reservoirs of infection there, experts said.
What makes the situation harder to control is that many of the workers who are infected are likely to show no symptoms, allowing the virus to continue spreading.
While most of the dorms have been ring-fenced - with workers segregated and isolated - the spread can continue in an undetectable fashion since most foreign workers are young and healthy, and those who get Covid-19 may be asymptomatic or very mildly symptomatic, said Professor Teo Yik Ying, dean of the National University of Singapore's Saw Swee Hock School of Public Health.
He had previously predicted that the cases in foreign worker dormitories would fall below 100 by the end of last month.
However, such a drop is taking longer because the outbreak happened in an environment where transmission was faster than expected, with so many workers living in close proximity.
"It will take a while longer to clear all the dormitories," he said.
Dorm infections hit a high of around 1,400 new daily cases on April 20. They have been hovering between 100 and 200 new cases daily since the end of last month but rose to 306 new cases yesterday.
The Health Ministry said that the higher number of cases yesterday is mainly due to fewer tests being conducted by Covid-19 testing laboratories over the public holiday last Friday as well as on Saturday, with the backlog of samples being cleared from Sunday.
Dorms have been steadily cleared with about 70 to 80 per cent of the workers set to be cleared by the end of this month, multi-ministry task force co-chair Lawrence Wong said at a briefing on June 25.
Not all dorms have the same rates of infection, with some more badly affected than others.
Said Associate Professor Alex Cook, vice-dean of research at the NUS Saw Swee Hock School of Public Health: "Not all dorms have been affected to the same degree, and the least badly affected ones may still have potential for infection to 'sneak in' through asymptomatic residents."
Infectious diseases expert Leong Hoe Nam is also disappointed at the numbers that continue to test positive in dormitories, as he had expected infections to have tapered off by now.
Noting that some of the workers he had encountered had only very mild symptoms, he said: "These workers did not meet anyone or go to work, and were confined in their quarters, suggesting that there is active asymptomatic transmission among them.
"We can do better."