The circuit breaker measures are working - but not as fast as hoped for, said Professor Teo Yik Ying, dean of the NUS Saw Swee Hock School of Public Health.
After more than two weeks, the number of new cases in the community should have been lower, he said.
Health Minister Gan Kim Yong said on Tuesday that Singapore has averaged 29 new cases a day over the past week. If the cases of foreign workers not living in dormitories are included, then the number doubles.
Said Prof Teo: "This really points to some leakage happening in the community, either amongst people who are supposed to be home but have been venturing out without taking the necessary precautions, or due to some of the essential workers who are still out there working and getting infected."
Evidence over the past four days suggests that workers providing essential services are getting infected, since construction workers and staff from fast-food chain McDonald's were among the newer cases.
Prof Teo said this is why there is a need to tighten the circuit breaker measures and to narrow the definition of essential services which are allowed to remain operational.
"The trend for infections outside the community still points to workplaces and perhaps some household transmission," he said.
Professor Leo Yee Sin, executive director of the National Centre for Infectious Diseases, was equally disappointed with the number of new, community-based cases in recent days: "We have not seen a satisfactory drop in local transmissions."
But looking at the experience of other countries, with many extending their period of lockdown, Prof Leo said: "Two weeks is not a (long enough) period to adequately reduce the number of cases."
So neither of them was surprised that the circuit breaker measures have been extended by another four weeks to June 1.
Prof Leo said the virus "will find ways to remain in the human population and try to find a way that it can transmit and continue to stay with us.
"So it's not a simple virus for us to deal with. There's human-to-human transmission. It can transmit when we are not aware."
How long should the circuit breaker measures be extended?
Prof Leo said: "If we don't change our behaviour, it's no longer (about) how long your circuit breaker is going to be because once we bounce back to our own normal selves, the virus is going to come back."
