SINGAPORE - The dip in new local Covid-19 cases is a good sign but this slide needs to continue for a least a week before Singapore can say it has turned the corner, experts said.

Nine community cases, excluding migrant workers, were confirmed on Saturday, marking the first time numbers have dropped to the single digits this month.

Experts told The Straits Times that while this shows the circuit breaker which started on April 7 is working, the trend must be sustained until at least the first week of May - and even then, the clusters at dormitories may remain a challenge.

Dr Leong Hoe Nam, an infectious diseases expert from Mount Elizabeth Novena Hospital, said he would like to see fewer than five local cases daily in the coming week, and then several days of zero transmissions in the first week of May.

"Only then would I say we are truly out of the woods. We realised that all you need is a tiny spark in the right setting and it can blow up, so even one new case is one too many," he said.

Similarly, Professor Wang Linfa, director of the emerging infectious diseases programme at the Duke-NUS (National University of Singapore) Medical School, said one to two weeks of continuous data is needed to make sure the "trend is real".

He gave May 10 as a benchmark date, explaining that this is both due to the incubation period of the coronavirus, which averages 14 days, and to account for the effect of social distancing measures.

Associate Professor Hsu Li Yang, who leads the infectious diseases programme at NUS' Saw Swee Hock School of Public Health, said at least a week of below 10 new cases a day would be a sign that Singapore is on the right track, as the virus' serial interval - how fast it spreads from person to person - is between three and five days.

But Prof Paul Tambyah at NUS' Yong Loo Lin School of Medicine said the numbers of cases are hard to interpret without knowing how many people are tested.

The Health Ministry (MOH) also monitors and collects data on severe pneumonia and influenza-like illnesses from polyclinics and general practitioners, he noted.

"When (both of) these are down significantly, perhaps close to zero, then we can be confident that we have turned the corner," he said.

Related Story Coronavirus cases in Singapore: What we know so far

The experts also cautioned that the recent dip in Covid-19 numbers among migrant workers is no cause for celebration, attributing it to reduced testing at some dormitories.

Numbers at dormitories still make up the vast majority of confirmed cases but dropped to 597 on Saturday, down from about an average of a thousand daily in the five days before.

Dr Leong, Prof Hsu and Prof Tambyah all cited Prof Dale Fisher, chair of infection control at the National University Hospital, who spoke about a shift in diagnostic criteria at the dormitories during a CNA interview on Saturday.

"The numbers are not really coming down. It's a function of the tests. In some of the dormitories, the infection rate or the positivity rate of the tests is so high, you get to the point where you don't need to test anymore," said Prof Fisher.

He added that instead, those with clinical respiratory illness in such sites are almost certain to have Covid-19, and are placed into isolation straightaway.

ST has contacted the MOH for more information.

Agreeing with Prof Fisher, Dr Leong said the strategy will "save lives and save resources", but also artificially suppress numbers.

In the same vein, Prof Hsu said: "Certainly, the intervention to move some of the workers out will enable physical distancing measures to be implemented properly at a few of the dormitories, but also numbers will fall as we do less testing, and will not reflect true numbers at the dormitories."

Overall, Dr Leong said, Singapore could now be seeing its version of "day 11", referring to the day by which Prime Minister Lee Hsien Loong said he hoped the circuit breaker measures would show results.

New Zealand Prime Minister Jacinda Ardern had earlier told him that cases turned around for her country on day 11 after a national lockdown on March 25.

Singapore's day 11 would have been April 17.

"Why are we late? We were not adhering to rules of the circuit breaker faithfully. Even now people are still breaking the rules," said Dr Leong, adding that now, people are paying more attention.

But Prof Tambyah said it is not helpful for Singapore to compare itself with New Zealand, Taiwan, or Hong Kong as none of these places have large migrant worker populations living in dormitories. He suggested looking at countries like Qatar or Saudi Arabia, which have comparable outbreaks involving similar sites.