Singapore is midway through a critical two-week stage in its battle against the coronavirus, an infectious diseases expert here has said.

This comes as Covid-19 infections have crossed the 1,000 mark, and local cases now make up almost half of that figure.

A new cluster in an old folk's home could potentially fan the fire that seems to be building steadily.

Plus, two other groups of patients appear to be of particular concern now - foreign workers living in dormitories, and scattered cases of doctors, nurses and other staff at hospitals, including at the National Centre for Infectious Diseases.

A total of 49 new cases were reported yesterday. There is the possibility of the outbreak getting out of hand if new cases were to shoot past 100 a day for a few days running next week, said Associate Professor Hsu Li Yang, programme leader of the Infectious Diseases Programme at the National University of Singapore's Saw Swee Hock School of Public Health.

"If we are able to get through these two weeks without a big surge in new cases, such as above 100 a day for a few days, we will be relatively okay," he said.

"If the number of new cases is below 100 like now, it does not mean that the situation is not serious. It just means that our healthcare system can cope and won't get overwhelmed by the new cases."

This means the hospitals have enough ventilators and intensive care unit beds, he said. While there is no way of knowing exactly how many patients will end up needing intensive care, research shows that about 5 per cent of patients will be critically ill. Some 15 per cent will be moderately ill, and the remaining 80 per cent will be mildly ill, of which a minority will be asymptomatic.

National Development Minister Lawrence Wong said on Wednesday that Singapore is at a "very critical time" on the infection curve.

The number of cases in Singapore started to surge last week as residents returned from overseas. These included those who travelled during the March school holidays and students who returned home as their dormitories were closing.

"We expect some of the new cases to generate new clusters. To be frank, this is expected, once you realise there is a surge of people coming in since the end of the March holidays," said Prof Hsu.

"We were worried once the school holidays started, and we saw cases in the United States and Italy rise significantly."

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It meant that those who had been in the US and Europe were at a higher risk of getting infected.

Singapore then moved to get Singapore residents returning from Britain and the US to serve their 14-day stay-home notice at hotels from 11.59pm on March 25.

However, some of those who had returned a few days before that and did not exhibit any symptoms would have gone home. They may then have gone on to unknowingly infect others.

The incubation period is up to 14 days, though many people show symptoms in about five days.

While new clusters can be expected, the fact that one is in a nursing home is more of a worry because the residents are elderly and have multiple health complications. These residents are put in nursing homes because they need help with daily living activities.

They are thus at increased risk of becoming critically ill from the coronavirus and dying from it.

Elsewhere, nursing homes are hotbeds for Covid-19 infection.

As for clusters at foreign worker dormitories, these can grow as the dorms are home to thousands of workers.

The consolation is that the residents are largely young and healthy, so while the risk of individuals getting infected is high, the chances of becoming critically ill are low, said Prof Hsu.

Workers may be housed in cramped spaces, but if the close contacts of the patients are quarantined early, the outbreak can be nipped in the bud, he said.