SINGAPORE - Nine in 10 imported Covid-19 cases in Singapore between Wednesday (March 18) and Friday last week did not show symptoms, like fever, when they had passed through the border checkpoints.

As a result, the cases were detected only later on, the Health Ministry said on Monday (March 23) in response to queries.

It explained that about 90 per cent of the imported cases reported in Singapore during the three-day period saw a doctor at a hospital or general practitioner's clinic after they returned home.

"They had not displayed symptoms when at the checkpoints," it added.

Travellers into Singapore have to walk past thermal scanners, which would help identify those who are feverish. Those who are observed to be so, or have other respiratory symptoms will be required to do a Covid-19 swab test.

Singapore had 119 cases altogether in those three days, with 87 of them being imported cases, the ministry's figures show.

There have been mounting concerns that people with coronavirus may not show symptoms, such as fever, dry cough and shortness of breath.

Scientists are calling for urgent studies to determine the proportion of people with coronavirus who show no, or delayed, symptoms, amid fears that the number of silent carriers may be greater than previously thought.

The South China Morning Post reported on Sunday that as many as one-third of the people who test positive show delayed symptoms or none at all, citing classified data from the Chinese government.

In line with stiffer border controls to help stop the spread of the coronavirus, travellers entering Singapore, regardless of where they come from, have been given a 14-day stay-home notice since 11.59pm on Friday. It is mandatory even for those who are well.

The ministry had also said on Sunday that an additional precaution will be taken for those who come into close contact with a confirmed case on their flight back.

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They will be quarantined.

A quarantine order requires the person to stay put in a dedicated room with a toilet.

It is the strictest among the measures and is given to those who are, or are suspected of being, a carrier of the coronavirus.

In contrast, people serving a stay-home notice will not require a dedicated room and toilet, but they must stay home at all times.

Border control measures in Singapore will be intensified from 11.59pm on Monday (March 23) in a further effort to reduce the risk of importing coronavirus cases.

All short-term visitors, including tourists, will be barred from entering or transiting through Singapore.

Also, work-pass holders and their dependants will be allowed to return to Singapore only if they are employed in sectors that provide essential services, such as healthcare and transport.