Some at-risk groups remain a potential source of Covid-19 infections and should be watched carefully, even while the focus is on containing others.

The risk of virus spread is still present, but there are "blind spots" where it is underestimated and overlooked, said Associate Professor Hsu Li Yang, who leads the National University of Singapore's Saw Swee Hock School of Public Health's infectious diseases programme.

Some commonalities of these vulnerabilities include crowded areas, places with high human traffic, "forgotten" sectors and those who live in a semi-closed community, experts told The Straits Times.

"Over time, those preparing and planning for outbreaks have learnt to identify and compensate for several of these blind spots, but clearly not all, as seen by the huge outbreak among our foreign workers," Prof Hsu said.

These vulnerabilities have changed through the different pandemics weathered by the country, Prof Hsu noted.

In the 2003 severe acute respiratory syndrome (Sars) outbreak, for instance, healthcare workers were the blind spots. "Forty-one per cent of those infected were healthcare staff in hospitals, primarily Tan Tock Seng Hospital, where the index case was hospitalised," he said.

Domestic helpers were a possible vulnerability in this pandemic, as large numbers congregate in specific areas such as Lucky Plaza, Prof Hsu said.

"This was identified and acted upon relatively late (April 11), with the Ministry of Manpower mandating that they have to stay home during their rest days. Fortunately, no significant spread of the virus occurred."

A sector often "forgotten" would be those in the sex industry. Thankfully, licensed brothels were shut along with nightclubs, bars and cinemas on March 26. It is possible that such activities have gone underground, but the risk of virus spread would be greatly reduced, Prof Hsu said.

Communities who are closed off also pose a high risk of rapid virus spread.

A group quite badly affected during the 2009 H1N1 outbreak was the Singapore Armed Forces (SAF), said Associate Professor Alex Cook, vice-dean of the Saw Swee Hock School of Public Health.

SAF units share features similar to foreign worker dormitories, in that they are semi-closed communities that would be at risk of respiratory diseases transmission.

Another closed group are prison inmates, Prof Cook said. "I would be worried about what might happen if Covid-19 were to get a foothold in the prison population as the disease can spread well through the community being closed off. We have seen other countries releasing lower-risk prisoners to minimise the risk of institutional outbreaks."

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Communities on board ships are also vulnerable.

"As Singapore is a maritime state, ships play a big role in our commerce and defence, and could be vulnerable to outbreaks," Prof Cook said.

Meanwhile, as Singapore hunkers down with heightened measures to curb virus transmission until June 1, the number of essential workers - who are also vulnerable - has been whittled down from 20 per cent to 15 per cent of the workforce.

Infectious diseases expert Leong Hoe Nam said: "The only way to stop the chain of transmission is to stop these workers from meeting people... If this cannot be avoided, one way to lower risk is to have everyone wear a mask properly and consistently."

The work environment can also be broken into various teams - those who deliver food should not meet those who prepare food, and those involved in food preparation in different outlets should not mingle, and can be further split into even smaller teams, Dr Leong added. "They should come at different times, through different entrances and never let their guard down even after work has ended.

"The more barriers you put at work, the more barriers you put for virus transmission."

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Ultimately, it is all about thinking how people can become spreaders, especially since infection can still spread even with minimal symptoms, said Associate Professor Josip Car, director of the Centre for Population Health Sciences at Nanyang Technological University's Lee Kong Chian School of Medicine.

"There really is no magic bullet to stop transmission. Some services are essential because they are indispensable to our way of life, and safe distancing measures, effective as they are, are not perfect...

"We all have a personal responsibility for ourselves and those around us," Prof Car said.