SINGAPORE - Ring-fencing strategies that aim to limit Covid-19 transmissions and contain their spread have been intensified recently, helping Singapore stave off a second circuit breaker for now.

Experts note that larger areas are now being closed off and more people made to undergo swab testing.

Professor Teo Yik Ying, dean of the National University of Singapore's (NUS) Saw Swee Hock School of Public Health, said that ring-fencing can help to flatten the curve of infections by preventing transmissions from spilling over to other sectors.

"It is a form of sectoral lockdown so that all the people within a location or sector can minimise further interactions with the public," he said, adding that it allows the authorities to understand the extent of the outbreak in that area.

However, it has to be executed promptly upon the discovery of cases, said Prof Teo, though the speed of execution is relative to how fast transmissions can occur.

"This depends on the extent of human-to-human interactions, personal safe management measures and the characteristics of the coronavirus infection, such as whether infected cases have high viral loads," he said.

Ring-fencing was used in 2003 amid the severe acute respiratory syndrome outbreak, with close contacts of patients quarantined at home.

More recently, the approach was used in Tan Tock Seng Hospital (TTSH) with some success. When a 46-year-old nurse followed by a few patients in Ward 9D were diagnosed with Covid-19, the hospital moved quickly to close the ward.

It expanded the ring fence when it emerged that there was potential exposure to these confirmed cases in other parts of the hospital.

By April 30, three days after the first infection was reported, the hospital had locked down four wards. On May 4, it stopped admitting new patients.

All close contacts were placed on quarantine, with TTSH staff and patients in the affected areas tested.

There was also increased clinical surveillance of all patients who developed fever and acute respiratory infection symptoms, and deep cleaning was conducted.

Close to 28,000 people were tested for Covid-19, including 12,000 TTSH staff and 1,000 patients, as well as close to 2,500 individuals who were quarantined following contact tracing.

Contact tracing, mandatory testing and deep cleaning are carried out for areas that are ring-fenced, followed by monitoring and heightened vigilance, said Professor Josip Car, director of the Centre for Population Health Science at Nanyang Technological University's Lee Kong Chian School of Medicine

But he noted that Singapore's ring-fencing strategy has been tweaked since last year.

"The areas being ring-fenced are now bigger for each potential outbreak. For instance, last year, outbreaks in schools first started with classes or levels being ring-fenced, but this year, the response is swifter and wider," he said.

Last July, then Education Minister Ong Ye Kung said his ministry's approach was to ring-fence schools on a small scale, and to put students and staff on leave of absence instead of fully closing schools. The approach was to close a class to keep the school safe and, if need be, the school.

But amid the current surge which has seen more children across several schools infected this year than last year, the ministry moved schools to home-based learning from May 19 to May 28.

On Friday night, the Education Ministry reported the first case of school-based transmission - a second student of Anglo-Chinese School (Junior) had tested positive for Covid-19, after his classmate had done so on May 19.

When multiple areas are concurrently affected, Prof Car said broader measures covering larger areas may be needed to stem or slow the spread.

This would also require the co-operation of those who were potentially exposed to the virus to go for testing, and to see a doctor if they develop any symptoms, he added.

Take White Sands shopping mall, which was listed 14 times on the Health Ministry's list of locations visited by Covid-19 community cases during their infectious period. The authorities moved to get people who visited the mall from May 2 to May 11 tested.

Through the TraceTogether app and token, close contacts were identified and notified by SMS.

However, a more aggressive approach was taken for residents of Block 506 Hougang Avenue 8, after a number of residents there tested positive. For the first time, it was made mandatory for all 116 households to get tested.

Prof Car said that in scenarios of limited transmissions, locking down areas not exposed to the source of the virus could overstretch resources and may not be more effective than ring-fencing in breaking the chain of transmissions.

"Whole-country lockdowns may be necessary, but only in the event of many localised outbreaks or uncontrolled spread where contact tracing resources are overstretched," he said, adding that Singapore has been following this playbook and extending the ring-fence outwards when needed.

Unlinked cases

As at Sunday, Singapore has had over 90 unlinked local Covid-19 cases reported since May 7, compared with a total of 68 unlinked cases reported from the start of the year till before the spike.

Associate Professor Hsu Li Yang, an infectious diseases expert from the NUS Saw Swee Hock School of Public Health, said: "If unlinked cases start appearing in the majority of sectors in the economy, it will probably trigger a more general lockdown."

Agreeing, Prof Car said that these unlinked cases signal leakages from containment areas, and could be a result of higher infectivity of the new viral strain, rather than the non-effectiveness of the localised ring-fencing strategy.

Prof Teo said: "If the pace and scale of the spillover is so great that the contact tracing and ring-fencing are basically trying to play catch-up, then a complete lockdown becomes the only viable option to arrest the growth of the outbreak in the community."