Almost three in four Covid-19 cases involving Singaporeans and permanent residents and with links that have yet to be established surfaced in the past 14 days, the latest situation report by the Health Ministry (MOH) showed.

From March 30 till Sunday, 205 local cases were detected that are still being investigated for links. They form 72 per cent of the 284 cases still under such investigation since the start of the outbreak.

MOH has also classified a total of 312 cases where no link as yet has been found as unlinked, with numbers rising in the last two weeks.

Professor Teo Yik Ying, dean of the National University of Singapore's Saw Swee Hock School of Public Health, pointed to two possible reasons - the trend could reflect greater community transmission, or it could be that contact tracing is lagging behind actual figures, given the high number of daily cases now.

With new imported cases tapering off, much attention has turned to the rise in unlinked cases. All returning travellers have to serve their 14 day stay-home notice at government facilities now, and this has helped to cut off transmission from that source, said Associate Professor Hsu Li Yang, infectious diseases programme leader at the Saw Swee Hock School of Public Health.

However, the continued relentless rise in unlinked cases is reason for the strict measures to break the circuit of transmission, infectious diseases expert Leong Hoe Nam told The Straits Times. Without those, the number of unlinked cases could double or even triple.

Asymptomatic infections would also contribute to the rise in unlinked cases, experts added.

Latest studies have shown that the virus is more contagious than originally thought and recent epidemiology models have found that around half of Covid-19 cases might have arisen from carriers who have yet to show 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.

Dr Leong noted: "When you do not know if a person has the virus, you let them near you. The virus can be inside the body for only around four weeks; if it fails to spread, it dies. Each time it spreads, its lifespan is extended. That is why we must strictly adhere to the circuit breaker measures to stop transmission,"

It is also too early to conclude if circuit breaker measures have been effective as it takes around 14 days to see the impact of confinement on the infection figures, experts said.

However, results seem optimistic, Dr Leong added. The difference between the two graphs for local unlinked cases - one based on when cases are announced to the public and the other based on when patients actually began experiencing symptoms or were confirmed to be positive for Covid-19 - represents the time lag between public knowledge and what is actually happening on the ground, Dr Leong explained.

"This delay is necessary - it gives the authorities time to confirm that the patients are positive before it is being announced. We actually see a dip in local unlinked cases in the last few days if you were to look at the ground situation - an indication that the circuit breaker might be beginning to show little signs of success."

But Singaporeans must cooperate to fully stop transmission, and continue to remain individually vigilant during and even after this phase, Prof Car said.

He added that though the circuit breaker has already altered lifestyles, in the way people work and learn, and how they express themselves as social beings, "to use a sports language analogy, we're only 20 minutes into the first half of the football match and, while we're winning with an excellent goal difference, much of the fight is still ahead of us".