SINGAPORE - A dozen pre-school staff have tested positive for Covid-19 since active screening for that group began this month, said the Health Ministry's director of medical services Kenneth Mak during a virtual press conference on Thursday (May 28).

However, subsequent testing showed that 11 of these were old infections and the staff members had recovered - meaning they were no longer infectious, Prof Mak added. Results of the serology test, which detects antibodies to tell if a person has been infected, are pending for the last person.

Patients have antibodies around two weeks after they recover from the infection and will not have them at the point of infection.

More than 33,100 swab tests for the coronavirus have been performed on pre-school staff as of Tuesday (May 26), of which 12 tests - 0.036 per cent - have been positive.

All pre-school staff have to undergo a one-time swab test for Covid-19 as a precautionary measure before the centres reopen next month.

"The results of the serology tests, together with the polymerase chain reaction (PCR) tests that we performed, were then critically evaluated. The evidence suggests that these are not active, fresh infections, but old infections," Prof Mak said.

"The test results suggest that these staff members have all recovered from the infection. As a result, it is our evaluation... that none of these staff members are currently a risk to either other colleagues within the pre-school setting, or to the children who are supported and cared for by pre-school staff. And this of course means that they are not infectious..."

They are unable to say for sure when these people had been infected, he said. The majority of the staff members were asymptomatic.

"It reaffirms our assessment that previously, prior to the circuit breaker, there was some level of community spread. But with the circuit breaker in place, we've seen the number of community active cases coming down very significantly, consistent with our assessment that the circuit breaker has been effective."

In a statement issued late on Thursday night, the Early Childhood Development Agency said there will be precautions when pre-schools re-open.

One of these is a "risk-based testing system" that will be in place after services have resumed fully. Staff and children returning from Quarantine Order or Stay-Home Notice, for example, will have to be tested for Covid-19 - and deemed non-infectious - before they return. This also applies to new pre-school staff.

The country's circuit breaker measures will be eased in three phases from June 2 onwards.