SINGAPORE — The Ministry of Health (MOH) has concluded Covid-19 testing of all 405 residents and visitors of Block 506, Hougang Avenue 8, and two people have been found to be infected with the coronavirus from the exercise.

Test results for 403 people have returned negative, MOH said on Sunday (May 23).

The two infected persons were added to the nine positive cases that prompted the authorities to test all residents of and visitors to the block.

One of the two positive cases had been reported earlier on Sunday when partial results were released.

The authorities on Sunday also began testing residents at Block 559, Pasir Ris Street 51.

MOH said that 188 people have been tested, with 155 individuals having negative results and 33 more waiting for test results. So far, no one has tested positive for Covid-19.

"Initial investigations have revealed that the units with confirmed cases do not share the same stack," MOH said, referring to the flats not being in the same column in the housing block.

"Epidemiological investigations are ongoing to determine linkages and the source of transmission."

In response to media queries on the threshold that would trigger mass testing of a public housing block, MOH said that it carries out precautionary testing when "epidemiology investigations have assessed that there is a possibility of ongoing transmission".

"There is no one single trigger," it said. "Risk assessment is based on a number of factors, such as the total number of cases within the block, the potential risk of transmission by these individuals, and likely modes of transmission."