SINGAPORE - Four people returned positive tests for Covid-19 after swabs were conducted on more than 1,000 residents and visitors of three Housing Board blocks in Yishun and Hougang.

Three of the four were detected after 509 residents and visitors of Block 745 Yishun Street 72 were tested, the Ministry of Health (MOH) said on Wednesday (June 2).

The fourth case was found among the 681 people tested at Block 501 and Block 507 Hougang Avenue 8 as of Wednesday.

Of those, 665 people were negative, and 15 more test results were pending.

All residents of the three HDB blocks were asked to undergo swab tests after surveillance measures and wastewater testing detected likely cases of Covid-19 transmission there.

Investigations are ongoing to determine linkages and the source of transmission.

Infectious disease experts told The Straits Times it was not possible for residents of one block of flats to catch Covid-19 from residents of a neighbouring block via the wastewater system.

Professor Paul Tambyah, president of the Asia-Pacific Society of Clinical Microbiology and Infection, said: "Detection of Covid-19 fragments in wastewater does not mean that was the mode of transmission - it is a screening tool to pick up the presence of individuals who are shedding the virus."

A PUB spokesman also said that sanitary systems here are "closed systems" where the pipes are airtight and watertight to ensure no foul air, viruses or bacteria can travel between household or blocks.