The Covid-19 viral strain that caused the cluster at Block 506 Hougang Avenue 8 has been found to be the same as the one detected at other blocks in the vicinity.

This points to either a common exposure to the virus or some people being initially infected and then subsequently spreading it through their household or via common areas such as lifts and staircases, Singapore's director of medical services Kenneth Mak said yesterday.

"It does not necessarily mean that the virus has spread through sewage systems or even airflow," he said at a virtual news conference by the multi-ministry task force on Covid-19.

But investigations are still ongoing, and the Ministry of Health has yet to arrive at a firm decision on what the mode of transmission is.

It is also looking at the other cases that have arisen in neighbouring blocks.

Tests suggest that the viral strain for these infections is the same.

The virus' genome is first sequenced before a phylogenetic analysis is done, which allows for cases to be linked to clusters and variants of concern to be identified.

"We are also focusing on the blocks around Block 506…

"The initial testing we did was for that particular block, but we have extended it now as we have also picked up other cases in the vicinity," said Associate Professor Mak.

Block 506 was first announced as a new cluster on June 3.

As at yesterday, the cluster has 13 cases.

A special testing operation was also carried out at the block on May 21 and May 22 after the earlier cases were detected.

Covid-19 viral fragments were detected in wastewater samples collected from Blocks 501 and 507 Hougang Avenue 8.