The cluster of cases at the bus interchanges is likely to be a workplace transmission, though investigations so far have not identified specific modes of transmission, said Singapore's director of medical services Kenneth Mak yesterday.
"It is highly likely that spread has occurred within the work environment, possibly due to mask-off interactions during rest periods at the bus interchanges," he added.
The results of preliminary phylogenetic tests support the view that the cases are likely a workplace transmission event, Associate Professor Mak said at a press conference held by the multi-ministry task force on Covid-19.
Phylogenetic testing compares different virus genomes and helps scientists deduce the index case of a Covid-19 cluster.
The number of Covid-19 cases linked to eight bus interchanges have ballooned to more than 300 people. The interchanges are: Toa Payoh, Boon Lay, Punggol, Jurong East, Bishan, Sengkang, Tampines and Clementi.
However, it is not clear how infection had first been introduced to the bus interchanges.
"But given the higher number of cases in the community presently, it is not unreasonable to expect that introduction of infection occurred from the community," he added.
He noted some bus captains may have been infected in the community, and then infected other bus captains in the interchange.
"We've also seen some infected bus captains bring infection back into their households, infecting their own household members."
There is currently no evidence of there being any passengers or users of public transport being exposed and infected with Covid-19, so the clusters still appear to be confined to the workplace at the bus interchanges, noted Prof Mak.
"The measures announced by the LTA will prevent potential further spread of infection at the workplace, which helps us to better manage the clusters," he said.
The Land Transport Authority on Thursday introduced a slew of tighter measures to curb Covid-19 spread in the interchanges.
This includes segregating resting and dining areas at the bus interchanges, with only one staff member allowed to sit at each table for meals.
Bus drivers and staff manning service counters in bus interchanges will also have to undergo routine testing at least once a week.
"We are mindful of the need to be targeted and focused in our public health actions, so as not to unduly compromise essential transport services while seeking to disrupt further transmission at the interchanges," said Prof Mak.
He added that the Ministry of Health will continue to work closely with the LTA and the bus companies for bus captains to be tested regularly using antigen rapid test kits, which will supplement the testing operations that are already being conducted at some of the bus interchanges.
More than 7,000 of the staff at the interchanges have been tested so far, with about 2 per cent of them having tested positive for Covid-19.
"It will take the rest of this week for the situation to stabilise and we anticipate that the number of cases from the interchanges may start coming down in the following week," he added.
Reiterating this, Health Minister Ong Ye Kung said that the number of new cases at the interchanges has been slowly coming down.
At Boon Lay Bus Interchange, there were 15 new cases on Monday, compared with five new cases on Thursday.
"The cases are actually coming down, but the overall figures are going up, mainly contributed by unlinked cases and those needing classification, but these are not cases detected through swab operations at the bus interchange," he said.
He added that measures such as testing, isolating, and contact tracing at the interchanges have been working in suppressing infections.