A total of 120 bus drivers had been infected with Covid-19 from clusters involving seven bus interchanges as at noon on Sunday, said the Land Transport Authority (LTA).
This is about 1.3 per cent of the 9,500 bus captains here, said LTA, which is working with public transport operators to minimise the impact on services.
The number of clusters grew after the first two - in Bishan and Sengkang - were announced on Aug 14.
An LTA spokesman told The Straits Times yesterday that many of the cases were picked up early and clusters isolated as a result of community surveillance testing and proactive regular testing.
Last night, another cluster was announced involving staff from Tampines Bus Interchange, brin-ging the total number of clusters linked to bus interchanges to eight.
It was not immediately clear how many more bus drivers had been infected.
Of the 120 infected bus drivers previously announced, four were unvaccinated.
The rest are fully inoculated, and most are asymptomatic or showing only mild symptoms.
Overall, 99 per cent of front-line public transport workers have completed their first dose of a vaccine, and more than 95 per cent are fully inoculated, said the spokesman.
He said: "Once a cluster at a public transport node is detected, deep cleaning and disinfection are carried out at the location as well as on all affected buses and common facilities. This is in addition to the stepped-up cleaning regime that operators have put in place since last year."
Safe management measures are also strictly enforced, and in the light of the recent developments, workers will have to take their meals and smoke breaks alone even if they are fully vaccinated, said the spokesman.
Professor Paul Tambyah, senior consultant at the National University Hospital's infectious diseases division, said the clusters are a "slight cause for concern" as the mode of transmission is still unknown.
"But even if we are unable to figure out how these clusters are arising, the number of cases is still unlikely to be more than the number infected in the markets linked to the Jurong Fishery Port," he added.
He said the next few days will be crucial in allowing experts to estimate the potential size of the clusters linked to the bus interchanges.
Professor Teo Yik Ying, dean of the National University of Singapore's Saw Swee Hock School of Public Health, said the infections may have spread through bus drivers who reside in the same households, or could be due to the fact that bus captains rotate between at least two interchanges each.
"Commuters can play their part by making sure they wear their masks properly while moving about in public, including on public transportation," he said.