SINGAPORE - Concerns about the risk of Covid-19 transmission to passengers by the Singapore Airlines steward who worked as a Grab driver are valid, but mask wearing and other precautions taken, as well as the robust contact tracing system here, will mitigate the likelihood of a cluster forming.

Professor Teo Yik Ying, dean of the Saw Swee Hock School of Public Health at the National University of Singapore (NUS) said while there is indeed a higher risk of infection for passengers picked up by the infected Grab driver, contact tracing and isolating those exposed will limit a spread.

"There will always be such incidents cropping up in future, and that is exactly why everyone in the community needs to remain vigilant, and our testing and contact tracing efforts cannot cease," he noted.

The 48-year-old steward was detected through proactive testing arranged by the Civil Aviation Authority of Singapore (CAAS) for air crew who had returned from overseas flights.

He had travelled to the United States for work from Dec 12 to Dec 16. After his return, he worked part-time as a Grab driver from Dec 17 to 24.

The man was tested on Dec 23 and the results were returned as inconclusive on Dec 25. A second test on Dec 25 also came back inconclusive.

He remained at home till Dec 27, when he was confirmed to be infected.

The Health Ministry (MOH) on Wednesday (Dec 30) said that passengers who took the Grab ride are being tracked down through contact tracing.

It added that any passenger deemed to be a close contact of the man will have to be quarantined.

Ride-hailing firm Grab, which is supporting contact tracing efforts, said the man had been temporarily suspended from its platform as a safety precaution.

Associate Professor Alex Cook, vice-dean of research at the Saw Swee Hock School of Public Health, said that the situation is "definitely a concern".

"It's hard to imagine a worse possible situation than having someone potentially exposed to viruses overseas, being able to bypass quarantine, and coming into contact with a large number of people in a closed environment like a private-hire car," he added.

But he noted that Grab had put in place precautions like mask wearing. Passengers are also not allowed to sit in the front seat, where droplets are more likely to spread.

Prof Cook said the firm is also able to identify passengers who rode in the particular vehicle during that week.

However, given the close proximity of driver and passengers in a car, there is a higher risk of infection, said Prof Teo.

"The decision to quarantine the affected passengers is the right one, and the Grab platform will facilitate the contact tracing process in identifying all affected passengers."

He said while there is a small risk of a community cluster developing, if a passenger is infected and spreads the virus to others in his or her social network, contact tracing will be able to identify the rest and they will be isolated as well.

Dr Leong Hoe Nam, an infectious diseases expert at Mount Elizabeth Novena Hospital, said the incident suggests that safe measures for those who work overseas, such as Singapore Airlines crew, may not be sufficient.

He laid out options for more measures to be introduced for air crew - implementing a 14-day stay-home notice period, though it would impair the airline's ability to function; more frequent Covid-19 testing; or mandatory vaccination.

The CAAS announced last Wednesday that it was tightening measures to ensure the safety and well-being of air crew and to safeguard public health in Singapore.

It came after two SIA personnel - a pilot and the steward who worked as a part-time Grab driver - tested positive for the virus.

Airline crew here had been exempt from stay-home notices since March, provided precautions were taken to minimise their risk of exposure to the virus.

CAAS also said those who layover in high-risk destinations will be required to undergo polymerase chain reaction (PCR) tests on arrival and on the third and seventh day following their return to Singapore.

As for public transport passengers, including those who tap ride-hailing services, Prof Teo said masks should be kept on at all times.

"This is especially important in taxis and private-hire vehicles. It is also a good practice to disinfect hands before and after the journey in any public transport, not just in taxis or private-hire vehicles but also in buses and MRT trains.

"All these habits go towards reducing infection risks," he added.

A passenger of the SIA steward on Dec 23 wrote to ST on Tuesday, expressing his concern that the steward was allowed to drive the private-hire car immediately after returning from overseas.

"Further bans of flight crew with recent travel history from being drivers of private-hire drivers should be implemented as well," said the man, who did not want to be named.

He was placed under quarantine on Monday and has so far tested negative for Covid-19.

"Grab should also have a check on all their drivers to make sure they do not have any travel history over the past 21 days," he added.