While the risk of contracting Covid-19 during a flight has been reduced with measures such as air filtering in place, passengers should continue to take precautions while on board.

Infectious diseases experts urged caution when flying, saying that the risk of getting infected from somebody else on board cannot be eliminated completely.

Their comments come after industry group International Air Transport Association (Iata) released findings this month stating that the risk of Covid-19 transmission on board planes appears to be "very low", and that current figures "are extremely reassuring".

Airplane manufacturers Airbus, Boeing and Embraer had backed the findings with their own studies.

Professor Teo Yik Ying, dean of the National University of Singapore's Saw Swee Hock School of Public Health, who noted that there have been documented instances of transmissions during flights, said: "Travellers should pay heed that it is entirely possible to be infected, and they should take every precaution to protect themselves should they be travelling on a plane."

Wearing face masks properly and maintaining good hygiene will greatly reduce the risk, he said.

"The only concern will be when passengers remove their masks during meal times, but the risk is also reduced compared with normal meals at home or in restaurants since passengers typically are facing forward and not each other."

Dr Leong Hoe Nam, an infectious diseases expert at Mount Elizabeth Novena Hospital, said that one potential source of risk could be when passengers use the airplane toilets.

To manage this risk, passengers can clean surfaces inside the toilet before touching them, and also wash their hands thoroughly after.

Passengers can also sit farther away from the aisle when possible to reduce the risk of coming into direct contact with an infected person, he added.

Dr Wong Sin Yew of the Infectious Disease Specialists Group said that current data does indicate a low risk of Covid-19 transmission when the use of a mask is mandated, referring to a study on the risk of in-flight transmission that was published in the Journal of Travel Medicine last month.

The study by Dr David Freedman, a United States infectious diseases specialist, and Dr Annelies Wilder-Smith of the London School of Hygiene and Tropical Medicine found that compulsory face coverings on flights were associated with lower transmission risks.

It also concluded that the absence of large numbers of confirmed and published in-flight transmissions of Covid-19 is encouraging but is not definitive evidence that plane passengers are safe.

Iata had drawn upon the research to suggest that it is safe to fly, and also suggested a low risk of trans-mission based on figures centred on the documenting of just 44 identified potential cases among 1.2 billion travellers this year.

But Dr Freedman told news agency Reuters that Iata's calculation "was bad maths" as most of the passengers were not tested. The absence of evidence is not evidence of absence, he added.

The World Health Organisation has said that the lack of extensive documentation of in-flight transmission does not mean that it does not happen, although it noted that the risk appears to be very low.

When asked about cases of in-flight transmission that occurred on its planes, Singapore Airlines said it was unable to provide information about it.

Budget carrier Jetstar Asia said there have been no reported cases of passengers or crew being infected on board its flights.

Prof Teo told ST: "It is impor-tant that we interpret the present data carefully.

"Given that many flights operating during this period have kept to some degree of social distancing between passengers, for example, by keeping the middle seats empty, the data that has been collated to date may not adequately reflect transmission risk when airlines start to resume normal operations and fill all possible seats."

Experts agreed that moving forward, pre-flight testing for Covid-19 is key in reducing the chances of an infected person getting on board.

Dr Leong said that even if antigen testing - which delivers faster results but is less accurate than the polymerase chain reaction tests - is used to screen passengers, it can help to detect the bulk of those infected.

Dr Wong said: "Pre-departure testing for Covid-19 is very important in reducing transmission, and compulsory testing should be seriously considered for all flights."

But he cautioned that the safe resumption of air travel on a large scale is "nowhere near", with several hundreds of thousands of cases occurring daily.

Prof Teo said: "I anticipate that air travel can resume in an almost unrestricted manner in the form of travel bubbles between cities that are able to keep Covid-19 at bay.

"For other cities that are struggling to keep the coronavirus under control, it is unlikely that large-scale air travel can resume, simply because the demand will not be there."