Airborne, droplet or surface transmission: How Covid-19 could have spread in Jem and Westgate

Investigations on how the virus managed to spread and result in clusters at Westgate and Jem, as well as Changi Airport, are still ongoing.
Investigations on how the virus managed to spread and result in clusters at Westgate and Jem, as well as Changi Airport, are still ongoing.ST PHOTO: DESMOND FOO

SINGAPORE - The ways in which the Covid-19 virus can be transmitted have come under renewed scrutiny after a 53-year-old cleaner who works at a stay-home notice facility was identified as a possible source of infection that led to the latest community cluster at Westgate and Jem malls in Jurong East.

Infectious diseases expert Paul Tambyah said the cleaner - if infected at her workplace - was unlikely to have direct contact with the stay-home notice visitors, so the virus would have somehow carried itself through the air or via a contaminated surface to get to the cleaner.

There are three main modes of transmission: airborne transmission - where people get infected over long distances by inhaling very fine respiratory droplets carried by the air and aerosolised particles; transmission through contact with droplets; and the touching of contaminated surfaces.

Most scientists and researchers agree on droplet transmission but debate surrounds airborne and surface transmissions. For instance, an article published by British medical journal The Lancet on May 1 detailed 10 scientific reasons in support of airborne transmission. The World Health Organisation and the United States' Centres for Disease Control and Prevention also recently accepted that the coronavirus can spread through air.

However, some experts whom The Straits Times spoke to felt that droplet and surface transmissions remain the key modes by which the virus spreads, with airborne transmission occurring under more unusual settings such as aerosol-generating environments in hospitals.

Investigations on how the virus managed to spread and result in clusters at Westgate and Jem, as well as Changi Airport, are still ongoing, hence it is possible that multiple modes of transmission were involved. There could also be lapses in personal hygiene, experts said.

Professor Tambyah, who is the president of the Asia Pacific Society of Clinical Microbiology and Infection, explored two schools of thought for the case of the cleaner: airborne versus surface transmission.

The former mode argues that the cleaner was infected because she was breathing in the air that was exhaled by somebody who was infected, and the latter mode of transmission - which Prof Tambyah believes took place - is that the virus was transmitted through contact of contaminated surfaces.

"Those in the first camp would assume that the infected person could have been coughing and the virus was carried through the air and the poor cleaner ended up breathing it in, even though she was wearing a surgical mask. The assumption is that the mask, if worn properly, still failed to block out very small virus particles," Prof Tambyah explained.

Cleaners, unfortunately, come into contact with a lot of very dirty surfaces, especially in toilets, which could lead to virus spread via surface transmission, he added.

"The mode of transmission is something which we have been debating for one year and we still haven't come to a conclusive answer."

There is also the possibility of lapses in personal hygiene and mask wearing, Associate Professor Alex Cook, vice-dean of research at the NUS Saw Swee Hock School of Public Health, said.

"It is quite impossible for rules to be adhered to strictly 100 per cent. The index case could have taken off her mask to rub her nose after touching a surface and that could have resulted in her getting infected," Prof Cook said.

With the jury still out, measures such as the temporary closure of the affected malls and zoning of the airport will help to prevent further spread.

For those who have visited the affected malls, going for a swab test is a good idea, Prof Cook said. As a precaution, Prof Cook himself started using a povidone-iodine throat spray three times a day, as a trial previously conducted by the National University Hospital has shown it to reduce the likelihood of infection by over 20 per cent.

These additional precautions will also help the authorities buy time while investigations are under way, Prof Tambyah said.

"The measures will also provide staff the reassurance that something is being done. The actual work is behind the scenes, where things like molecular genetics studies and environmental sampling are done to identify the cause of these clusters," Prof Tambyah added.

Clusters aside, how likely Sars-CoV-2 will spread also depends on the environment, such as if it is indoors, outdoors and how well the ventilation is, Professor Teo Yik Ying, dean of the Saw Swee Hock School of Public Health, said.

"Some have been concerned that the virus can be spread through transient contact, something which has no fixed definition," Prof Teo said.

If the contact between an infected and a healthy person takes place in a poorly ventilated, enclosed environment indoors, and if the infected person was previously coughing or sneezing in the room, then there may be a good chance of infection even if the healthy person was merely walking past the infected person.

Conversely, if the same degree of interaction took place in an outdoor environment, such as a park or garden, then the risk of infection would be much lower.

Ultimately, it is tough to properly define which is riskier, Prof Teo said, such as comparing a five-minute conversation between colleagues in a pantry as opposed to a five-minute interaction between a customer and a supermarket cashier.

"It really comes down to respiratory etiquette, hand hygiene, proper mask wearing and minimising extended interactions within an enclosed, confined space that is poorly ventilated."