Chinese bus offers new evidence of airborne coronavirus spread, study says

A passenger rides on a bus amid rainfall in Beijing, China.
A passenger rides on a bus amid rainfall in Beijing, China.PHOTO: REUTERS

WASHINGTON (AFP) - A person on a poorly ventilated Chinese bus infected nearly two dozen other passengers with coronavirus even though many weren't sitting close by, according to research published on Tuesday (Sept 1) that offers fresh evidence the disease can spread in the air.

Health authorities had initially discounted the possibility that simply breathing could send infectious micro-droplets into the air, but did a U-turn as experts piled on pressure and evidence mounted.

The article published on Tuesday in JAMA Internal Medicine probes the threat of airborne infection by taking a close look at passengers who made a 50-minute trip to a Buddhist event in the eastern Chinese city of Ningbo aboard two buses in January before face masks became routine against the virus.

Researchers believe a passenger, whose gender was not identified, was likely patient zero because the person had been in contact with people from Wuhan, the city where the contagion emerged late last year.

The scientists managed to map out where the other passengers sat, and also test them for the virus, with 23 of 68 passengers subsequently confirmed as infected on the same bus.

What is notable is that the sickness infected people in the front and back of the bus, outside the perimeter of 1-2 metres that authorities and experts say infectious droplets can travel.

On top of that, the sick passenger was not yet showing symptoms of the disease, such as a cough, when the group made their trip to a religious event.

Researchers also noted the air conditioning simply recirculated the air inside the bus, which likely contributed to spreading of the virus.

"The investigations suggest that, in closed environments with air recirculation, Sars-CoV-2 is a highly transmissible pathogen," they wrote, referring to the name of the virus.

"Our finding of potential airborne transmission has important public health significance."

Their study, which includes a diagram showing where each infected passenger sat, adds to the evidence of airborne transmission, including research into how the virus spread between diners' tables at a restaurant in the southern Chinese city of Guangzhou.

Vice-dean of research at the National University of Singapore's Saw Swee Hock School of Public Health (SSHSPH), Associate Professor Alex Cook, said while the study adds further evidence to support aerosol-based transmission, it "doesn't say anything about the relative importance of droplet versus aerosol transmission, which is one of the big questions we have now".

"It seems that the bus was poorly ventilated with recirculating warm air, because it was still winter and awareness of the emergence of the pandemic was low, and neither the index case nor the other passengers wore masks. The cluster demonstrates the importance of wearing masks in public, especially on public transport, and ensuring adequate ventilation," Prof Cook noted.

SSHSPH dean, Professor Teo Yik Ying, said the report comes as no surprise as it is a known fact that there are 3Cs that drive super spreading events: closed off environment with poor ventilation, crowded spaces and close contact settings.

"The bus would have hit the 3Cs if it was crowded," said Prof Teo.

"This incident is exactly the reason why we continue to highlight to the public that if they do not feel well, they really should not be travelling outside except to see a doctor; and also the reason why we continue to remind people on the importance of mask wearing and personal hygiene."

Additional reporting by Clara Chong