Twenty-four passengers on the bus in Zhejiang Province tested positive for the virus after a single hour-and-a-half journey on January 19.
Scientists believe the air conditioning unit allowed the transmission of the virus by spreading viral droplets right through the bus.
Some air conditioners take in air from outdoors and expel it again, while others, called split units, recirculate the same air, which was the case in this instance.
British experts last month warned such units need to be either turned off or used with the windows open to prevent the spread of Covid-19.
And offices, cinemas, theatres and other indoor amenities in the UK are required to have newly serviced air con units that ensure fresh air is being pushed out.
Buses have once again been earmarked as potential breeding grounds for Covid-19 after two dozen cases were linked back to a single passenger in China (stock image of a London bus)
The latest study, published in JAMA Internal Medicine, was carried out by scientists at the Zhejiang Provincial Center for Disease Control and Prevention.
They traced the outbreak back to a single passenger on a bus in the eastern province on January 19, days before China locked down.
Passengers were not socially distanced but some were thought to have been wearing face masks.
AIR TRANSMISSION OF COVID-19 BEING ‘UNDERESTIMATED’
Air transmission of Covid-19 is being underestimated – which could impact guidelines on social distancing, ventilation systems and shared spaces, warns new research.
Researchers at Heriot-Wyatt University and University of Edinburgh found evidence that both small and large droplets can travel relatively long distances through the air – and not always in predictable directions with airflow.
The World Health Organisation (WHO) warned aerosol transmission of Covid-19 is being underestimated as a study reveals droplet spread from humans does not always follow airflow.
Researchers say the new findings on droplet migration may have important implications for understanding the spread of airborne diseases such as Covid-19.
It comes after top US infectious disease specialist Dr Anthony Fauci admitted during a Monday JAMA interview that there is much unknown about how coronavirus spreads through the air and that he himself needs to ‘study’ papers that suggest big droplets can travel further than six feet.
Scientists of the study at Heriot-Watt University and the University of Edinburgh in Scotland echoed his sentiments that a better understanding of different droplet behaviors and their spread based on droplet size is also needed.
A total of 68 passengers rode the bus for an hour and forty minutes to attend a Buddhist worship event.
Twenty-four passengers tested positive for Covid-19, with 18 of them suffering ‘moderate illness’. The other six either had mild sickness or were asymptomatic.
Passengers sat closest to the original patient were not more likely to get the disease than those at the opposite end of the bus.
This suggests the air con played a role in picking up the viral particles and recycling them elsewhere through vents on the bus.
Writing in the paper, the authors said: ‘The investigations suggest that, in closed environments with air recirculation, SARS-CoV-2 is a highly transmissible pathogen.
‘Our finding of potential airborne transmission has important public health significance, and future efforts at prevention and control should consider the potential for airborne spread of COVID-19.’
The outbreak comes on the back of warnings by British experts that so-called split AC units need to be either turned off or used with the windows open.
Dr. Shaun Fitzgerald, a fellow at the Royal Academy of Engineering, said that opening a window while having the air conditioning turned on may be the best way to reduce the risk.
‘The recommended strategy now, if you have one of these split units, is to throw the window open and sacrifice your desire for a cold or cooler environment,’ Fitzgerald told the Telegraph.
‘If there is a modicum of wind it will move the air around. If you can’t open a window turn the unit off.’
The Chartered Institution of Building Service Engineers warns on its website: ‘Air conditioning units that do not have a ‘dedicated source of outside air supply into a room… could be responsible for recirculating and spreading airborne viral particles into the path of socially distanced users.’
Researchers in April blamed the air conditioning unit for the spread of coronavirus among at least nine other diners who were eating in a restaurant in Guangzhou, China, in January.
A research paper which appeared in the journal Emerging Infectious Diseases investigated the incident at the eatery in January, where a family had arrived from Wuhan – the city where the Covid-19 pandemic began.
Researchers say one member of that family had an asymptomatic case, and barely two weeks later, the patient along with nine others, including members of their family, as well as two other groups on nearby tables in the restaurant, had all become ill with the virus.
The affected tables in the windowless venue were around three feet away from each other as the authors claim the most likely cause of this outbreak was droplet transmission.
However, they say that droplets only remain in the air for a short time and only travel short distances.
Therefore, they concluded, the air conditioner was likely to have spread the virus further between the affected tables.