This story takes us from a bus full of religious worshippers travelling to an outdoor event in eastern China, to the bathrooms of a high-rise apartment block in southern China.
On the surface the two scenarios may not seem to have much in common, except that they’re both examples of community outbreaks of COVID-19 most likely caused by airborne transmission.
So what can we learn from these two case studies?
Catching COVID on a bus
On a sunny winter’s day on January 19 this year, 126 people took two buses to an outdoor worship event being held at a temple in Yinzhou District in Ningbo.
No confirmed cases of COVID-19 had yet been reported in the city, but unbeknownst to the passengers on one of the buses, one of their number had been infected with the disease a couple of days before, but the person wasn’t showing any symptoms.
After the bus ride to and from the temple 23 other passengers on the bus would also develop COVID-19. No-one from the other bus was infected.
The bus trip had the ingredients for COVID-19 to spread via airborne transmission, says Lidia Morawska, director of the International Laboratory for Air Quality and Health at Queensland University of Technology.
There was an infected person on the bus and the air was recirculated inside the bus with no proper ventilation.
“This is a perfect prescription for an outbreak,” Professor Morawska says.
Adding further weight to the theory that people were infected by airborne transmission of the virus, is that the researchers studying the case grouped passengers into high and low risk zones depending on their proximity to the infected person, but this didn’t have a significant impact on whether they caught the disease or not.
Apartment plumbing connects cases
A couple of weeks later further south in Guangzhou, an outbreak of COVID-19 occurred in a high-rise apartment block.
Members of a family living on the 15th floor of the building had recently been to Wuhan where the pandemic started.
On their return, they used the toilet in their master bathroom.
Some days after they first showed symptoms of COVID-19, neighbours on the 25th and 27th floors in apartments directly above theirs were also diagnosed with COVID-19.
It is thought virus particles in the faeces of the first family were aerosolised and travelled up the building via the shared drainage system, and through dried out water seals into the other apartments’ master bathrooms.
While we probably don’t yet know all the details of this case, because outbreak studies are always retrospective, Professor Morawska says the research clearly points to airborne transmission again being the culprit.
And we do have precedence for such disease transmission occurring.
In an infamous case from the 2003 SARS outbreak known as the Amoy Garden cluster, 321 people from a private apartment complex in Hong Kong were infected from one person visiting the complex a couple of times and using the toilet in his brother’s apartment on both occasions.
It is reasonable to expect that something like this could have happened in Guangzhou too, Professor Morawska says.
Particularly as the load of the virus in faecal aerosols can be higher than in expelled breath.
While it’s normal for Australian high-rise apartment buildings to also have shared drainage systems, it would be highly unlikely for such a scenario to occur here because of our very stringent standards, says hydraulic engineer and Engineers Australia spokesperson, Adrian Zirps.
“It is very unlikely for any traps to basically dry out, unless there’s a major blockage or there’s an error in the pipework, which usually will be fixed fairly quickly because people notice that straight away,” Mr Zirps says.
“But it’s not a normal occurrence.”
How to reduce your chance of catching COVID
From the mounting evidence of cases like this, airborne transmission of COVID-19 is the most probable cause, Professor Morawska says.
To reduce your own risk of catching COVID-19 this way, she recommends wearing a mask if you’re going to take a bus trip or similar, and open the windows if you can.
Transport operators should look at how they can increase ventilation on their vehicles.
At home, make sure you always close the lid before you flush the toilet.
“This, actually should be something which is done all the time,” Professor Morawska says.
“We shouldn’t aerosolise anything from the toilet.”