Is it plausible that the new cases contracted in the Pullman hotel in Auckland were transmitted via pipes? Here’s what the science tells us.

As the investigation to determine how three people became infected with Covid-19 during their stay in New Zealand’s managed isolation and quarantine system gets away, it is worth considering a few studies published recently that point to drainage and ventilation systems as a possible route of transmission. That isn’t to say this is the case for the MIQ hotels but is definitely warrants investigating and ruling out.

China’s Block X – transmission through the drainage system?

Min Kang and colleagues investigated how Covid-19 might have spread between three families living in a high-rise apartment building in Guangzhou, China, referred to as Block X. The first family had travelled to Wuhan. The other two families had not travelled but developed symptoms later. Block X has 83 apartments spread over 29 floors. One possible point of transmission could have been a shared space like a lift. But here’s a big clue that it wasn’t a lift. The three families lived on different floors of the building, but their apartments were vertically aligned and connected by drainage pipes in the master bathrooms.

The working theory put forward by the researchers is that aerosols of virus generated from toilet wastewater – remember, some people excrete virus in their poo – was able to travel between the apartments through the connecting pipes. To test this, the researchers released the gas ethane through the drainage system to see where it went. Surprise, surprise, they could detect the ethane in the bathrooms connected by the same pipes.

One thing the researchers pointed out is that the building’s drainage system is designed to have water traps in the U-bends – presumably to stops smells/aerosols moving between apartments. But in interviews the residents admitted to not topping up the water every month. That means it’s likely many of the apartments didn’t have these water traps in place. Even so, the researchers were able to detect the ethane in apartments with water traps.

The researcher’s focused their attention on aerosols generated by flushing toilets. They may well have written their paper back before we thought the Covid-19 virus was airborne. Now we know differently. I’m curious whether aerosols from people just breathing/talking/sneezing/coughing in the bathrooms would also have been able to travel through the drainage system. I don’t know enough about ventilation and drainage systems to know whether that would be the case so feel free to get in touch if this is your area of expertise.

South Korean apartment block – transmission through the ventilation system

Seo Eun Hwang and colleagues have recently published their study of infections in a high-rise apartment block in South Korea. The block is shaped like a horseshoe with between nine and 15 storeys and as many as 21 apartments on each floor. The building also has natural ventilation shafts that run along each vertical line from the rooftop down to the bottom floor. Each shaft is connected to the apartments through what the authors describe as a “blowhole” in the bathroom.

In August 2020, a family living on the sixth floor of the block were diagnosed with Covid-19. Shortly afterwards a child living on the fifth floor also tested positive. Then a woman living on the fourth floor. All their apartments were on the same vertical line. At that point officials decided to test everyone in the building. More than 400 people from 267 households were tested and five more people were found to be positive: the mother of the child on the fifth floor, residents living on the 10th and 11th floor of the same vertical line, and residents living on the second and 11th floor of the next vertical line.

Despite them all using the same lifts, the only people who tested positive were those who lived on two vertical lines of the building. The families didn’t know each other so hadn’t been in each other’s homes. That’s pretty compelling evidence that the virus spread through the floor drains or air ducts.

Both of these studies are really good examples of how countries with low or minimal community transmission and well-functioning testing and contact-tracing systems are helping us understand Covid-19 and how the virus transmits. Our latest cases will be the same.

Should we be worried about these latest cases?

It’s worth remembering that over 100,000 people have come through New Zealand’s MIQ system since it was put in place. Yet we’ve experienced just a small number of border incursions. Each one has been an opportunity to learn and improve. And as our understanding of Covid-19 increases, there will always be room for improvement. But we should also remember that this is the real world, and no system is 100% guaranteed to work all the time. I don’t agree that we should be stopping New Zealanders and permanent residents from being able to return home to remove the risk of any border incursions.

Building new MIQ facilities in the middle of nowhere might sound sensible but is completely impractical. It would be like building a small town from scratch. And even that wouldn’t remove the risk entirely. To do that we’d need our new town to have buildings properly engineered to prevent an airborne microbe from transmitting. I’ve been involved in the design and build of a lab like that when I worked on tuberculosis. The lab has airlocks and doors that won’t open until others have closed. The flow of air within it is completely controlled. And that air can’t leave until it has passed through special filters that are designed to capture particles as small as a bacterium or virus. It’s incredible. And very, very, very expensive.

We can manage the risk of Covid-19. And we can stop any cases that do get through MIQ from turning into large outbreaks. But that will require our team of five million to keep working together. So, let’s all remember the Emmental/Swiss Cheese Model, and play our part.