New Delhi: More than a year into the Covid pandemic some questions about the spread of the SARS-CoV-2 virus remain. As several states in India went back into lockdown to combat the second surge in infections around April, experts are yet to conclusively understand whether the virus can spread through plumbing in high-rise apartments.
Scientists have been speculating about the vertical transmission of the coronavirus — that is whether an infected person in a high-rise building put families living on the floors above or below them, at risk of infection?
While several studies have pointed out that under specific circumstances such spread is possible, scientists are yet to conclusively establish a Covid case that can be attributed to vertical transmission.
In theory, such transmission is possible. In the past, during the 2003 SARS outbreak, one such case of vertical transmission was identified in Hong Kong.
While Hong Kong had been reporting 50 cases a day at the time, one apartment block in Hong Kong’s Amoy Gardens saw over 200 people contract the virus almost overnight. Their only connection was that they lived on the floors above or below each other in the same apartment block.
A total of 329 residents at Amoy Gardens came down with SARS and 42 of them died. Of these, 22 were in the Block E.
Experts found evidence that the building’s sewage system was involved in the vertical spread of the virus. Intense diarrhoea from one of the patients was believed to have spread the disease through defective piping in the building.
A break in E block’s flush-water system earlier that month had meant the water-sealed S-bend in some of the apartments’ toilets had been dry for an extended period, allowing virus-laden droplets to collect from the system’s soil pipe.
Bucket flushing — pouring buckets of water to flush out the contents of the toilets — by residents may have disturbed and released contaminated droplets.
In case of SARS-CoV-2, a study published in December 2020 also pointed to the possibility of a similar incident of spread through a connected sewage system.
A high-rise apartment building in Guangzhou, China that was home to over 200 people, was studied by a team of scientists. Nine people from three families living in the building were infected with Covid-19.
While the first family had a history of travel to Wuhan — the epicenter of the Covid pandemic — the other two families had no travel history and a later onset of symptoms.
The team found no evidence of transmission via the elevator or anywhere else.
However, the families lived in three vertically aligned flats connected by drainage pipes in the master bathrooms.
The researchers concluded that the observed infections and the locations of positive environmental samples pointed to the vertical spread of virus-laden aerosols via these stacks and vents.
Unlike the Amoy Gardens incident, in this case, however, the research team was unable to determine whether the water seals were dried out in the flats of the infected families. But other alternate explanations of transmission — such as a shared elevator or physical proximity of the residents — were all ruled out after the scientists looked at the CCTV footage from the apartment.
In an accompanying editorial in the same journal, Michael Gormley, a researcher at the Heriot-Watt University in the UK, noted that there are challenges to conducting epidemiologic studies on transmission in high-rise buildings.
“Establishing infectivity is much more complicated than establishing the presence of viral RNA, so more definitive evidence is anticipated to emerge with time,” Gormley noted.
He also stated that although evidence on such transmission is building, “it is not yet strong enough to warrant wide-scale intervention — but it does warrant some precautions.”
What are the odds
Rakesh Mishra, director, Centre for Cellular & Molecular Biology (CCMB) of the Council of Scientific and Industrial Research (CSIR), has been part of a team that has been surveilling the presence of the virus in wastewater in sewage systems in Hyderabad.
Mishra said that the virus particles found in the wastewater were not viable — that is, it could not transmit the disease. He added that scientists have been unable to grow the virus in the lab using samples collected from faecal matter.
“Generally, the materials that we find are RNA [Ribonucleic acid, genetic material of the virus] particles – which can help us estimate how much virus may have been there in the community,” Mishra told ThePrint.
“There is no evidence that Covid can be transmitted through water,” he added. He also noted that the virus itself is very susceptible. It gets destroyed in the sewage systems the moment it is exposed to detergents.
While the virus does enter the sewage system through faecal matter, the way it spreads is very different — the exposure has to be through the respiratory tract, Mishra explained.
“If the toilet water is somehow immediately transformed into aerosol form (after an infected person used it), then there is a chance of the virus getting into the respiratory tract,” Mishra said.
A study in June last year did show that flushing a toilet can create a cloud of virus-containing aerosol droplets that lasts long enough for others to breathe in.
However, this study used computer models to simulate water and air flows in a flushing toilet and the resulting droplet cloud. It did not look at the real-world instance of such a spread.
The only way the formation of such virus-laden aerosol can happen is when a person flushes the toilet, concurs S. Venkata Mohan, a researcher from CSIR-Indian Institute of Chemical Technology (IICT).
“When an infected person flushes the toilet after using it, virus laden particles are likely to rise out of the toilet bowl. If the bathroom is connected to the air duct, then people from top floor to ground floor are at risk of infection,” Mohan told ThePrint.
However, Mohan noted that this was speculative.
“In the Indian context, we have not yet found any such cases.”
However, his team was planning to conduct such studies — although Mohan said that so far getting permission from all residents in a building to participate in such studies has been difficult.
Since flushing is the only route that will transmit the aerosol, Mohan recommended some simple precautions that people living in high-rises could take.
“Close the cover of the toilet before flushing to prevent the aerosol formation. If a person is infected or suspected to be infected, then disinfectant sprays in the bathroom and closing the toilet lid while flushing can reduce the risk for other residents,” he said.
Turning on the exhaust fan in bathrooms after using it, would also ensure that the aerosols were dispersed into the outside environment, he added.
However, there has been so far no direct evidence of vertical transmission, and researchers believe that the risk of such vertical transmission was extremely low.
(Edited by Poulomi Banerjee)
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