Watch your back: Chinese study finds Covid-19 could be on your tail

Commuters leave a train in Hong Kong.
PHOTO: South China Morning Post

Chinese scientists say they have discovered an unexpected pathway to infection from Covid-19 – with more than 80 per cent of the tiny floating particles of virus on the breath landing on the back and buttocks of a passer-by.

Until now, the highest risk of exposure was commonly believed to be the nose, mouth and eyes .

But Weng Wenguo and his colleagues at Tsinghua University’s engineering physics department suggest the virus can also approach from behind, by riding on the wake of turbulence produced by the body’s movements.

What’s more, it can jump on to a moving person in a surprisingly brief period of time, according to the team’s paper published last month in the peer-reviewed Journal of Tsinghua University (Science and Technology).

When sharing a room with possibly infected people, the best strategy to reduce the risk of air transmission was not to run through them, but to “stay put”, they said.

According to the researchers, the reason is that virus-carrying aerosols can build up near a seated infected person. The wake turbulence caused by someone walking past can draw the infected particles towards the rear of the passer-by.

Laboratory experiments and computer simulations showed the turbulence rose to its maximum soon after passing the seated patient, moving the particles at twice the speed of the walking pace.

The researchers found it could take just four seconds for most of the fresh virus emitted by the patient to reach the walker’s clothing, with the rest reaching even greater distances, according to the paper.

The researchers said their results suggested everyone sitting in a meeting room should wear a mask, while more powerful ventilation could also reduce the amount of virus particles drifting in the air.

But most importantly, people should reduce their activities, the paper said. “It is recommended reducing cross-section movement in infected areas.” Weng’s team could not immediately be reached for comment.

A computer simulation of how the coronavirus droplets may spread from a seated patient to someone passing by. Photo: Tsinghua University

An aerodynamics professor at the Beijing University of Aeronautics and Astronautics – also known as Beihang University – said wake turbulence was a common phenomena, but highly variable.

“It’s the reason for the dust that builds up over the rear window on a car running off-road,” said the scientist, who asked not to be named because of the sensitivity around the pandemic issue.

“The air transmission physics of the coronavirus can be quite complex. There are still a number of unanswered questions regarding how it spreads from one place to another.”

But the behaviour of wake turbulence and its effects could vary widely in different environments, so the results reported by Weng’s team might not work in all conditions, he said.

Researchers use dummies in a lab experiment to determine if coronavirus particles can spread from a seated person to a passer-by

According to the paper, the researchers used a pair of dummies to simulate the seated person with Covid-19 and the passer-by – the latter fixed to a mobile platform. Experimental observation and computer analysis showed warm, lightweight aerosols would rise from the patient’s nose and mouth towards the ceiling.

But some larger, heavier droplets would fall and build up near the ground, they said. The motion of someone walking by would create a wake turbulence, sending most of the light aerosols to the walker’s back. More than half of the larger droplets – fewer and less mobile – reached the legs and bottom, the team found.

In most countries, a brief encounter with a Covid-10 patient is not regarded as a risk – US health advice defines close contact as 15 minutes or more, for example.

China’s health authorities have been more cautious, requiring anyone who comes into contact with an infected person to spend two weeks in home quarantine, regardless of the encounter’s duration. There is so far no scientific study to support the extra precautions.

There is also no scientific evidence that the virus can travel on the wind, as claimed by some Shanghai residents during the city’s prolonged outbreak of the Omicron variant.

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This article was first published in South China Morning Post.