Coronavirus can travel twice as far as official ‘safe distance’ and stay in air for 30 minutes, Chinese study finds
- Authorities advise people to stay 1-2 metres apart, but researchers found that a bus passenger infected fellow travellers sitting 4.5 metres away
- The scientists behind the research said their investigation also highlighted the importance of wearing face masks because of the length of time it can linger
Note: The study at the centre of this article on the transmission of the coronavirus was retracted on Tuesday by the journal Practical Preventive Medicine without giving a reason. The South China Morning Post has reached out to the paper's authors and will update the article.
The coronavirus that causes Covid-19 can linger in the air for at least 30 minutes and travel up to 4.5 metres – further than the “safe distance” advised by health authorities around the world, according to a study by a team of Chinese government epidemiologists.
The researchers also found that it can last for days on a surface where respiratory droplets land, raising the risk of transmission if unsuspecting people touch it and then rub their face.
The length of time it lasts on the surface depends on factors such as temperature and the type of surface, for example at around 37C (98F), it can survive for two to three days on glass, fabric, metal, plastic or paper.
These findings, from a group of official researchers from Hunan province investigating a cluster case, challenge the advice from health authorities around the world that people should remain apart at a “safe distance” of one to two metres (three to six and a half feet).
Their work was based on a local outbreak case on January 22 during the peak Lunar New Year travel season. A passenger, known as “A”, boarded a fully booked long-distance coach and settled down on the second row from the back.
The passenger already felt sick at that point but it was before China had declared the coronavirus outbreak a national crisis, so “A” did not wear a mask, nor did most of the other passengers or the driver on the 48-seat bus.
China requires closed circuit television cameras to be installed on all long-distance buses, which provided valuable footage for researchers to reconstruct the spread of the virus on the bus, whose windows were all closed.
“It can be confirmed that in a closed environment with air-conditioning, the transmission distance of the new coronavirus will exceed the commonly recognised safe distance,” the researchers wrote in a paper published in peer-review journal Practical Preventive Medicine last Friday.
The paper also highlighted the risk that the virus could remain afloat even after the carrier had left the bus.
The scientists warned that the coronavirus could survive more than five days in human faeces or bodily fluids.
They said the study proves the importance of washing hands and wearing face masks in public places because the virus can linger in the air attached to fine droplet particles.
“Our advice is to wear a face mask all the way [through the bus ride],” they added.
Hu Shixiong, the lead author of the study who works for the Hunan Provincial Centre for Diseases Control and Prevention, said the security camera footage showed patient “A” did not interact with others throughout the four-hour ride.
But by the time the bus stopped at the next city, the virus had already jumped from the carrier to seven other passengers.
These included not only people sitting relatively close to “patient zero”, but also a couple of victims six rows from him – roughly 4.5 metres away.
They all later tested positive, including one passenger who displayed no symptoms of the disease.
After these passengers left, another group got on the bus about 30 minutes later. One passenger sitting in the front row on the other side of the aisle also became infected.
Hu said the patient, who was not wearing a mask, was likely to have inhaled aerosols, or tiny particles, breathed out by the infected passengers from the previous group.
Aerosols are light-weighted particles that are formed from tiny droplets of bodily fluids.
“The possible reason is that in a completely enclosed space, the airflow is mainly driven by the hot air generated by the air conditioning. The rise of the hot air can transport the virus-laden droplets to a greater distance,” said the paper.
After getting off the shuttle bus, the initial carrier got on a minibus and travelled for another hour. The virus jumped to two other passengers, one of whom was also sitting 4.5 metres away from patient “A”.
By the time the study was finished in mid February, patient “A” had infected at least 13 people.
It is generally believed that the airborne transmission of Covid-19 is limited because the tiny droplets produced by patients will quickly sink to the ground.
This belief has prompted the Chinese health authorities to suggest that people should stay a metre apart in public and the US Centres for Disease Control recommend a safe distance of six feet (about 1.8 metres).
The researchers also found that none of those passengers in the two buses who wore face masks were infected.
They said it vindicated the decision to ask people to wear a face mask in public.
“When riding on more closed public transportation such as subways, cars, planes, etc, you should wear a mask all the time, and at the same time, minimise the contact between your hands and public areas, and avoid touching your face before cleaning,” they said.
The researchers also suggested improving sanitation on public transport and adjusting the air conditioning to maximise the volume of fresh air supplied.
They also said interiors should cleaned and disinfected once or twice a day, especially after passengers arrive at the terminal.
A doctor in Beijing involved in the diagnosis and treatment of Covid-19 patients said the study had left some questions unanswered.
For instance, the passengers sitting immediately next to the carriers were not infected, though they were suffering the highest exposure to the disease-bearing aerosols.
“Our knowledge about this virus’s transmission is still limited,” he said.