Is it safe to travel by train for hours during the Covid-19 pandemic? Will fellow passengers be infected if a traveller turns out to be a patient?
There are no simple, direct answers to these questions, but a study by Chinese researchers who analysed data from passengers sitting in seats close to a confirmed patient might offer a glimpse into the potential risks.
Passengers in seats on the same row as the infected person averaged a 1.5 per cent chance of catching the virus, about 10 times higher than seats one and two rows apart.
Surprisingly, researchers found that only 0.075 per cent of people who used a seat previously occupied by an infected passenger went on to contract Covid-19.
Time spent with the "index patient" matters, too, according to the study. The risk, measured as attack rate, increased by 0.15 per cent for every hour that a person travelled with an index patient. The risk jumped to a much higher 1.3 per cent for passengers sitting next to the index patient.
The researchers believe that passengers within the same row might be easily infected by each other because of the higher risk of face-to-face contact and because viruses attached on aerosols and droplets are more likely to spread at close range.
Transmission in the same column is less significant possibly because the backrests that separate rows are a good barrier to slow the spread of virus-laden aerosols, they wrote.
Researchers concluded that one metre of social distance would be safe for journeys of an hour or less.
"To prevent Covid-19 spread during an outbreak, the recommended distance is at least two seats apart within the same row, with travel time limited to three hours," researchers wrote. The high-speed train seats face the same direction, and each seat is about 0.5 metres wide.
"Our study shows that although there is an increased risk of Covid-19 transmission on trains, a person's seat location and travel time in relation to an infectious person can make a big difference as to whether it is passed on," said Dr Lai Shengjie, senior research fellow in the geography and environmental science department at the University of Southampton.
"The findings suggest that during the Covid-19 epidemic it is important to reduce the density of passengers and promote personal hygiene measures, the use of face coverings and possibly carry out temperature checks before boarding," he said.
Researchers also split close contacts who shared the same departure-destination stations as the index patients and those who did not; they found the risk was much higher for the first group. The pattern of higher risks with longer travel time in the first group was not observed in the second.
Lai said the decrease might have to do with the small number of close contacts for the second group, only 1,355-strong, most of whom travelled for less than two hours. On the other hand, the data did not identify social status so there was a chance that the higher risk for the first group reflected transmission threat among family members travelling together.
Although the data was collected only for train travel, Lai said it could be used as a reference for other situations such as schools, cinemas and restaurants.
"Considering the similar setting of train in a closed space, the findings from our study can provide some evidence for Covid-19 interventions in these scenarios during the outbreak," Lai wrote.
A high-speed rail passenger has her information checked upon arrival at Beijing West Railway Station in Beijing in April. Photo: Xinhua alt=A high-speed rail passenger has her information checked upon arrival at Beijing West Railway Station in Beijing in April. Photo: Xinhua
The study was jointly conducted by the Chinese Academy of Sciences, China Academy of Electronics and Information Technology, Chinese Centre for Disease Control and Prevention and the University of Southampton, using anonymous data from epidemiological investigations by the Chinese Centre for Disease Control and Prevention.
Researchers identified 2,334 passengers as index patients in different coaches who had reported travelling on China's high-speed trains 14 days from December 19 to February 25 before symptom onset. The data, however, did not take into account if the passengers wore masks during their journey.
Those patients led to 72,093 close contacts whose seats were within three rows of an index patient, and 234 passengers were subsequently confirmed as secondary Covid-19 cases.
The risk is calculated as the "attack rate" for each seat - the number of passengers in a given seat diagnosed with Covid-19, divided by the total number of passengers travelling in the same seat during the same period.
About 150 million passengers travelled by train across China from January 10 through January 23, when the Chinese government imposed a full lockdown on Wuhan and other cities in Hubei province. At least 1,058 people infected with Covid-19 might have travelled by train before Wuhan's lockdown.
This article originally appeared in the South China Morning Post (SCMP), the most authoritative voice reporting on China and Asia for more than a century. For more SCMP stories, please explore the SCMP app or visit the SCMP's Facebook and Twitter pages. Copyright © 2020 South China Morning Post Publishers Ltd. All rights reserved.
Copyright (c) 2020. South China Morning Post Publishers Ltd. All rights reserved.