A superspreader transmits coronavirus to 23 others in air-conditioned bus. So how safe will metro rail travel be?

A single person travelling in an air-conditioned bus along with 67 other passengers in Zhejiang province in eastern China on a 100-minute round trip spread the virus to 23 people. With India’s Unlock 4 guidelines permitting metro rail services operation from September 7 outside the containment zones, how safe will metro rides be.

With the Central government issuing the Unlock 4 guidelines that will allow many activities that attract large crowds to gather in areas outside the containment zones, including the operation of metro rail services from September 7, there is heightened risk of a surge in coronavirus cases. Metro rail coaches with sealed windows, closed doors and air-conditioning seem to be particularly prone to spreading the virus.

In the third week of January, a single person travelling in an air-conditioned bus along with 67 other passengers in Zhejiang province in eastern China on a 100-minute round trip spread the virus to 23 people. At the same time, 60 passengers travelling in another bus to the same venue and where no one was infected did not report any cases. In both buses, the air conditioners were set in recirculation mode. In addition to 128 bus passengers, another set of 44 people too attended a worship event. Besides the 23 passengers of the bus, another seven who attended the worship event and were in close contact with the index patient during the event too got infected. One reason why just seven people who did not travel by the same bus got infected could be because the worship was an outdoor event.

According to a paper published in the JAMA Internal Medicine, the travel duration to and from the temple was 50 minutes each way. All the passengers, including the index patient, were seated in their own seats both during the onward and return journey. None of the event participants wore masks during the rides and worshiping. 

Two days before travelling in the bus, the index patient aged over 60 years had come in close contact with 10 individuals, four of whom had travel histories to Hubei province.  The index patient was asymptomatic during the bus trip but developed symptoms the same day after returning from the temple. The index patent also infected the spouse and child.

“Through detailed epidemiologic analysis, airborne transmission within a bus with recycled air seems likely to have contributed to a COVID-19 outbreak,” the authors who are physicians with the Chinese Center for Disease Control and Prevention write. That passengers seated close to the index patient, including the passenger seated next to him did not have statistically higher risk compared with passengers sitting further away including those in the very last row of the bus, seven rows behind the index patient suggests that more than close contact or respiratory droplets airborne transmission, aerosol transmission would have been responsible for the virus spread.

The study adds to the existing body of evidence that suggests a possible spread by tiny particles that stay in the air for a longer time and not just through large respiratory droplets. “Our finding of potential airborne transmission has important public health significance, and future efforts at prevention and control should consider the potential for airborne spread of COVID-19,” the authors write.

Also, the fact that the index patient was asymptomatic during the bus journey suggests that “individuals with infection may be able to shed virus by breathing”, as reported by few other studies.

The World Health Organization on July 9 acknowledged that novel coronavirus can be airborne in closed settings and spread from one person to another. It also acknowledged that people without symptoms can spread the virus.

Published in the Hindu on September 5, 2020