Sep 03, 2020 08:44 PM

Study of Chinese Bus Passengers Finds Evidence of Covid-19 ‘Airborne Transmission’

As a bus full of Buddhists neared a temple in the Zhejiang port city of Ningbo in late January, the 68 people on board could not have known that one of them was carrying and spreading a new and frightening virus.

Twenty-four of the passengers would later be diagnosed with Covid-19. Ningbo and Zhejiang public health officials suspect they contracted it during the 100-minute round trip.

But it is the way the virus appears to have spread among those onboard — in part via the bus’s air conditioner system, which was set to recirculation mode — that has experts concerned. They say it adds convincing new evidence that the pandemic virus can spread through the air in closed environments, instead of just through droplets sneezed or coughed out by an infected person.

The Jan. 19 bus journey and another one by 60 people who rode to the same event on the same day, but none of whom were later diagnosed with Covid-19, forms the basis of a natural experiment documented by Chinese and U.S. scientists in a paper published this week in JAMA Internal Medicine — a peer reviewed journal published by the American Medical Association.

Previous studies have suggested SARS-CoV-2, the virus that causes Covid-19, can survive for up to 16 hours in the air. But evidence those airborne virus particles can actually infect people has been more difficult to find.

In this study, the researchers, led by Ye Shen, a PhD student at University of Georgia College of Public Health, mapped the bus according to where each individual passenger was sitting. They found that people sitting closer to the source patient did not have a significantly higher risk of later falling ill than those who sat further away.

They relied on demographic and contact tracing data collected by local epidemiologists at the Centers for Disease Control and Prevention in Ningbo, and at the Zhejiang provincial level.

Confirming who introduced the virus was one of the difficulties of the study, and the authors said they could not completely rule out other sources. But they said the so-called “index patient,” a worshipper in their 60s, was the only person present who had come into contact with people from the area where the outbreak began.

Two days earlier that person had dinner with four people who had recently been to Hubei, the province around the epidemic ground zero of Wuhan, 700 kilometers (434 miles) west of Ningbo. Wuhan would be locked down days later. The index patient was also the first person present to be diagnosed with the disease.

The fact that around 300 people who participated in the outdoor worship event had a far lower rate of infection also provides evidence to support their conclusions, the researchers say.

The event lasted for two-and-a-half hours and passengers on the infected bus were evenly distributed throughout it, the researchers said. But only seven people who were not on the bus were infected.

The research has implications for public health responses to coronavirus, including on public transport and in hospital disease control efforts. Virus particles floating in the air, instead of in respiratory droplets, are small enough to pass through most protective masks. That means in certain circumstances, masks may not be enough to protect people from Covid-19 transmission.

Corresponding author Feng Ling of the Zhejiang Provincial Center for Disease Control and Prevention declined to be interviewed.

Contact reporter Flynn Murphy (

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