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In what could be more evidence for SARS-CoV-2 airborne transmission, passengers who rode on a bus with someone who had COVID-19 had a higher risk of contracting the virus versus passengers who rode on a different bus to the same event, researchers in China found.

In January, 24 of 68 individuals on a bus with one case of COVID-19 in the city of Ningbo, including the index case, were diagnosed with COVID-19 afterwards, whereas none of 60 individuals on the second bus, which had no apparent index case, contracted the virus in the weeks following, reported Feng Ling, MD, of Zhejiang Provincial Center for Disease Control and Prevention in Hangzhou, China, and colleagues.

Whether proximity to the index case mattered was unclear: people seated closest to the individual were at 60% higher risk of infection versus those in "low risk" areas, but this was not statistically significant (relative risk 1.6, 95% CI 0.8-3.2), the authors wrote in .

Importantly, they noted there was no mask wearing or prevention during the rides and service, due to no public awareness of COVID-19 in Ningbo at that time.

"Through detailed epidemiologic analysis, airborne transmission within a bus with recycled air seems likely to have contributed to a COVID-19 outbreak in eastern China," Ling and colleagues wrote.

Whether airborne transmission is responsible for many COVID-19 cases is still debated. A panel of experts at the National Academies of Science and Medicine described current research as "circumstantial evidence," leaving researchers to continue seeking the proverbial "smoking gun."

Ling's group attempted to add to the evidence by examining 126 bus passengers and two drivers in Ningbo, about 45 miles from Wuhan, who traveled to a worship event on January 19. Bus 1 had 60 people aboard, including the driver; 68, including the driver, were in bus 2. The trip was 50 minutes each way.

Passengers "remained seated in their own seats ... and did not change seats on the way back," according to the report. The worship service was 150 minutes, and there was a 15-30 minute luncheon with 10 attendees each at a "dining table in a spacious room with no recirculating central air conditioning systems on."

The index patient was a man in his 60s who was seated in the middle of row 8 on a 15-row bus. He was asymptomatic during the bus ride, but developed cough, chills, and myalgia after returning from the worship event. His wife and child were hospitalized for quarantine on January 22, and tested positive for SARS-CoV-2 on January 28.

Researchers discussed evidence implicating bus 2 as the source of transmission. Of the 68 individuals, 35.3% including the index patient, were diagnosed with COVID-19 after the event, but none of the 60 individuals on bus 1 were diagnosed with the virus.

Of the 172 individuals at the event who were not on either bus, seven were diagnosed with COVID-19, and all described "close contact" with the index patient. In total, 30 individuals contracted the virus during the event, excluding the index case.

Passengers on bus 2 were 42 times more likely to develop COVID-19, albeit with a wide confidence interval (95% CI 2.6-679.3), with a 34.3 percentage-point difference in absolute risk. Compared to all individuals who attended the worship event, passengers on bus 2 were 11.4 times more likely to contract the virus, again with a wide confidence interval (95% CI 5.1-25.4).

Of the cases who contracted the virus on the bus, two were asymptomatic, three had mild symptoms, and the remaining 17 had moderate symptoms. Only one passenger seated near an "openable" bus window developed symptoms, but the driver and passengers sitting near the bus door did not.

Ling and colleagues wrote that since the index case was seated in row 8, and passengers in the last row developed infection, "airborne transmission is likely to be a partial transmission route."

But the group acknowledged that alternative sources of infection cannot be ruled out. In addition, the small sample size may have contributed to the nonsignificant association between distance from the index patient and infection risk.

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