Airborne spread of COVID-19 possible among bus riders

Commuters who had been exposed to an index case with COVID-19 during a bus journey were 42 times more likely to be infected than control riders in another bus of similar journey — adding further evidence to the notion that airborne transmission of the virus is possible, especially in an enclosed area.

The study compared 68 passengers in one bus (bus A) containing the index case to 60 passengers in another bus (bus B; unexposed comparators) who made the same 100-minute round trip to attend a worship event on 19 January, 2020 in a city in Eastern China, based on tracing investigation after the index case was diagnosed. Another 172 participants at the worship event (not from the two buses) were also included as comparators for an outdoor setting. [JAMA Intern Med 2020;doi:10.1001/jamainternmed.2020.5225]

Twenty-four people (35.5 percent) on bus A were infected compared with none on bus B. Among the other 172 participants at the worship event, seven (4.1 percent) were subsequently diagnosed with COVID-19.

All those who were infected at the worship event reported having been in close contact with the index case.

When translated to risk calculation, passengers in bus A were 42.2 times more likely to have COVID-19 (relative risk [RR], 42.2; p<0.01) when compared with unexposed individuals in bus B, and 11.4 times more likely when compared with all others at the worship event (outdoor comparators; RR, 11.4; p<0.01).

“The index patient on bus A, likely a super spreader of the outbreak, only developed symptoms on the evening after returning from the temple and was asymptomatic during the bus rides, suggesting that individuals with infection may be able to shed virus by breathing and cause secondary cases before they become symptomatic, echoing the findings from earlier pre-symptomatic reports,” the researchers explained.

The researchers further divided the seats in bus A to high-risk zone (ie, three rows in front and behind the index case) and the rest as low-risk zone.

While passengers seated in the high-risk zone had an increased risk of contracting COVID-19 (RR, 1.6–1.8; p>0.05) than those in the low-risk zone, the difference was not statistically significant.

“If COVID-19 transmission occurred solely through close contact or respiratory droplets during this outbreak, the risk of COVID-19 would likely be associated with distance from the index patient, and high-risk zones on the bus would have more infected cases,” the researchers pointed out.

“[However,] passengers sitting closer to the index patient on the exposed bus did not have statistically higher risks of COVID-19 as those sitting further away,” they noted.

Also, no confirmed cases have occurred in the city yet before 19 January — hence, the possibility of having another source of infection was unlikely. “The index patient was the only person exposed to individuals from Wuhan and the first at the event to receive a diagnosis of COVID-19, suggesting a high probability that they were the source of the outbreak,” said the researchers.

Therefore, airborne transmission is likely to contribute partly to the spread of the virus among the bus riders, especially when there were passengers seated at the last row who were infected as well.

“The results suggested that the transmission largely occurred in the exposed bus, where a much higher attack rate in a closed environment with recirculating air was observed,” the researchers stated.

“[As] forced, circulating air might play an important role in airborne spread of the virus, gatherings in enclosed settings with minimal air ventilation should be limited,” they added. “Future efforts at prevention and control must consider the potential for airborne spread of the virus.”