A new study published in the journal JAMA Network suggests airborne transmission in a bus in China led to one infected individual spreading of COVID-19 to 23 other fellow passengers.
Analysing community transmission in China’s Zhejiang province, the study reports that 128 individuals took two buses on January 19, 2020 — 60 in bus 1 and 68 in bus 2 — on a 100 minute round trip to attend a 150-minute worship event.
The source patient was a passenger on bus 2 and both the buses had central air conditioners functioning in indoor recirculation mode. Among these 128 individuals, 15 were men, 113 were women with a mean age of 58.6 years. On bus 2, 24 individuals turned out to be positive after the event, while none of the individuals in bus 1 were affected. Seven others who turned positive after the outdoor event had all come close to the index patient.
Authors of the study conclude that individuals on bus 2 had a 34.3 per cent higher risk of getting COVID-19 compared with those on bus 1 and were 11.4 times more likely to have COVID-19 compared with all the others attending the event.
The dynamics of the spread of disease in the bus
Bus 2 had 15 rows of seats, starting from the third row, each row had three seats on one side and two seats on the other. The index patient was sitting in the middle seat on the three-seat side of row eight. While those sitting close to the index patient were infected, other cases were spread out across the bus. Significantly, apart from the passenger sitting next to the index patient, none of the passengers sitting in seats close to the bus window developed an infection.
Further, the driver and passenger sitting close to the bus door did not develop infection and only one passenger who was sitting close to an openable window developed an infection.
Among the passengers from bus 2 who eventually developed COVID-19, two were asymptomatic, three had mild symptoms, remaining 17 had moderate symptoms. The index patient developed moderate symptoms as well and had been exposed to individuals from Wuhan. The index patient developed symptoms on the evening after returning from the visit and was asymptomatic during the bus rides.
Were any of the passengers wearing masks?
None of the passengers and the participants at the worship event wore masks during the ride or at the event as there was no public awareness of COVID-19 in the city at the time.
What does this mean?
In conclusion, the authors say that among the cohort of individuals studied, those that travelled in bus 2 were at a higher risk of contracting the infection than those who did not, implying that the airborne spread of COVID-19 likely contributed to the higher attack rate in the exposed bus.
What does airborne transmission mean?
As per the WHO, airborne transmission is defined as the spread of an infectious agent caused by the dissemination of aerosols that remain infectious when suspended in the air over long distances and time. Airborne transmission can happen during aerosol-generating medical procedures and even through speaking and singing.
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What do we know about the airborne spread of COVID-19?
In July, the WHO published an updated version of its document on the modes of transmission of SARS-CoV-2 and acknowledged that that the novel coronavirus can remain in the air in crowded indoor spaces, where “short-range aerosol transmission… cannot be ruled out”.
The updated brief came out three days after a group of 239 scientists from 32 countries published a commentary titled ‘It is Time to Address Airborne Transmission of COVID-19’, in which they issued an “appeal to the medical community and to the relevant national and international bodies to recognize the potential for airborne spread of COVID-19”.
Significantly, in May, the US Centers for Disease Control and Prevention (CDC), published a study titled ‘High SARS-CoV-2 Attack Rate Following Exposure at a Choir Practice’. The researchers, who studied “superspreading events”, found that following 2.5-hour choir practice attended by 61 persons, including a symptomatic index patient, 32 confirmed and 20 probable secondary Covid-19 cases occurred; three patients were hospitalised, and two died. Other such outbreaks have been reported in indoor crowded spaces possibly through aerosols in restaurants and fitness classes.
In its section on ‘How to prevent transmission’, the WHO brief said that apart from hand washing and physical distancing, one should “avoid crowded places, close-contact settings and confined and enclosed spaces with poor ventilation”, and “wear fabric masks when in closed, overcrowded spaces to protect others; and ensure good environmental ventilation in all closed settings and appropriate environmental cleaning and disinfection”.
What are the limitations of the study?
The authors note that “while the high attack rate and the distribution of cases on bus 2 is consistent with airborne transmission, there is no way to rule out a common surface, such as a pole, because of possible insufficient recall. However, given that there were participants with infection sitting in the last row, airborne transmission is likely to be a partial transmission route.”