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Community Outbreak Investigation of SARS-CoV-2 Transmission Among Bus Riders in Eastern China

Our take —

Investigators found that those most at risk of infection during a SARS-CoV-2 outbreak at Buddhist worship event in eastern China were among those who traveled to and from the event on the same bus as the index case. The paper supports the notion that there is a higher risk of transmission of SARS-CoV-2 infection in enclosed spaces compared to outdoor spaces. Also, the fact that no one in the bus without the index case became infected even after attending the event suggests that outdoor, socially distant events are lower risk. However, investigators cannot rule out other means of transmission as they did not collect samples of surfaces from the buses or from the event.

Study design

Prospective Cohort

Study population and setting

Between January 27 and February 23, 2020, the Centers of Disease Control and Prevention in China investigated an outbreak in Zhejiang province in China. On January 19, 2020, 293 lay Buddhists attended an outdoor worship service with 128 of worshipers traveling 2 hours roundtrip in two separate buses. One bus (bus 2) contained a passenger who had SARS-CoV-2 infection (the assumed index case). Investigators compared the attack rates of those who took bus 1 (n=60), those who took bus 2 (n=67, excluding the index case), and the other worshipers who did not take the buses on the way to the worship event (n=172). Both buses had the same infrastructure and had central air conditioners that recirculated indoor air. Participants also interacted during a luncheon. Investigators tested worshippers for evidence of SARS-CoV-2 using PCR testing via throat swabs and compared the attack rates of those who were seated closer to the index case on bus 2 (“high-risk zones” defined as being in the same row or within 2 or 3 rows of the index case) compared to those who were seated further away (i.e. “low-risk zones” which included seats outside of 3 rows of the index case).

Summary of Main Findings

Among the 68 people on bus 2, 24 (including the index case) were diagnosed with SARS-CoV-2 infection following the worship service (35.2%). There were no confirmed cases of SARS-CoV-2 infection among those on bus 1. There were 7 people who were diagnosed with SARS-CoV-2 infection among the 172 other worshippers who traveled to the event through different transportation (4.1%). Thus, compared with all of the other worshippers at the event, those who were in bus 2 had 11.4 (95% CI: 5.1, 25.4) times the risk of developing SARS-CoV-2 infection over follow up. Within bus 2, investigators found that there was an increased risk (though not statistically significant) among those sitting in a “high-risk zone” compared to a “low-risk zone”) (RR, 1.8; 95% CI: 0.9, 3.3).

Study Strengths

A strength of this study is the level of demographic and epidemiologic detail that investigators were able to collect from the individuals involved in the worship service. Investigators were able to identify an index case as the one person who had contact with Wuhan residents two days prior to the service, and thus researchers were able to retrospectively compare an “exposed” group with an “unexposed” group. Additionally, the entire population at risk was well enumerated and followed.

Limitations

A limitation is that the study cannot exclude the possibility of other sources or modes of transmission. Investigators report that the fact that there was no association between sitting in a “high-risk zone” seat on bus 2 and developing SARS-CoV-2 infection suggests aerosol transmission of the infection. However, whether transmission primarily occurred through touching droplets on surfaces throughout the bus or through breathing in aerosolized droplets cannot be determined as investigators were not able to collect samples of surfaces on the bus or obtain an accurate depiction of what and where passengers touched on the bus.

Value added

This study provides more evidence to suggest higher risk of transmission of SARS-CoV-2 infection in contained spaces (i.e. inside of a bus), likely through aerosolized particles, compared to being in an open, outdoor space.

This review was posted on: 2 October 2020