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The risk of COVID-19 transmission in train passengers: an epidemiological and modelling study

Our take —

This study used contact tracing data conducted through the Chinese Centers for Disease Control. The overall attack rate was low (<1%); however, this study provides information that is not well documented (i.e. comparing secondary attack rates based on spatial positioning and travel time among passengers on a train). Though this is one of few studies to make such comparisons, the findings are only among symptomatic cases and do not represent subsequent infections among asymptomatic individuals.

Study design

Cross-Sectional

Study population and setting

The Chinese and local Centers for Disease Control and Prevention conducted contact tracing for 2,568 confirmed COVID-19 cases who traveled from December 19, 2019 to March 6, 2020 on the G train, which is the mostly widely used train in China and accounts for over 60% of China’s rail passengers. The Chinese government started issuing lockdowns and public transit travel restrictions in different cities, starting with Wuhan, China and other hot spots in the Hubei province starting January 23, 2020. Researchers aimed to examine spatial distance (ranging from seats A (window seat), B, C (aisle seat), D (aisle seat), and F (window seat) within each row of the train) of contacts and duration of travel time in relation to viral transmission risk. Someone was a contact of a case if they had traveled within a three-row seat distance (either in front or behind) and 5 columns of an index case within 14 days of the case’s symptom onset. Investigators calculated the attack rate for different seat locations adjusting for co-travel time in order to explore individual level transmission risks on public transportation.

Summary of Main Findings

The overall attack rate among contacts was 0.32% (234/72,093) (95% CI: 0.29, 0.37). The attack rate for those seated in seat A from was 0.28% (95% CI: 0.21, 0.39), from seat B: 0.41% (95% CI: 0.31, 0.54), from seat C: 0.34% (95% CI: 0.26, 0.45), seat D: 0.34% (95% CI: 0.26, 0.45) and seat F: 0.27% (95% CI: 0.20%, 0.36%). Attack rates were different depending on the row a contact was sitting. Contacts in the same row as an index case had an attack rate of 1.5%, which is 10 times higher than the attack rate of being 1 or 2 rows away from the index case. Travel time was associated with the attack rate as there was an increase by 0.15% per hour of travel time, though this relationship strengthened after a contact traveled for more than 4 hours.

Study Strengths

This study provides useful surveillance data that is usually not available in other settings (i.e. information concerning potential contacts of confirmed cases from train travel). The date of symptom onset and date of diagnosis were available for all cases, making this a rich data source for contact tracing analyses.

Limitations

Those without symptoms were not tested, and thus results represent an underestimation of travel risk since asymptomatic infections are not represented. Considering that the majority of cases have mild or no symptoms, it is unclear if the data represent the true infectivity of cases during train travel. Additionally, although there are separate attack rates within each row depending on which seat a contact was sitting in (i.e. A vs. B vs. C, etc.), the results do not account for how close a contact was to a given case (i.e. if a contact was sitting in seat A and was sitting next to the case versus if the contact was sitting in seat A and sitting several seats away from a given case).

Value added

This study provides information concerning the likelihood of contracting SARS-CoV-2 virus while traveling on a train. The evidence can be useful in informing people’s decisions when trying to travel or informing safety guidelines to reduce transmission on trains.

This review was posted on: 24 August 2020