Spread of SARS-CoV-2 aerosols via two connected drainage stacks in a high-rise housing outbreak of COVID-19

Abstract

Vertical transmission of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) along a vertical column of flats has been documented in several outbreaks of coronavirus disease 2019 (COVID-19) in Guangdong and Hong Kong. We describe an outbreak in Luk Chuen House, involving two vertical columns of flats associated with an unusually connected two-stack drainage system, in which nine individuals from seven households were infected. The index case resided in Flat 812 (8 th floor, Unit 12), two flats (813, 817) on its opposite side reported one case each (i.e., a horizontal sub-cluster). All other flats with infected residents were vertically associated, forming a vertical sub-cluster. We injected tracer gas (SF 6 ) into drainage stacks via toilet or balcony of Flat 812, monitored gas concentrations in roof vent, toilet, façade, and living room in four of the seven flats with infected residents and four flats with no infected residents. The measured gas concentration distributions agreed with the observed distribution of affected flats. Aerosols leaking into drainage stacks may generate the vertical sub-cluster, whereas airflow across the corridor probably caused the horizontal sub-cluster. Sequencing and phylogenetic analyses also revealed a common point-source. The findings provided additional evidence of probable roles of drainage systems in SARS-CoV-2 transmission.

Probable cross-corridor transmission of SARS-CoV-2 due to cross airflows and its control

“…The outbreak occurred during a time when there were no other locally transmitted cases in Hong Kong [ 19 ]. The building has 15 storeys, with residential flats located on 2F–15F.…”

“…For example, the −10 flats and the −12 flats, which are adjacent to each other, share a two-stack drainage pipe; this was found to be the likely cause of the vertical spread of the virus from flat 812 (and possibly flat 810) to flats 710, 1012 and 1112 [ 19 ]. Each of the two corridors is divided into 10 corridor zones, i.e., corridor A divided into corridor zones 1–10 and B into 11–20.…”

“…In June 2020, we carried out field measurements of the dispersion of tracer gas in Luk Chuen House in Hong Kong [ 19 ]. A COVID-19 outbreak had occurred here at the end of May 2020, and a cross-corridor virus transmission was suspected ( Fig.…”

“…1 b, [ 20 ]. The outbreak included two spatial clusters [ 19 ]: a horizontal cluster on the eighth floor, including flat 812 where the index case lived and flats 813 and 817 where secondary infections occurred ( Fig. 1 a); and a vertical cluster connected by the building's shared drainage system (including flats 710, 810, 1012 and 1112).…”

“…1 a); and a vertical cluster connected by the building's shared drainage system (including flats 710, 810, 1012 and 1112). Wang et al [ 19 ] conducted a detailed investigation of the role of the unusual two-stack drainage system that connected the flats in spreading the virus in the vertical cluster. The potential role of airflow between flats in facilitating cross-corridor virus transmission in the horizontal cluster was discussed in this study.…”