Possible inter-room aerosol-mediated COVID-19 transmission in a quarantine hotel: A case study

In 2021, COVID-19 infections were reported in two individuals in separate, vertically adjacent rooms in a quarantine hotel. Epidemiological analysis and virus sequencing linked the two individuals despite no physical contact, prompting investigation of a possible vertical aerosol transmission route between the two rooms through field assessments including aerosol tracing and visual inspections. Our investigation suggested the vertical inter-room transmission was due to air backflow from the exhaust system. Aerosols dosed in the source room travelled to the recipient room (mean 9.4 mg/m 3 , at least 2.6 times higher than other rooms tested along the same stack), likely via the toilet exhaust into the exhaust shaft, then back-flowing into the ceiling plenum via gaps in brick exhaust shaft wall; a fan coil unit drawing air from the plenum then facilitated aerosol entry into the bedroom. Poor exhaust in the recipient’s room (0.40 m/s) compared to other tested rooms (averaging 1.24 m/s) may have exacerbated the risk. Deploying a HEPA air purifier in the recipient’s room accelerated aerosol decay by 3.1 times, from 0.0014 s⁻¹ to 0.0043 s⁻¹, supporting its use as a mitigation measure. Together with virus whole-genome sequences and ruling out other epidemiological links, the findings suggest inter-room COVID-19 aerosol transmission. The study provides evidence that air leakage from the exhaust shaft and internal partition walls may form a potential route for inter-room aerosol transmission along a shared stack, highlighting the importance of understanding risk factors and implementing mitigation measures in multi-storey buildings used as quarantine facilities.