Assessing vertical transmission of SARS-CoV-2 in high-rise apartments via a joint epidemiologic and modelling investigation

Samuel Chong^1*Chin Chun Ooi²Muhammad Ismail Abdul Malek³Zhengwei Ge²Derrick Low²Chang Wei Kang²Calvin J. Chiew⁴Sae-Rom Chae⁴Yee Leong Teoh⁴Kelvin Bryan Tan^1,3,5,6,7

• ¹Ministry of Health, Singapore, Singapore
• ²Agency for Science Technology and Research, Singapore, Singapore
• ³Tan Tock Seng Hospital National Centre for Infectious Diseases, Singapore, Singapore
• ⁴Singapore Communicable Diseases Agency and Planning Office, Singapore, Singapore
• ⁵National University Singapore Saw Swee Hock School of Public Health, Singapore, Singapore
• ⁶Duke-NUS Medical School, Singapore, Singapore
• ⁷Lee Kong Chian School of Medicine, Singapore, Singapore

High-rise apartments (HRAs) present a complex environment with multiple infection routes, especially for respiratory pathogens like SARS-CoV-2. With HRAs serving as the dominant housing modality in many dense urban regions worldwide, particularly in dense Asian cities such as Singapore where high-rise living is normative, understanding transmission within such settings is essential for both global and region-specific public health preparedness. In this study, we assessed transmission risks and potential routes of transmission within HRAs based on observed SARS-CoV-2 clusters in Singapore. We analyzed SARS-CoV-2 incidence in HRAs subjected to government-mandated mass screenings to evaluate the transmission risk associated with various relative positions within a HRA, and found significantly elevated risk of transmission for residents living within the same vertical stack as a potential index case. A computational fluid dynamics (CFD) model was further developed for a HRA with the highest vertical transmission risk to elucidate potential aerosol transmission routes. Interestingly, the epidemiological analysis indicated increased infection risk for residents living within two levels above an infected case, correlating with CFD observations that aerosolized particles can move vertically up the stack and remain at elevated concentrations in the two levels above a potential index case. The analysis and modelling provide additional insights into alternative vertical transmission within HRAs, distinct from prior work that has primarily hypothesized transmission via drainage stacks. Nonetheless, factors such as wind direction and individual unit configurations are shown in CFD to have significant influence on the potential spread of aerosolized particles in such settings, highlighting the need for additional in-depth investigation. This work further demonstrates the importance of joint epidemiology and numerical modelling to better understand different potential mechanisms of particle spread, especially in the HRA setting.