Explosive outbreak of SARS-CoV-2 Omicron variant is associated with vertical transmission in high-rise residential buildings in Hong Kong

Vincent Chi-Chung Cheng; Shuk-Ching Wong; Albert Ka-Wing Au; Cheng Zhang; Jonathan Hon-Kwan Chen; Simon Yung-Chun So; Xin Li; Qun Wang; Kelvin Keru Lu; David Christopher Lung; Vivien Wai-Man Chuang; Eric Schuldenfrei; Gilman Kit-Hang Siu; Kelvin Kai-Wang To; Yuguo Li; Kwok-Yung Yuen

doi:10.1016/j.buildenv.2022.109323

Explosive outbreak of SARS-CoV-2 Omicron variant is associated with vertical transmission in high-rise residential buildings in Hong Kong

Build Environ. 2022 Aug 1:221:109323. doi: 10.1016/j.buildenv.2022.109323. Epub 2022 Jun 24.

Authors

Vincent Chi-Chung Cheng^{1

2}, Shuk-Ching Wong¹, Albert Ka-Wing Au³, Cheng Zhang⁴, Jonathan Hon-Kwan Chen², Simon Yung-Chun So², Xin Li⁵, Qun Wang⁴, Kelvin Keru Lu⁶, David Christopher Lung⁷, Vivien Wai-Man Chuang⁸, Eric Schuldenfrei⁹, Gilman Kit-Hang Siu⁶, Kelvin Kai-Wang To⁵, Yuguo Li⁴, Kwok-Yung Yuen⁵

Affiliations

¹ Infection Control Team, Queen Mary Hospital, Hong Kong West Cluster, Hong Kong Special Administrative Region, China.
² Department of Microbiology, Queen Mary Hospital, Hong Kong Special Administrative Region, China.
³ Centre for Health Protection, Department of Health, Hong Kong Special Administrative Region, China.
⁴ Department of Mechanical Engineering, Faculty of Engineering, The University of Hong Kong, Hong Kong Special Administrative Region, China.
⁵ Department of Microbiology, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region, China.
⁶ Department of Health Technology and Informatics, The Hong Kong Polytechnic University, Hong Kong Special Administrative Region, China.
⁷ Department of Pathology, Queen Elizabeth Hospital, Hong Kong Special Administrative Region, China.
⁸ Quality & Safety Division, Hospital Authority, Hong Kong Special Administrative Region, China.
⁹ Department of Architecture, Faculty of Architecture, The University of Hong Kong, Hong Kong Special Administrative Region, China.

Abstract

The phenomenon of vertical transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in high-rise residential buildings (HRRBs) is unique in our densely populated cosmopolitan city. The compulsory testing of a whole building under the scheme of restriction-testing declaration (RTD) during the fourth wave (non-Omicron variant) and fifth wave (mostly Omicron variant) of COVID-19 outbreak in Hong Kong allowed us to study the prevalence of this phenomenon, which may represent a form of airborne transmission. From 23 January 2021 to 24 March 2022, 25,450 (5.8%) of 436,397 residents from 223 (63.0%) of 354 HRRBs under RTD were test-positive for SARS-CoV-2. Using the clustering of cases among vertically aligned flats with shared drainage stack and lightwell as a surrogate marker of vertical transmission, the number of vertically aligned flats with positive COVID-19 cases was significantly higher in the fifth wave compared with the fourth wave (14.2%, 6471/45,531 vs 0.24%, 3/1272; p < 0.001; or 2212 vs 1 per-million-flats; p < 0.001). Excluding 22,801 residents from 38 HRRBs who were tested negative outside the 12-week periods selected in fourth and fifth waves, the positive rate among residents was significantly higher among residents during the fifth wave than the fourth wave (6.5%, 25,434/389,700 vs 0.07%, 16/23,896; p < 0.001). Within the flats with COVID-19 cases, the proportion of vertically aligned flats was also significantly higher in the fifth wave than in the fourth wave (95.6%, 6471/6766 vs 30.0%, 3/10, p < 0.001). The proportion of HRRBs with COVID-19 cases was significantly higher during the corresponding 12-week period chosen for comparison (78.2%, 219/280 vs 11.1%, 4/36; p < 0.001). Whole-genome phylogenetic analysis of 332 viral genomes showed that Omicron BA.2 was the predominant strain, supporting the high transmissibility of BA.2 by airborne excreta-aerosol route in HRRBs of Hong Kong.

Keywords: BA.2; COVID-19; Residential building; SARS-CoV-2 Omicron variant.