COVID-19 outbreaks in high-rise buildings suggested the transmission route of fecal-aerosol-inhalation due to the involvement of viral aerosols in sewer stacks. The vertical transmission is likely due to the failure of water traps that allow viral aerosols to spread through sewer stacks. This process can be further facilitated by the chimney effect in vent stack, extract ventilation in bathrooms, or wind-induced air pressure fluctuations. To eliminate the risk of such vertical disease spread, the installation of protective devices is highly encouraged in high-rise buildings. Although the mechanism of vertical pathogen spread through drainage pipeline has been illustrated by tracer gas or microbial experiments and numerical modeling, more research is needed to support the update of regulatory and design standards for sewerage facilities.
During the coronavirus disease-19 (COVID-19) pandemic, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) viruses have been spread around the world causing respiratory symptoms, pneumonia, and deaths [1,2]. Many national governments limited social interactions and restrained economic activities to mitigate COVID-19 transmission, leading to isolation and hardship. The rapidly emerging new variants of concern, such as Delta and Omicron, create new challenges for countries even with a high vaccination rate .
The highly contagious SARS-CoV-2 exploited many routes to be transmitted among people, including human-to-human transmission under close contact by sprayed large droplets, or aerosol inhalation, and fomite transmission by touching contaminated surfaces with viral deposition . In addition, SARS-CoV-2 viruses are excreted into the stool of infected individuals and enter sewers after toilet flushing [5,6], as confirmed by the detection of SARS-CoV-2 gene markers in wastewater samples globally [7, 8, 9, 10, 11, 12, 13]. Many countries have implemented routine wastewater surveillance programs for the presence and concentration of SARS-CoV-2 in different sewer catchments or wastewater treatment plants (WWTPs) [14, 15, 16]. Hence, the wastewater in sewers is also a potential transmission vehicle for SARS-CoV-2.
The possibility of fecal-oral transmission has been proposed and discussed widely [17, 18, 19, 20, 21, 22, 23], supported by the isolation of viable viruses from human feces . Apart from fecal shedding, other sources containing active SARS-CoV-2 viruses, including sputum, nasal mucus, blood, and saliva, may also enter the wastewater in sewers [25, 26, 27]. Some strains of coronavirus could preserve infectivity in bulk wastewater for a few days [28,29], and in sewers for hours , thus potentially making small sewage droplets as a transmission pathway of COVID-19.
High-rise apartment buildings are common residential arrangements in many densely populated cities worldwide. The flats/units of high-rise buildings are usually connected to the same drainage system. This makes it possible that the aerosolized viral particles generated from toilet flushing can be transmitted through sewer stacks, including linked sewage and ventilation pipes [31, 32, 33], causing infections distributed vertically in the building towers linked with the same sewer stacks. This paper reviewed the reported cases of vertical outbreaks of COVID-19 and studies related to the viral transmission mechanisms in sewer stacks of high-rise buildings, most of which are within the last two years. Based on this, some recommendations and research needs were proposed for preventing the transmission of COVID-19 in high-rise buildings.
Vertical outbreaks of COVID-19
The vertical outbreak came into the spotlight as a serious severe acute respiratory syndrome (SARS) outbreak that happened back in 2003 at a private residential apartment in Hong Kong, resulting in 42 deaths and 321 infected cases . For the current COVID-19 pandemic, 15 vertical outbreaks in residential buildings were reported in Hong Kong and Guangzhou, China [35,36], and one outbreak in Seoul, South Korea  (Table 1). The infected cases were detected along the vertical line in the buildings connected by the same plumbing pipe. Moreover, most of the secondary infections resided on upper floors above the index case (Table 1). Additionally, the high risk of in-building infection is also a considerable concern for quarantine hotels and hospitals, where infected people frequently discharge viruses into the interconnected drainage system [38,39]. Therefore, it is likely that there have been more unrecognized vertical outbreaks as well.
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