Dominant transmission route of SARS-CoV-2 and its implication to indoor environment

Abstract

The exact main transmission route(s) for SARS-CoV-2 remains to be determined after more than 12 months into the once-in-a-century Coronavirus Disease 2019 (COVID-19) pandemic Following the first reported infected case of COVID-19 a year ago, different intervention strategies have been adopted worldwide A just in case approach may overkill with significant economic and societal losses, while an underkill approach may result in significant infection An adequate intervention demands the knowledge of the dominant transmission route(s) of SARS-CoV-2 In this perspective, the definition and categorization of transmission routes for respiratory infection are discussed, and a new categorization is proposed Among the traditional definitions of various transmission routes, the concept of contact transmission appears to be the most problematic The short-range aerosol route was not considered, certainly not in the traditional aerosol transmission, which generally implies a possible long-range transmission It is not desirable to lump short-range aerosol transmission into aerosol (airborne) transmission as the preventive measures differ between the two In a recent mechanistic study, the short-range inhalation route was shown to dominate the exposure of respiratory infection during close contact, while spray transmission is only significant at a very close distance (< 0 5 m) Our suggested new categorization of basic transmission routes of respiratory infection is transfer-process-focused The three basic routes are the (drop) spray route as infection transmitted by spray and deposition of virus-laden respiratory drops expelled from an infected person onto the mucosal surfaces (e g , eyes, nose and mouth) of a susceptible individual, (aerosol) inhalation route as infection of a susceptible individual via inhalation of virus-laden respiratory droplets at both short-range and long-range, i e , aerosols that can suspend in the air, and (surface) touch route as infection transmitted by hand touch of a surface (can be both animate and inanimate) which was contaminated with the virus-laden respiratory drops or droplets, followed by his/her hands touching mucosa, resulting in self-inoculation The threshold diameter of drops in spray transmission is 50-100 μm These three newly defined routes may be simply referred to as spray, inhalation and touch transmission An opportunistic airborne transmission mechanism is hypothesized for SARS-CoV-2 spread, which can explain the following major observations of the infection pattern so far, i e , transmission at close range seems to dominate and social (physical) distancing has worked;distant transmission has been reported occasionally, which often leads to super spreading events;mass face masking has also worked;most infection occurred indoors;and finally infection that occurred outdoors has been rare The opportunistic airborne transmission hypothesis is also supported by a recent mechanistic study of exposure of the susceptible to exhaled droplets If such a hypothesis can be further verified, the importance of building ventilation becomes obvious The research needs of the improved indoor environment in the post-pandemic period are also speculated Mankind will reflect deeply on the needs of future indoor environment, or broadly city design, in terms of balancing the needs of economics and sustainability of humanity, asking what has gone wrong indoors, and refocusing on human health, particularly for possible environmental intervention mode of operation for COVID-19 and other novel infectious diseases Opportunities or possibilities exist that the buildings may be equipped with better ventilation and filtration, e g to equip the building system with pandemic time operation mode (e g much more enhanced ventilation, availability of HEPA, UVGI), to design a better control of indoor humidity, to ensure recirculation of air with good filtration or use fan-coil system with a separate ventilation system, to develop safe drainage systems in high-rise buildings with monitored foul gas low, to apply non-touchable high touch surfaces such as automatic door opening and remote lift button, automatic occupancy control, to reconsider open plan office design, and to design for easy future facility management and upgrade etc Opportunities of determining transmission routes using big data, artificial intelligence (AI) and the internet to things (IoT) are also briefly mentioned so that next time the exact transmission routes can be determined rapidly and accurately © 2021, Science Press All right reserved