Transmissibility and transmission of respiratory viruses

Abstract

Human respiratory virus infections lead to a spectrum of respiratory symptoms and disease severity, contributing to substantial morbidity, mortality and economic losses worldwide, as seen in the COVID-19 pandemic. Belonging to diverse families, respiratory viruses differ in how easy they spread (transmissibility) and the mechanism (modes) of transmission. Transmissibility as estimated by the basic reproduction number (R₀) or secondary attack rate is heterogeneous for the same virus. Respiratory viruses can be transmitted via four major modes of transmission: direct (physical) contact, indirect contact (fomite), (large) droplets and (fine) aerosols. We know little about the relative contribution of each mode to the transmission of a particular virus in different settings, and how its variation affects transmissibility and transmission dynamics. Discussion on the particle size threshold between droplets and aerosols and the importance of aerosol transmission for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and influenza virus is ongoing. Mechanistic evidence supports the efficacies of non-pharmaceutical interventions with regard to virus reduction; however, more data are needed on their effectiveness in reducing transmission. Understanding the relative contribution of different modes to transmission is crucial to inform the effectiveness of non-pharmaceutical interventions in the population. Intervening against multiple modes of transmission should be more effective than acting on a single mode.

Fig. 1. Major modes of transmission of respiratory viruses during short-range and long-range transmission.

During an acute respiratory virus infection, an infected individual (infector; red) may shed virus in exhaled breath droplets and aerosols, and may also contaminate their immediate bodily surfaces (for example, skin and clothes) or surrounding objects and surfaces (for example, tables) with their respiratory secretions. In general, if a susceptible individual (infectee; grey) is close to the infector, short-range transmission may occur when the infectee breathes in the virus-laden droplets or aerosols released by the infector, during direct (physical) contact with the infector or during physical contact with objects or surfaces contaminated (fomite) by the infector. If the infectee is at a distance from the infector, long-range transmission may occur when the infectee breathes in the virus-laden aerosols released by the infector or during physical contact with a fomite. However, the terminology and the defining features of each mode of respiratory virus transmission, especially regarding redefining the particle size threshold between droplets and aerosols, is under active discussion (see the section Terminology and defining features of modes of transmission).

Fig. 2. Viral, environmental and host determinants of respiratory virus transmission.

Virus, environmental and host factors influence whether a successful transmission occurs by governing the infectivity of the respiratory virus, the contagiousness of the infected person, the environmental stress on the virus, which affects its persistence and survival during transmission, and the susceptibility of the exposed person.