Transmission routes of respiratory viruses among humans

Abstract

Respiratory tract infections can be caused by a wide variety of viruses. Airborne transmission via droplets and aerosols enables some of these viruses to spread efficiently among humans, causing outbreaks that are difficult to control. Many outbreaks have been investigated retrospectively to study the possible routes of inter-human virus transmission. The results of these studies are often inconclusive and at the same time data from controlled experiments is sparse. Therefore, fundamental knowledge on transmission routes that could be used to improve intervention strategies is still missing. We here present an overview of the available data from experimental and observational studies on the transmission routes of respiratory viruses between humans, identify knowledge gaps, and discuss how the available knowledge is currently implemented in isolation guidelines in health care settings.

Fig. 1

Isolation guidelines for respiratory virus infections in comparison to experimental evidence on transmission routes. Isolation guidelines for all respiratory viruses discussed in this review from National (Working Group Infection Prevention (WIP) [108], from the Netherlands National Institute for Public Health and the Environment (RIVM)), European (‘The Blue Book’ [109]), American (‘The Red Book’ [110] and the Centers for Disease Control (CDC) [3]) and International (UpToDate [111]) organizations are shown on the X-axis, together with the experimental evidence on transmission routes (Table 2). The categories on the Y-axis are the different transmission routes (contact, droplet or aerosol), the absence of guidelines for infection prevention (‘No guideline’), or the limited availability of experimental data (‘Lim. exp. data’). The information shown for influenza virus reflects the guidelines on seasonal influenza virus. Closed squares (■): isolation guidelines for the respective respiratory virus. Open squares (□): guidelines are only for children ≤6 years old. Open circles (○): data from stability experiments only. Open triangles (▵): specific CDC guidelines for Healthcare Professionals [115] (not the isolation guideline [3] used in this review).