Drivers of airborne human-to-human pathogen transmission

Abstract

Airborne pathogens - either transmitted via aerosol or droplets - include a wide variety of highly infectious and dangerous microbes such as variola virus, measles virus, influenza A viruses, Mycobacterium tuberculosis, Streptococcus pneumoniae, and Bordetella pertussis. Emerging zoonotic pathogens, for example, MERS coronavirus, avian influenza viruses, Coxiella, and Francisella, would have pandemic potential were they to acquire efficient human-to-human transmissibility. Here, we synthesize insights from microbiological, medical, social, and economic sciences to provide known mechanisms of aerosolized transmissibility and identify knowledge gaps that limit emergency preparedness plans. In particular, we propose a framework of drivers facilitating human-to-human transmission with the airspace between individuals as an intermediate stage. The model is expected to enhance identification and risk assessment of novel pathogens.

Figure 1

Circle of events leading to human-to-human transmission of airborne pathogens. (1) The pathogen is associated with either small aerosols/large droplets or dust particles when transported through the air from donor to recipient. This may be directly, or via an intermediate stage involving settlement on a surface and re-emergence into the air later. (2) A low infectious dose is sufficient for deposition of the pathogen in the respiratory tract of the recipient. (3) After infection of susceptible cells, pathogens may amplify at the site of deposition only (localized site), or are disseminated from the primary deposition site to peripheral tissues (secondary site) where additional amplification takes place. (4) Eventually, the recipient becomes the donor and pathogens are expelled from the exit site. This is (usually) the respiratory tract, but may also be the secondary site of replication. High infectious loads of the pathogen emerge in the air, and the transmission cycle is repeated.

Figure 2

Framework for the classification of drivers of human-to-human transmission of zoonotic pathogens by the airborne route (interhuman barrier). These drivers operate on tissue, individual, community, country, and global levels. The presented framework also makes provisions for the multitude of influences between levels highlighting that some drivers have implications for several lower level drivers and in sum may be equally important to drivers considered to act from an outer level.