Abstract: Pathogens often follow more than one transmission route during outbreaks—from needle sharing plus sexual transmission of HIV to small droplet aerosol plus fomite transmission of influenza. Thus, controlling an infectious disease outbreak often requires characterizing the risk associated with multiple mechanisms of transmission. For example, during the Ebola virus outbreak in West Africa, weighing the relative importance of funeral versus health care worker transmission was essential to stopping disease spread. As a result, strategic policy decisions regarding interventions must rely on accurately characterizing risks associated with multiple transmission routes. The ongoing Zika virus (ZIKV) outbreak challenges our conventional methodologies for translating case-counts into route-specific transmission risk. Critically, most approaches will fail to accurately estimate the risk of sustained sexual transmission of a pathogen that is primarily vectored by a mosquito—such as the risk of sustained sexual transmission of ZIKV. By computationally investigating a novel mathematical approach for multi-route pathogens, our results suggest that previous epidemic threshold estimates could under-estimate the risk of sustained sexual transmission by at least an order of magnitude. This result, coupled with emerging clinical, epidemiological, and experimental evidence for an increased risk of sexual transmission, would strongly support recent calls to classify ZIKV as a sexually transmitted infection.
Abstract: Zika virus (ZIKV) continues to be a threat to countries with conditions suitable for transmission, namely adequate temperatures and the presence of competent mosquito vectors. Estimates of risk in other countries based on the sexual transmission of ZIKV may be underestimated because of inadequate surveillance. Here, we formulate random network models of sexual transmission of ZIKV with asymmetric transmission (men being infectious for longer than women) and show that, contrary to previous work, there exists two epidemic thresholds and that certain men who have sex with men communities could sustain transmission on their own. Our results also shed light on a class of processes on random networks by providing a complete analysis of dynamics with multiple critical points.