The risk of sustained sexual transmission of Zika is underestimated - Fig 2

left panel: Considering a scenario in which women are 10 times less likely to transmit ZIKV than men (ε = 10), the dotted, dashed and solid lines show three estimates of the basic reproductive number R₀ associated with sexual transmissions: (i) is the average number of secondary infections due to all infected individuals, including by the vector, (ii) is the average number of secondary infections resulting from individuals infected sexually, and (iii) is the average number of secondary infections resulting from individuals in the MSM community who were infected sexually. These estimates are computed by counting the number of sexual infections caused by the individuals infected by the vector and/or their direct neighbors (see Methods). These various R₀ estimates indicate that the transmission probability is at the epidemic threshold when they are equal to one (large dots). The community-specific is the only observable that adequately measures when ZIKV could invade the MSM community; the other metrics falsely imply that sustained sexual transmission is unlikely; critically, leads to an overestimation of the epidemic threshold, T_c, by an order of magnitude (i.e., 0.35 versus 0.03).right panel: Prevalence of ZIKV in the whole population and in two sub-populations (MSM and all heterosexuals) as a function of T using ε = 2 and only considering individuals with more than one sexual partner (as a rough approximation of the sexually active population). The vertical, dashed-gray lines show the threshold values for a self-sustained epidemic within the MSM community (with subcritical spillovers in the rest of the population, shaded area), and within the entire population (i.e., supercritical outbreaks exist both within and outside of the MSM community), see Eqs (1) and (2). Note that the prevalence outside of the MSM community start rising at the threshold for endemic transmission in the MSM population because of sub-critical spillovers. Note also that we used ε = 2 instead of ε = 10 for pedagogical reasons since it facilitated the presentation of the results and is clearly conservative with respect to our conclusions.