Fig. 2

Regions of R_d and R_z where dengue vaccination increases/reduces Zika infections. a The red curve represents the variation of ΔZ with respect to P_v when R_z and R_d falls into the red region in (b). The blue curves refer to the case when R_z and R_d fall into the blue region, \(P_{v}^{*}\) and \(P_{v}^{c}\) are the optimal and critical dengue vaccine rates to reduce Zika infections. Here κ=2. b Dengue vaccine can always increase Zika infections if the basic reproduction numbers R_d and R_z are in the red region. The vaccine can reduce Zika infections if the vaccine coverage rate is within a certain interval when R_d and R_z are in the blue region