Fig. 1. Factors determining the global risk from ZIKV.

(A) As long as ZIKV circulates anywhere, periodic introductions into ZIKV free regions will occur. Whether these lead to an epidemic depends on the reproductive number, R, a measure of transmission efficiency determined by local ecology and population susceptibility to ZIKV. (B) When R>1, introductions can result in significant epidemics, after which the virus may go locally extinct or become endemic. (C) ZIKV could be maintained endemically either in local non-human primates (the sylvatic cycle) or through ongoing human transmission. (D) Most ZIKV infections (75–80%) are asymptomatic, and those with symptoms are likely at highest risk for rare neurological complications (6, 63, 92), particularly Guillain-Barré (45). Adverse fetal outcomes, notably microcephaly, may also be more common when the mother is symptomatic. Owing to its association with pregnancy, ZIKV’s health impact depends on the fertility rate and the age distribution of infections. The age distribution mirrors the general population in ZIKV free (A) and epidemic (B) settings, but is a function of the force of infection in endemic settings (C) (4, 45). Appropriate control measures can reduce R, decreasing the probability of successful ZIKV invasion (A) and its subsequent impact (B–C) (see 116).