Epidemic dynamics are key and data gaps must be addressed As evidence grew for a causal link between Zika infection and microcephaly and other serious congenital anomalies ( 1 ), the World Health Organization (WHO) declared the Latin American Zika epidemic a public health emergency of international concern in February 2016 ( 2 ). The speed of spread see the figure, top, and the supplementary materials (SM) has made effective public health responses challenging. Immediate responses have included vector control ( 3 ) and advice to delay pregnancy in a few countries ( 4 ), followed by an extended recommendation to all affected countries by WHO in June 2016. These have merits but are likely to have limited effectiveness ( 5 ) and may interact antagonistically. Fuller understanding of dynamics and drivers of the epidemic is needed to assess longer-term risks to prioritize interventions.