  Background: In 2015, the Zika arbovirus (ZIKV) began circulating in the
Americas, rapidly expanding its global geographic range in explosive outbreaks.
Unusual among mosquito-borne diseases, ZIKV has been shown to also be sexually
transmitted, although sustained autochthonous transmission due to sexual
transmission alone has not been observed, indicating the reproduction number
(R0) for sexual transmission alone is less than 1. Critical to the assessment
of outbreak risk, estimation of the potential attack rates, and assessment of
control measures, are estimates of the basic reproduction number, R0. Methods:
We estimated the R0 of the 2015 ZIKV outbreak in Barranquilla, Colombia,
through an analysis of the exponential rise in clinically identified ZIKV cases
(n = 359 to the end of November, 2015). Findings: The rate of exponential rise
in cases was rho=0.076 days-1, with 95 percent CI [0.066,0.087] days-1. We used
a vector-borne disease model with additional direct transmission to estimate
the R0; assuming the R0 of sexual transmission alone is less than 1, we
estimated the total R0 = 3.8 [2.4,5.6], and that the fraction of cases due to
sexual transmission was 0.23 [0.01,0.47] with 95 percent confidence.
Interpretation: This is among the first estimates of R0 for a ZIKV outbreak in
the Americas, and also among the first quantifications of the relative impact
of sexual transmission.
