Abstract
BackgroundLocal mosquito-borne Zika virus (ZIKV) transmission has been reported in two counties in the contiguous United States (US), prompting the issuance of travel, prevention, and testing guidance across the contiguous US. Large uncertainty, however, surrounds the quantification of the actual risk of ZIKV introduction and autochthonous transmission across different areas of the US.MethodsWe present a framework for the projection of ZIKV autochthonous transmission in the contiguous US during the 2015–2016 epidemic using a data-driven stochastic and spatial epidemic model accounting for seasonal, environmental, and detailed population data. The model generates an ensemble of travel-related case counts and simulates their potential to have triggered local transmission at the individual level in the 2015–2016 ZIKV epidemic.ResultsWe estimate the risk of ZIKV introduction and local transmission at the county level and at the 0.025° × 0.025° cell level across the contiguous US. We provide a risk measure based on the probability of observing local transmission in a specific location during a ZIKV epidemic modeled after the epidemic observed during the years 2015–2016. The high spatial and temporal resolution of the model allows us to generate statistical estimates of the number of ZIKV introductions leading to local transmission in each location. We find that the risk was spatially heterogeneously distributed and concentrated in a few specific areas that account for less than 1% of the contiguous US population. Locations in Texas and Florida that have actually experienced local ZIKV transmission were among the places at highest risk according to our results. We also provide an analysis of the key determinants for local transmission and identify the key introduction routes and their contributions to ZIKV transmission in the contiguous US.ConclusionsThis framework provides quantitative risk estimates, fully captures the stochasticity of ZIKV introduction events, and is not biased by the under-ascertainment of cases due to asymptomatic cases. It provides general information on key risk determinants and data with potential uses in defining public health recommendations and guidance about ZIKV risk in the US.
Highlights
Local mosquito-borne Zika virus (ZIKV) transmission has been reported in two counties in the contiguous United States (US), prompting the issuance of travel, prevention, and testing guidance across the contiguous US
We report the regression coefficient for each of the explanatory variables along with R squared, based on n = 1220 cells ntr average number of local ZIKV transmissions within each 0.25° × 0.25° cell from January 1, 2015, to December 31, 2016, Nim number of ZIKV importations, f20° fraction of days with temperature higher than 20 °C, Gross domestic product (GDP) gross domestic product per capita in purchasing power parity ***p < 0.001
Our model estimates that approximately 68.9% of the people in the contiguous US live in areas that are environmentally suitable for ZIKV transmission, in line with other models’ estimates [36]
Summary
Local mosquito-borne Zika virus (ZIKV) transmission has been reported in two counties in the contiguous United States (US), prompting the issuance of travel, prevention, and testing guidance across the contiguous US. From 2015 to 2016, the Zika virus (ZIKV) epidemic spread across most countries in the Americas, including the United States (US) [1,2,3]. Laboratory evidence of possible ZIKV infections has been found in 4900 pregnant women from US territories, 167 of whom have had pregnancy outcomes with ZIKV-related birth defects [3, 5, 6]. The US states and District of Columbia have reported 5710 travel-associated ZIKV cases, including 2474 pregnant women with evidence of ZIKV infection and 116 ZIKV-related birth defects [3]. While the outbreaks in Florida and Texas were limited, the indirect impact on the local economy has been remarkable [9]
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