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Background: The Zika virus poses a unique health threat since it is transmitted both sexually and via Aedes aegypti mosquito bites. Understanding where sexually transmitted infections (STIs) and Zika virus are likely to occur in parallel across the U.S. can inform public health efforts to prevent the transmission of the Zika virus in the U.S. We sought to identify at-risk areas in the U.S. for the spread of the Zika virus by these two primary transmission routes.
Methods: Using STIs as a surrogate marker for unprotected sexual activity, we estimated 2014 county rates of STIs across the 3,108 counties in the contiguous 48 United States using a spatial Poisson regression model with adjustment for concentrated disadvantage, pregnancy rates and the proportion of the county’s population that is of childbearing age and an offset for county population size. We then combined the predicted county-level STI rates with the probability of the presence of the Aedes aegypti mosquito to estimate the total risk of Zika transmission per county. Counties with the highest potential Zika transmission risk were identified and mapped. We also tabulated the total estimated population at high risk for Zika virus transmission in the U.S.
Results: Our findings suggested that 2.8-95.6 million individuals could be at potential higher risk for Zika infection, of which 42,000-1.4 million may be pregnant women; the highest at risk group. These areas were mostly southern states extending northward along the Atlantic coast and in southern California, with the highest predicted risk in Mississippi counties.
Conclusions: Identifying areas in the U.S. that are at highest risk for Zika virus infection could inform primary disease prevention strategies and build infrastructure for diagnosis and treatment. Timely strategies are needed to communicate risk, control mosquito populations, and reduce disease transmission in these at-risk areas to prevent further large-scale outbreaks.