- Correspondence to Dr Jennifer S Smith, Department of Epidemiology, 2103 McGavran-Greenberg, Department of Epidemiology, UNC Gillings School of Global Public Health, Campus Box 7435, Chapel Hill, NC 27599, USA;
- Received 3 May 2017
- Revised 14 July 2017
- Accepted 30 July 2017
- Published online 2 September 2017.
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