- Correspondence to Dr Shu-Hong Zhu, Department of Family Medicine and Public Health, University of California, San Diego, 9500 Gilman Drive, MC 0905, La Jolla, CA 92093-0905, USA;
- Received 1 April 2016
- Accepted 3 July 2016
- Published in print 1 October 2016.
- Published online 3 October 2016.
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