Article History
First Online: 9 December 2019
Compliance with Ethical Standards
:
: Hongchao Li, Lei Lai and Bin Wu have no conflicts of interest to declare.
: No funding or sponsorship was received for this work.
: HL, LL and BW were involved in the design of the study, collected the data and performed the economic analysis. BW wrote the first draft of the manuscript, which was critically revised by LS.
: No additional data are available.