The interviewers were four trained public health and social science researchers. Institutional ethical approval was obtained from the Scientific Committee of Hanoi Medical University and written informed consent was obtained from all interviewees, who were invited to participate voluntarily. The interviews were conducted privately and anonymously. A code book was developed focusing on key findings and terminologies. The transcripts of the semistructured interviews were coded, entered and analysed by means of N-VIVO software. 