Building the hospital event-based surveillance system in Viet Nam: a qualitative study to identify potential facilitators and barriers for event reporting oleh Hien Do, Hien T Ho, Phu D Tran, Dang B Nguyen, Satoko Otsu, Cindy Chiu de Vázquez, Tan Q Dang, Quang D Tran, Van Anh Pham, Nanako Mikami, Masaya Kato