基金This study was supported by the National Health and Family Planning Commission of the People's Republic of China, the National Natural Science Foundation of China (grant No81330016), the China Medical Board (grant No 11-065), WHO (grant No: CHN-12-MCN-004888), UNICEF (grant No 2016EJH016), and the National "Twelfth Five-Year" Plan for Science and Technology Support (2014BA106B01).