Acknowledgement
We thank Wei-jie Guan (State Key Laboratory of Respiratory Disease and National Clinical Research Center for Respiratory Disease, The First Affiliated Hospital of Guangzhou Medical University, Guangzhou, China) for his assistance in language editing during revision. This article has supplementary material available from erj.ersjournals.com
Author contributions: Y-H. Gao designed the project. D. Yan, X-Y. Liu and B-T. Dan carried out the data collection. Y-H. Gao, Y-N. Zhu and L. Huang analysed the data and prepared the figures. Y-H. Gao, D. Yan, Y-N. Zhu and L. Huang drafted the manuscript. All the authors have revised the manuscript critically, approved the version submitted for publication and have agreed to be accountable for all aspects of the work.
Conflict of interest: D. Yan has nothing to disclose.
Conflict of interest: X-Y. Liu has nothing to disclose.
Conflict of interest: Y-N. Zhu has nothing to disclose.
Conflict of interest: L. Huang has nothing to disclose.
Conflict of interest: B-T. Dan has nothing to disclose.
Conflict of interest: G-J. Zhang has nothing to disclose.
Conflict of interest: Y-H. Gao has nothing to disclose.