He, Z, Xiang, HB, Manyande, Anne ORCID: https://orcid.org/0000-0002-8257-0722, Xu, W, Fan, l and Xiang, B (2021) Epidemiological characteristics of sporadic nosocomial COVID-19 infections from June 2020 to June 2021 in China: an overview of vaccine breakthrough infection events. Frontiers in Medicine, 8.