I. Introduction
Coronavirus disease 2019 (COVID-19) is an ongoing global pandemic affecting 221 countries and territories. According to World Health Organization reports [1], as of August 11, 2021, there have been 203.9 million confirmed COVID-19 cases with more than 4.31 million deaths around the world. To date, the number of COVID-19 cases worldwide is dramatically increasing every day. One of the key steps to fight against COVID-19 is to timely detect the infected patients. Clinically, there are mainly two ways for this task: one is reverse-transcription polymerase chain reaction (RT-PCR) testing, and the other is chest computed tomography (CT), where the latter shows higher sensitivity to COVID-19 and can be used as an effective screening or diagnostic tool in epidemic areas [2], [3], [4]. Unfortunately, manual diagnosis using CT images requires many radiologists to read the subtle imaging features of COVID-19, which is extremely labor-intensive and subjective [5].