A medical worker collects nucleic acid sample at a testing site in Harbin, northeast China's Heilongjiang Province, Jan. 22, 2021. Citizens in Harbin have been undergoing nucleic acid testing for the COVID-19 prevention and control. (Xinhua/Xie Jianfei)

A medical worker collects nucleic acid sample at a testing site in a middle school in Harbin, northeast China's Heilongjiang Province, Jan. 22, 2021. Citizens in Harbin have been undergoing nucleic acid testing for the COVID-19 prevention and control. (Xinhua/Wang Jianwei)

A medical worker collects nucleic acid sample at a testing site in a primary school in Harbin, northeast China's Heilongjiang Province, Jan. 22, 2021. Citizens in Harbin have been undergoing nucleic acid testing for the COVID-19 prevention and control. (Xinhua/Wang Jianwei)

Medical workers record information for citizens at a testing site in a primary school in Harbin, northeast China's Heilongjiang Province, Jan. 22, 2021. Citizens in Harbin have been undergoing nucleic acid testing for the COVID-19 prevention and control. (Xinhua/Wang Jianwei)

A citizen arrives to get nucleic acid testing at a testing site in a middle school in Harbin, northeast China's Heilongjiang Province, Jan. 22, 2021. Citizens in Harbin have been undergoing nucleic acid testing for the COVID-19 prevention and control. (Xinhua/Xie Jianfei)

A medical worker collects nucleic acid sample at a testing site in a primary school in Harbin, northeast China's Heilongjiang Province, Jan. 22, 2021. Citizens in Harbin have been undergoing nucleic acid testing for the COVID-19 prevention and control. (Xinhua/Wang Jianwei)

A medical worker collects nucleic acid sample at a testing site in a middle school in Harbin, northeast China's Heilongjiang Province, Jan. 22, 2021. Citizens in Harbin have been undergoing nucleic acid testing for the COVID-19 prevention and control. (Xinhua/Wang Jianwei)

A medical worker collects nucleic acid sample at a testing site in a primary school in Harbin, northeast China's Heilongjiang Province, Jan. 22, 2021. Citizens in Harbin have been undergoing nucleic acid testing for the COVID-19 prevention and control. (Xinhua/Wang Jianwei)

A medical worker collects nucleic acid sample at a testing site in a primary school in Harbin, northeast China's Heilongjiang Province, Jan. 22, 2021. Citizens in Harbin have been undergoing nucleic acid testing for the COVID-19 prevention and control. (Xinhua/Wang Jianwei)

Medical workers record information for citizens at a testing site in a middle school in Harbin, northeast China's Heilongjiang Province, Jan. 22, 2021. Citizens in Harbin have been undergoing nucleic acid testing for the COVID-19 prevention and control. (Xinhua/Xie Jianfei)

A medical worker checks a nucleic acid collecting tube at a testing site in a primary school in Harbin, northeast China's Heilongjiang Province, Jan. 22, 2021. Citizens in Harbin have been undergoing nucleic acid testing for the COVID-19 prevention and control. (Xinhua/Wang Jianwei)