A medical worker wearing protective suit works at a COVID-19 testing site in Hedong District in north China's Tianjin, Jan. 20, 2022. The city launched its fourth citywide nucleic acid testing at 6 a.m. Thursday, aiming to curb the further spread of the virus. (Xinhua/Sun Fanyue)

A medical worker takes a swab sample from a resident for nucleic acid test at a COVID-19 testing site in a residential community of Nankai District in north China's Tianjin, Jan. 20, 2022. The city launched its fourth citywide nucleic acid testing at 6 a.m. Thursday, aiming to curb the further spread of the virus. (Xinhua/Zhao Zishuo)

A medical worker takes a swab sample from a resident for nucleic acid test at a COVID-19 testing site in a residential community of Nankai District in north China's Tianjin, Jan. 20, 2022. The city launched its fourth citywide nucleic acid testing at 6 a.m. Thursday, aiming to curb the further spread of the virus. (Xinhua/Zhao Zishuo)

Residents and volunteers queue up for nucleic acid tests at a residential community in Nankai District of north China's Tianjin, Jan. 20, 2022. The city launched its fourth citywide nucleic acid testing at 6 a.m. Thursday, aiming to curb the further spread of the virus. (Xinhua/Zhao Zishuo)

A medical worker takes a swab sample from a resident for nucleic acid test at a COVID-19 testing site in Hedong District in north China's Tianjin, Jan. 20, 2022. The city launched its fourth citywide nucleic acid testing at 6 a.m. Thursday, aiming to curb the further spread of the virus. (Xinhua/Sun Fanyue)

Medical workers head for a COVID-19 testing site at dawn in north China's Tianjin, Jan. 20, 2022. The city launched its fourth citywide nucleic acid testing at 6 a.m. Thursday, aiming to curb the further spread of the virus.

(Xinhua/Sun Fanyue)

A medical worker takes a swab sample from a resident for nucleic acid test at a COVID-19 testing site in Hedong District in north China's Tianjin, Jan. 20, 2022. The city launched its fourth citywide nucleic acid testing at 6 a.m. Thursday, aiming to curb the further spread of the virus. (Xinhua/Zhao Zishuo)

A resident arrives to receive nucleic acid test at a COVID-19 testing site in a residential community of Nankai District in north China's Tianjin, Jan. 20, 2022. The city launched its fourth citywide nucleic acid testing at 6 a.m. Thursday, aiming to curb the further spread of the virus. (Xinhua/Zhao Zishuo)

Staff members prepare medical materials for nucleic acid tests at a COVID-19 testing site in a residential community of Nankai District in north China's Tianjin, Jan. 20, 2022. The city launched its fourth citywide nucleic acid testing at 6 a.m. Thursday, aiming to curb the further spread of the virus. (Xinhua/Zhao Zishuo)