Medical workers carry out door-to-door nucleic acid sampling on the elderly who have difficulty in moving in Shunyi District of Beijing, capital of China, Dec. 28, 2020. Beijing has expanded its nucleic acid testing and tightened COVID-19 prevention and control measures following new reports of local COVID-19 infections, local authorities said on Monday. (Xinhua/Ju Huanzong)

A medical worker collects sample to conduct nucleic acid test at a sampling site in Shunyi District of Beijing, capital of China, Dec. 28, 2020. Beijing has expanded its nucleic acid testing and tightened COVID-19 prevention and control measures following new reports of local COVID-19 infections, local authorities said on Monday. (Xinhua/Ju Huanzong)

Medical workers collect samples to conduct nucleic acid tests at a sampling site in Shunyi District of Beijing, capital of China, Dec. 28, 2020. Beijing has expanded its nucleic acid testing and tightened COVID-19 prevention and control measures following new reports of local COVID-19 infections, local authorities said on Monday. (Xinhua/Ju Huanzong)

A medical worker collects sample to conduct nucleic acid test at a sampling site in Shunyi District of Beijing, capital of China, Dec. 28, 2020. Beijing has expanded its nucleic acid testing and tightened COVID-19 prevention and control measures following new reports of local COVID-19 infections, local authorities said on Monday. (Xinhua/Ju Huanzong)

A medical worker collects sample to conduct nucleic acid test at a sampling site in Shunyi District of Beijing, capital of China, Dec. 28, 2020. Beijing has expanded its nucleic acid testing and tightened COVID-19 prevention and control measures following new reports of local COVID-19 infections, local authorities said on Monday. (Xinhua/Ju Huanzong)

Medical workers collect samples to conduct nucleic acid tests at a sampling site in Shunyi District of Beijing, capital of China, Dec. 28, 2020. Beijing has expanded its nucleic acid testing and tightened COVID-19 prevention and control measures following new reports of local COVID-19 infections, local authorities said on Monday. (Xinhua/Ju Huanzong)

A medical worker (1st, R) scans to register personal information at a sampling site in Shunyi District of Beijing, capital of China, Dec. 28, 2020. Beijing has expanded its nucleic acid testing and tightened COVID-19 prevention and control measures following new reports of local COVID-19 infections, local authorities said on Monday. (Xinhua/Ju Huanzong)