Photo taken on Dec. 28, 2021 shows a closed subway station in Xi'an, northwest China's Shaanxi Province. Authorities in Xi'an have upgraded epidemic control and prevention measures starting Monday, ordering all residents to stay indoors and keep away from gatherings except when taking nucleic acid tests. (Xinhua/Tao Ming)

Aerial photo taken on Dec. 28, 2021 shows the empty Great Tang All Day Mall in Xi'an, capital of northwest China's Shaanxi Province. Authorities in Xi'an have upgraded epidemic control and prevention measures starting Monday, ordering all residents to stay indoors and keep away from gatherings except when taking nucleic acid tests. (Xinhua/Tao Ming)

A staff member feeds pigeons at Xincheng Square in Xi'an, northwest China's Shaanxi Province, Dec. 28, 2021. Authorities in Xi'an have upgraded epidemic control and prevention measures starting Monday, ordering all residents to stay indoors and keep away from gatherings except when taking nucleic acid tests. (Xinhua/Tao Ming)

Aerial photo taken on Dec. 28, 2021 shows an empty overpass in Xi'an, northwest China's Shaanxi Province. Authorities in Xi'an have upgraded epidemic control and prevention measures starting Monday, ordering all residents to stay indoors and keep away from gatherings except when taking nucleic acid tests. (Xinhua/Tao Ming)

Aerial photo taken on Dec. 28, 2021 shows the surrounding area of Dayan Pagoda in Xi'an, northwest China's Shaanxi Province. Authorities in Xi'an have upgraded epidemic control and prevention measures starting Monday, ordering all residents to stay indoors and keep away from gatherings except when taking nucleic acid tests. (Xinhua/Tao Ming)

A deliveryman is seen on an empty street near Yongning Gate in Xi'an, northwest China's Shaanxi Province, Dec. 28, 2021. Authorities in Xi'an have upgraded epidemic control and prevention measures starting Monday, ordering all residents to stay indoors and keep away from gatherings except when taking nucleic acid tests. (Xinhua/Tao Ming)

Aerial photo taken on Dec. 28, 2021 shows empty streets near Yongning Gate in Xi'an, northwest China's Shaanxi Province. Authorities in Xi'an have upgraded epidemic control and prevention measures starting Monday, ordering all residents to stay indoors and keep away from gatherings except when taking nucleic acid tests. (Xinhua/Tao Ming)