A health worker marks the finger of a child after giving polio vaccine during a door-to-door polio vaccination campaign in southern Pakistani port city of Karachi on Dec. 3, 2020. (Xinhua/Ahmad Kamal)

A health worker gives polio vaccine to a child during a door-to-door polio vaccination campaign in southern Pakistani port city of Karachi on Dec. 3, 2020. (Xinhua/Ahmad Kamal)