The flights are one link in a vast global supply chain being assembled to tackle… United Airlines Holdings Inc. on Friday began operating charter flights to position doses of Pfizer Inc.’s Covid-19 vaccine for quick distribution if the shots are approved by regulators, according to people familiar with the matter. The initial flights are one link in a global supply chain being assembled to tackle the logistical challenge of distributing Covid-19 vaccines. Pfizer has been laying the groundwork to move quickly if it gets approval from the Food and Drug Administration and other regulators world-wide. Pfizer’s…