United Airlines Holdings Inc. UAL -0.74% on Friday began operating charter flights to position doses of Pfizer Inc.’s PFE 1.92% 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 distribution plan also includes refrigerated storage sites at the drugmaker’s final-assembly centers in Kalamazoo, Mich., and Puurs, Belgium, and expanding storage capacity at distribution sites in Pleasant Prairie, Wis., and in Karlsruhe, Germany, in addition to dozens of cargo flights and hundreds of truck trips each day…

United had sought permission to carry more dry ice than is typically allowed on flights to maintain the extremely low temperatures required to prevent Pfizer’s vaccine from spoiling. The FAA said it would allow United to carry 15,000 pounds of dry ice per flight—five times more than normally permitted. Regulators restrict the amount of dry ice that can be carried on passenger jets because they typically lack equipment to monitor and mitigate any leaked carbon dioxide.