Ewan McGregor is on a mission to the ends of the earth to immunise some of the hardest-to-reach children in the world. Against the odds, Ewan and his team deliver vaccines to the world's most remote children as health workers do every day. But their fragile nature means the vaccines must be kept constantly cold as they are passed along the supply chain, from freezer to freezer - wherever that may be. The routes these vaccines travel are known as cold chains, and a vast network of them exists across the globe.