Ewan is following three of the world's toughest cold chains, supported by Unicef, who immunise over half of the world's children. In the second episode, Ewan journeys up the mighty Congo River, where he passes through frontier towns in the wilds of Central Africa, encounters a unique subculture for whom style is a religion, and ventures deep into the jungle in search of Pygmy tribes. Ewan said of his experience: 'The cold chain routes these vaccines follow can be hugely challenging and adventurous, and I was keen to explore some of them to discover how they reach the world's most remote children.

• Ewan McGregor