The global plan to deliver COVID-19 vaccines to poorer countries faces a "very high" risk of failure, potentially leaving nations home to billions of people with no access to vaccines until as late as 2024, internal documents say.

The World Health Organization's COVAX program is the main global plan to vaccinate people in poor and middle income countries around the world against the coronavirus. It aims to deliver at least 2 billion vaccine doses by the end of 2021 to cover 20% of the most vulnerable people in 91 poor and middle-income countries, mostly in Africa, Asia and Latin America.

But in internal documents reviewed by Reuters, the plan's promoters say the program is struggling from a lack of funds, supply risks and complex contractual arrangements which could make it impossible to achieve its goals.