From UNICEF:

Lancet series launch: Breastfeeding saves lives and it’s time for action

Funded by the Gates Foundation and the Wellcome Trust, [the Lancet series on breastfeeding] is the most extensive piece of research into the effects of breastfeeding ever undertaken and should put an end to any doubts about the impact of breastfeeding on the health and well-being of populations.
Press release from The Lancet:

Increasing breastfeeding worldwide could prevent over 800,000 child deaths and 20,000 deaths from breast cancer every year

New estimates produced for the two-part Series reveal that increasing breastfeeding to near-universal levels for infants and young children could save over 800000 children’s lives a year worldwide, equivalent to 13% of all deaths in children under two, and prevent an extra 20000 deaths from breast cancer every year.