The world keeps millions of vaccines on ice. Is it worth it?

A child receives a cholera vaccine at a temporary treatment center during a past outbreak in Lusaka, Zambia.

It costs nearly $100 million a year to maintain global stockpiles of vaccines for Ebola, cholera, meningitis and yellow fever in case of emergency. A new study estimates how many lives they've saved.

(Image credit: Namukolo Siyumbwa)