Vaccines save at least 4 to 5 million lives every year. This remarkable success story is the result of massive immunization campaigns that have swept round the world in recent decades. Smallpox, which killed hundreds of millions, was eradicated in 1980, and polio cases have been reduced by more than 99% since 1988. Dr. Kate O’Brien, Immunization Director at the World Health Organization, takes us on a historical tour from the development of the first modern vaccine in the late 18th Century, to the new vaccines that are combatting today’s COVID pandemic. The United Nations, and its partners in the COVAX facility, are ensuring that vaccines reach every low-income country in the world. Dr. O’Brien was moved to devote her life to immunization after working in a pediatric ward in Haiti, where she saw that a third of the children were dying from diseases that were completely vaccine preventable.