"Today, the threat of yellow fever looms larger than ever before, especially for thousands of children across Africa," Stefan Peterson, chief of health at the United Nations children's agency (UNICEF), said in a statement.
A major vaccination campaign in Angola and Congo in 2016 brought one of the worst outbreaks of the disease in decades under control after more than 400 people died.
One reason the disease is spreading is because more people are moving from rural to urban areas, particularly to slums, said Robin Nandy, UNICEF's chief of immunisation.
"These areas tend to have high numbers of people living in close proximity with poor hygiene and sanitation - all the conditions that make it ripe for a disease outbreak," he told the Thomson Reuters Foundation in a telephone interview.
The virus also poses a serious risk in Sudan, South Sudan, Uganda and the Democratic Republic of Congo, he said.
The vaccination programme is a joint venture by the WHO, UNICEF, the GAVI global vaccine alliance and more than 50 health partners.