Hari Sreenivasan:

Not everyone in a community has to have the vaccine for protection. If 95 percent of a group gets immunized, there's what's called herd immunity, when the high number of vaccinated prevent the spread of an infection.

But in the Congo, only 57 percent of the population is vaccinated, according to UNICEF.

There's also the matter of the return visit. According to UNICEF, while 86 percent of the children around the world receive the first dose, fewer than 70 percent receive the recommended second dose.

Dr. Paul Offit is the director of the vaccine center at Children's Hospital of Philadelphia.