A decade ago in Nigeria, rumors spread that polio vaccines were surreptitious sterilization efforts. That led to a boycott of the vaccine in 2003 and a resurgence in the poliovirus three years later.
The story points up a key point about vaccines: Confidence is critical.
A new study of more than 20,000 people in five countries looks as why people aren't confident in vaccines. The reasons vary, from a belief the vaccine isn't safe to a bad experience with a previous vaccination.
And the results can be devastating.