Not vaccinating hard-to-reach populations results in high health and economic costs.

Key Evidence: A study in Kenya estimated that the failure to vaccinate the 21% of children considered hard-to-reach (living beyond a 5 km radius of a vaccination post) against measles would result — over 4 years — in more than 1,400 measles cases, 257 deaths, and cost nearly US$10 million, mainly in productivity losses from caretakers missing work.