Despite significant global efforts to improve vaccination coverage against major childhood diseases, vaccination rates are below 90%. To eradicate diseases such as measles, however, vaccination rates close to 95% are needed. We use a randomized experiment to investigate the effect of a demand incentive, a conditional cash transfer program, in improving vaccination coverage in rural Nicaragua. Double-difference estimates show the program led to large increases in vaccination coverage, and these resulted in vaccination levels greater than 95% for some vaccines. Effects were especially large for children who are typically harder to reach with traditional supply-side interventions.