This paper explores the use of social signaling as a policy tool to sustainably affect childhood immunization. In a 26-month field experiment with public clinics in Sierra Leone, the researcher introduces a verifiable signal—in the form of color-coded bracelets—given to children upon timely completion of the first four, or all five required vaccinations. Signals increase parents’ belief in the visibility of their actions and their knowledge of other children’s vaccine status. The impact of signals varies significantly with the cost and perceived benefits of the action: there are no discernible effects on timely and complete immunization when the signal is linked to an easier-to-complete vaccine with low perceived benefits, and large, positive effects when the signal is linked to a costlier-to-achieve vaccine with high perceived benefits. Parents adjust their behavior nine months prior to realizing the social image benefit, demonstrating the motivational strength of signaling incentives. Of substantive policy importance, bracelets increase full immunization at one year of age by 9 percentage points, with impacts persisting up to two years. At a cost of US$24.7 per additional fully immunized child, social signals can prove more cost-effective than financial or in-kind incentives.