Although the investigators have reached the Healthy People 2010 goal of 80% coverage among U.S. children 19-35 months of age for the primary immunization series, 20% of children still remain underimmunized. In addition, pockets of low immunization coverage and outbreaks of vaccine preventable disease persist. Immunization reminder-recalls have been shown to be successful, but have been limited by the ability to identify and reach large target populations in a real-time, automated fashion. Reminder-recalls via text message may offer an innovative opportunity to improve vaccination coverage. Currently, 91% of Americans own a cell phone. Prior research by our team suggests that text messages reminder recalls may be an effective tool to remind parents about their children's vaccines. In this study, the investigators will demonstrate the effectiveness of tailored text message appointment and immunization reminders linked to a well-established and functional immunization registry to increase coverage rates and timeliness of the sentinel vaccines of MMR and HepA. The investigators will target parents of young children from an underserved population that is primarily Latino, with low socioeconomic status.

Inclusion Criteria:

• Parents of child >9 months to 25 months
• Child with at least one visit to one of the participating clinical sites in the previous 12 months
• Parental cell phone number recorded in the registration system