The secondary outcome measure is represented by the children in the study having either no adverse effects or minimal adverse effects from the ProQuad® vaccine. Adverse effects will be monitored via Electronic Medical Records (EMR) for Emergency Department (ED), hospital or clinic visits, follow-up phone calls to the family/subjects, as well as the vaccination report card that the parents/legal guardians of the participants in the study will complete for their child for 7 days after he/she receives each dose of the vaccine.