The purpose of this study is to see if the MMR and chickenpox vaccines work as well in premature infants as in children that were carried to full term. A group of children who were carried full-term will be matched for age, sex, and race and will be used for comparison. Extremely premature infants (born at <28-30 weeks gestation) have lower antibody responses than full-term infants to several vaccines given at the postnatal ages recommended for full term infants. We propose to evaluate the immunogenicity of varicella and mumps-measles-rubella (MMR) vaccines in relatively healthy 15 month-old children born at <29 weeks gestation. This is a phase IV, observational study with 2 study arms having 16 infants each. The first group will enroll infants 9-12 months old that were born premature (<29 weeks gestation). The second group will be matched for sex, race, and postnatal age, but will have been full term (>= 37 weeks gestation) at birth. Infants will be vaccinated at visit 1 and post-vaccine serology will drawn at visit 2 (4 to 6 weeks after visit 1).