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.

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).

Inclusion Criteria:

1. Premature infant < 29 weeks gestation at birth or term infant >= 37 weeks gestation at birth.
2. Postnatal age < 16 months, 0 days.
3. Has not yet received MMR or varicella vaccines. (There are no restrictions on the administration of other vaccines at the time of MMR/varicella vaccination.)
4. Parental permission.
5. Agreement of primary care pediatrician/ health care provider.
6. Receives primary pediatric care within an approximate 25-mile radius of the University of Rochester.
7. Healthy status at enrollment.

Exclusion Criteria:

1. Known immunodeficiency.
2. Systemic corticosteroid therapy at the time of MMR/varicella vaccination.
3. Requiring oxygen therapy.
4. Clinically significant findings on review of medical history and physical exam determined by the investigator or sub-investigator to be sufficient for exclusion.
5. Any condition determined by the investigator that would interfere with the evaluation of the vaccine or be a potential health risk to the subject.