Background: An optimal vaccination schedule for VLBW infants is still under debate. The aim of the study was to analyze factors influencing the efficacy of hepatitis B vaccination in VLBW infants.Material/Methods: 211 newborns of HBsAg negative mothers (mean birth weight 1234±24g and mean gestational age 29.2±0.18 weeks) were observed. The patients who were vaccinated before their transfer to the tertiary NICU (mean age: 1 day,range: 0–9) were enrolled into the group A (early vaccination, N=100), the others into the group B (delayed vaccination, N=111). In the group B vaccination was postponed on average until the 53rd day of life (range: 11–179 day). According to the schedule of vaccination two subgroups of infants were identified - receiving 3 doses (at the age of 0, 1 and 6 months, n=81) or 4 doses of vaccines (at the age of 0, 1, 2 and 12 months, n=105). After 2, 12 and 36 months from the beginning of vaccination the serum level of antiHBs was measured.Results: A positive response (?10mIU/ml) was found after 2 months in 42 (51%) and 60 (69%) newborns (p

Keywords: hepatitis B vaccine, immunogenicity, prematurity, Antibodies