Evaluation of Hepatitis B Vaccine Immunogenicity in Low-Birth-Weight Infants After Complete Immunization: The Impact of Postnatal Catch-Up Growth and Maternal–Neonatal Characteristics

Background: Low-birth-weight (LBW, <2500 g) infants are at increased risk of suboptimal hepatitis B vaccine responses; yet, data on their immunogenicity patterns and modifiable determinants remain limited. This study aimed to assess hepatitis B vaccine immunogenicity in LBW infants and to examine whether postnatal catch-up growth and maternal–neonatal characteristics are independently associated with antibody levels. Methods: We enrolled 511 LBW infants who completed the recommended 3-dose hepatitis B vaccination series at 0, 1, and 6 months. Blood samples were collected 4–6 weeks after completion of the full vaccination series. Geometric mean concentration (GMC) and seroprotection rate (SPR, anti-HBs ≥ 10 mIU/mL) were evaluated. Catch-up growth was quantified as the change in weight-for-age Z-score between 6 and 8 months of age (ΔWAZ). Multivariable linear regression was used to identify independent predictors of log-transformed antibody titers, adjusting for gestational age, maternal hepatitis B surface antigen (HBsAg) positivity, maternal body mass index (BMI), maternal fasting glucose, maternal thyroid disease, infant hemoglobin at 6 months, and ΔWAZ. Results: The overall SPR was 99.41% (508/511), with a GMC of 1045.37 mIU/mL (95% CI: 916.24–1192.70). SPR remained consistently high across all subgroups. In multivariable analysis, ΔWAZ was not significantly associated with antibody levels (β = −0.063, p = 0.571). Maternal HBsAg positivity showed no significant association (β = −0.104, p = 0.792). Maternal thyroid disease was independently associated with higher antibody levels (β = 0.793, 95% CI: 0.213–1.373, p = 0.007). None of the other covariates reached statistical significance. Conclusions: Hepatitis B vaccination demonstrated high immunogenicity in LBW infants, with very high seroprotection rates. Postnatal catch-up growth did not independently influence antibody levels. The significant positive association between maternal thyroid disease and infant antibody response warrants further prospective investigation.