Baseline Hepatitis B Immunity and Vaccination Booster Response Among Medical Residents: A Longitudinal Study in a Spanish Tertiary Hospital

<b>Background:</b> Despite universal infant hepatitis B virus (HBV) vaccination, declining circulating anti-HBs levels are increasingly observed in young healthcare professionals, a high-risk group for occupational exposure. Although several studies have evaluated HBV antibody persistence in healthcare workers, data specifically addressing newly incorporated medical residents in the Spanish context remain limited. This study evaluated baseline anti-HBs levels and serological response to a vaccination booster dose in medical residents at a Spanish tertiary hospital. <b>Methods:</b> A retrospective longitudinal observational study was conducted among medical residents attending the Preventive Medicine Service of Hospital Universitario San Cecilio (Granada, Spain) between 2021 and 2024. Anti-HBs antibody titers were obtained at baseline and ≥10 mIU/mL were considered the conventional protective threshold. Residents with anti-HBs < 10 mIU/mL received an Engerix-B booster followed by repeat serology. Demographic and occupational variables were analyzed. Measles serostatus was collected for comparisons. <b>Results:</b> A total of 275 residents were included (mean age 25.4 years, SD = 2.3 years; 64% females). Baseline serology showed anti-HBs levels < 10 mIU/mL in 53.1% of participants. Lower baseline anti-HBs levels were associated with younger age (adjusted OR = 0.75; 95% CI: 0.64-0.88) and earlier residency year (R1-R2) (adjusted OR = 0.28; 95% CI: 0.13-0.61). Among 116 residents receiving a booster, 94.8% achieved anti-HBs ≥ 10 mIU/mL after booster administration. Measles serology was negative in 54.6% of participants. <b>Conclusions:</b> More than half of newly incorporated medical residents had anti-HBs levels below the conventional protective threshold (10 mIU/mL), yet almost all demonstrated a strong anamnestic response, supporting the persistence of immunological memory despite reduced circulating antibody concentrations. Systematic baseline screening combined with targeted booster vaccination appears to be an effective strategy to ensure occupational protection. Further research incorporating cellular immunity markers may refine future vaccination policies and booster strategies.