Seropositivity of Anti-HBs Titer Among College Students Several Years After Completion of the Primary Hepatitis B Vaccination Series

Background: The Philippines is hyperendemic for hepatitis B infection. Vaccination is crucial for protection. Local data on the antibody response after completion of the primary vaccination series is limited. Objective: This study aimed to measure the anti-HBs levels among college students who completed primary hepatitis B vaccination series, compare seropositivity across stratified groups and correlate anti-HBs levels with the time elapsed since the last vaccine dose. Methods: This cross-sectional study included 111 college students in health-related courses with immunization record showing the complete primary Hepatitis B vaccination series. Participants were stratified based on the following vaccination schedules: 0-1-6-month group; 0-1-2-month group; and booster group. Anti-HBs titers were determined. Data Analysis: Statistical analyses included One-way ANOVA, Kruskal-Wallis test, Fisher’s Exact test and Shapiro-Wilk normality test. Kaplan-Meier Survival Estimate assessed the probability of anti-HBs seropositivity over time. Data were analyzed using STATA 13.1. Results: The baseline characteristics of the study population were homogenous. The median anti-HBs titer several years after primary vaccination was low at 2.9 mIU/mL. Participants in the booster group had the highest seropositivity rate (57.14%) with a median titer of 30.16 mIU/mL. There was an inverse relationship between anti-HBs titer and elapsed time since the last vaccine dose. Kaplan-Meier Survival Estimate showed that the seropositivity decreases to 90.56% after 15.8 years, 51.3% after 17.5 years, and 2.97% at 18 years. Conclusion: This study revealed low anti-HBs titers among students who previously completed primary vaccination series, with no significant difference between two schedules. Booster doses resulted in the highest seropositivity. Over-all, seropositivity declines over time.