Hepatitis B Booster Study in Adolescence

Purpose:

The purpose of the study is to assess whether or not individuals immunized as infants with Hepatitis B vaccine have residual immunity in their pre-adolescent and adolescent years. Two age groups (10, 11 year olds and 15, 16 year olds) will be invited to have blood taken to measure their level of protection (antibody titer). Those without a minimum protective titer will be offered a Hepatitis B "test" vaccination to determine if they still have the capacity to recall an immune response (ie immune memory), as a second line of protection against infection. A second blood sample will be obtained to determine if a booster response occurred (immune memory persists, protection intact) or not. The study vaccine will be a pediatric dose of HB vaccine (EngerixB) made by GlaxoSmithKline Canada (10mcg), a licensed product used in the public immunization program in British Columbia (BC) currently.

Hypothesis:

The hypothesis being tested is that protection has been lost in a sufficient proportion of 15-16 year olds to warrant consideration of a pre-adolescent HB booster, modeled by the 10-11 year olds whose protection is expected to be uniformly reinforceable.

Justification:

This will be the first Canadian study of the duration of the HB immune memory after infant immunization. According to a systematic review of 46 studies, evidence of progressive loss of immune memory and protection decades after infant HB vaccination is clear. Provinces in Canada recently took a leap of faith in dropping their adolescent HB immunization programs, assuming that HB protection from infancy will be life-long. This project investigates residual HB immunity 10 and 15 years after infant vaccination and evaluates a prototype booster program to provide health officials with timely, strategic information for maintaining optimal, informed HB disease control.

Research Methods:

The study will be conducted in BC which has used a consistent infant HB dosing schedule throughout. Eligible participants will be healthy 10-11 or 15-16 year olds. After obtaining informed consent/assent, a blood sample (8 ml) will be obtained to measure serum anti-HBs (anti-HB surface antigen) titer. Participants with a titer ≥10 IU/L will be informed of their ongoing protection and will exit the study. Those with a titer of ≤10 IU/L will be invited to return for a second visit for an HB challenge immunization. Participants will be asked to return 28 days post-vaccination for another blood test. Those who developed a booster response will be informed that they are immune and will exit the study. Those who do not boost will be invited to return for an additional (remedial) HB vaccination with a final titer check available to confirm restoration of protection. Tests will be carried out at BC Center for Disease Control's Hepatitis Laboratory. Booster responses are expected to be strong (titers >100 IU/L). Samples with post-booster titers 10-99 IU/L may be retested to assess antibody avidity to distinguish between primary and secondary type responses.

Data will be assembled by the study center team. Electronic case report forms will capture case and laboratory information, using a commercial platform.

Statistical Analysis Plan The baseline blood test will identify participants with residual anti-HBs titers above or below the minimum protective threshold of 10 IU/L. Based on the meta-analysis of studies involving 10 or 15 years of follow-up after infant immunization, the investigators project that about 30% of younger and 60% of older participants will have titers <10 IU/L. The booster challenge will identify those who are able to mount an anamnestic/recall response (titer ≥100 IU/L), a possible primary response (titer 10-99 IU/L, low avidity) or no response (titers <10 IU/L). The investigators project that 25-30% of older booster recipients will not have an anamnestic response, representing 15-20% overall of that study population. To detect a susceptibility rate of 15% with a precision of ± 0.05 and 95% confidence intervals, it will be necessary to screen 220 individuals, allowing for a 10% drop-out rate. Among younger booster recipients, the investigators project that 10% will not have an anamnestic response, representing an overall susceptibility rate of ~3%. For a precision ± 0.03 and 95% confidence intervals, the number of younger subjects to be screened will be 140, allowing for a 10% drop-out rate.

Multivariable analysis will be undertaken to identify factors associated with loss of detectable protection, such as age at final dose, interval since vaccination, gender etc. The apparent rate of decline in titers between 10 and 16 years of age will be calculated.

Mathematical modelling and economic analysis of immunization program options will be included to aid subsequent decision-making by health authorities. Among the options to be modelled are no intervention (relying on herd immunity of adjacent age groups) and a single booster dose at age 10-11years (grade 6 in BC, the locus of the cancelled adolescent HB vaccination program).