Development of immunity following financial incentives for hepatitis B vaccination among people who inject drugs: A randomized controlled trial.
BACKGROUND
People who inject drugs (PWID) are at risk of hepatitis B virus (HBV) but have low rates of vaccination completion. The provision of modest financial incentives increases vaccination schedule completion, but their association with serological protection has yet to be determined.


OBJECTIVE
To investigate factors associated with vaccine-induced immunity among a sample of PWID randomly allocated to receive AUD$30 cash following receipt of doses two and three ('incentive condition') or standard care ('control condition') using an accelerated 3-dose (0,7,21 days) HBV vaccination schedule.


STUDY DESIGN
A randomised controlled trial among PWID attending two inner-city health services and a field site in Sydney, Australia, assessing vaccine-induced immunity measured by hepatitis B surface antibodies (HBsAb ≥ 10 mIU/ml) at 12 weeks. The cost of the financial incentives and the provision of the vaccine program are also reported.


RESULTS
Just over three-quarters of participants - 107/139 (77%)--completed the vaccination schedule and 79/139 (57%) were HBsAb ≥ 10 mIU/ml at 12 weeks. Vaccine series completion was the only variable significantly associated with vaccine-induced immunity in univariate analysis (62% vs 41%, p<0.035) but was not significant in multivariate analysis. There was no statistically discernible association between group allocation and series completion (62% vs 53%). The mean costs were AUD$150.5, (95% confidence interval [CI]: 142.7-158.3) and AUD$76.9 (95% CI: 72.6-81.3) for the intervention and control groups respectively.


CONCLUSION
Despite increasing HBV vaccination completion, provision of financial incentives was not associated with enhanced serological protection. Further research into factors which affect response rates and the optimal vaccination regimen and incentive schemes for this population are needed.