Our study will expand on Cardell's by looking specifically at those who have failed two complete series of vaccinations (6 or more doses) who would be otherwise labelled as non-responders according to the CDC criteria. Our hypothesis is that the combined hepatitis A and B vaccine (Twinrix®) given at double-dose as in Cardell's study will induce protective immunity in these non-responders at a rate much higher than the traditionally quoted 10% response rate.