LUM ET AL. RESPOND

Abstract

Diamond correctly points out the difficulty of making comparisons between the presence of hepatitis B surface antibodies (anti-HBs) among persons reporting prior immunizations (Seattle Young Men’s Survey [YMS]) and anti-HBs seroconversion measured after a documented immunization series (the UFO Study). Recall bias and waning anti-HBs could certainly have contributed to the lower proportion (70%) of subjects with isolated anti-HBs that YMS investigators observed.1 In the UFO Study, 62% of young injection drug users (IDUs) who reported 3 prior immunizations demonstrated isolated anti-HBs. We reported anti-HBs seroconversion in 78% of young IDUs after 3 doses of a monovalent recombinant hepatitis B virus (HBV) vaccine.2 Physiological factors that may blunt the immune response include lifestyle (e.g., smoking and obesity), biological factors (e.g., male sex and older age), and the coexistence of other immune-mediated diseases.3 Among homosexual and bisexual men, HIV infection has been associated with poorer HBV vaccine responses.4 The role of hepatitis C virus (HCV) antibody status in suboptimal HBV vaccine responses is still controversial. Some studies have reported an association,5,6 while others have found none.7,8 Our observation of a lower vaccine response among anti-HCV–positive subjects occurred in a subset too small for statistical significance. However, we plan to investigate this issue further in a new trial of hepatitis vaccine adherence among young IDUs in San Francisco. We agree with Diamond that prevaccination screening of high-prevalence populations for prior infection or immunity makes sense. At the San Francisco STD Clinic, screening was cost saving when seroprevalence was greater than 22% (J. D. Klausner, MD, MPH, STD Prevention and Control Services, San Francisco Department of Public Health; written communication; September 17, 2003). Because many young IDUs and young men who have sex with men are lost to follow-up and cannot afford to pay for either blood tests or vaccines, public health providers must often decide whether to assume these costs. Some providers have chosen a catch-as-catch-can approach, administering the first vaccine dose on the day of prevaccination screening. Finally, while understanding the factors that impair vaccine responses is important, 78% effectiveness is not all that bad. Future HIV vaccine candidates may be licensed with much lower partial efficacy.9 Strategies to improve vaccine series completion among high-risk adolescents and adults arguably present the greater challenge for the prevention of HBV transmission in the United States. Those strategies are likely to be useful not just for the delivery of HBV vaccines but also for the delivery of promising HIV vaccine candidates in the future.