When hepatitis B vaccine first became commercially available, it was extremely expensive. Clinical trials on the immunogenicity of reduced doses of the vaccine were therefore conducted in several ocentres. These studies showed that the dose of vaccines from some manufacturers could be reduced without affecting the vaccine-induced antibody response in Anewboms, infants and adults [1-7]. A reduced dose of 5 [mu]g of Merck, Sharp & Dohme (MSD) plasma-based vaccine is as immunogenic and efficacious as the standard dose of 10 [mu]g in preventing perinatal transmission of hepatitis B virus (HBV) . In healthy adults immunized with the MSD plasma-based vaccine, no significant difference in the immune response was observed between groups receiving doses of 40 pg, 20 pg, and 10 pg, and there was 92% seroconversion with the 5-[mu]g dose [9-12]. Similarly, clinical trials with MSD yeast-derived HBV vaccine demonstrated that, for pre-exposure prophylaxis in children aged 1-12 years, the immunogenicity of lower doses (0.6 [mu]g, 1.25 [mu]g, and 2.5 [mu]g was as good as that of the recommended dose (5 [mu]g) . The immunogenicity of reduced doses was also demonstrated for teenagers (2.5 [mu]g) [14-15] and adults  (5.0 [mu]g). These findings are of practical importance, especially in developing countries where hepatitis B virus infection is endemic, since the use of a lower dose of vaccine without compromising its immunogenicity and efficacy would reduce considerably the cost of immunization programmes.
In the study, we followed up for 5-6 years two cohorts of seronegative adults who were immunized with a reduced dose of 10 [mu]g of plasma-based HBV vaccine. To the best of our knowledge this is the first long-term follow-up of vaccinees administered such a dose.
Materials and methods
The population groups followed up consisted of preclinical medical and dental students and national service recruits. The Singapore Expert Committee on the Immunisation Programme has recommended that these groups should be protected against viral hepatitis B on a voluntary basis .
The purpose of die study was carefully explained to the volunteers and their informed consent was obtained. Prior to vaccination, 5 ml of venous blood was collected from each subject, using disposable needles and syringes, for analyses of various HBV markers. Blood collection was carried out at the Student Health Service Clinic, National University of Singapore, for the students, and at the medical centre of one of the army camps, for the national service recruits.
The blood samples were immediately dispatched to the Hepatitis and Liver Cancer Research Unit, Department of Clinical Research, Singapore General Hospital. The sera were separated, transferred to labelled polypropylene tubes, and stored at -70 [degrees]C before being analysed in batches. The samples were tested for hepatitis B surface antigen (HBsAg), antibody to hepatitis B core antigen (anti-HBc), and anti-body to hepatitis b surface antigen (anti-HBs) using commercially available enzyme immunoassay kits (AUZYME II, CORZYMEE and AUSAB, resp. [sup.a].
Subjects who were negative for HBsAg, anti-HBc and anti-HBs and who had no previous history of hepatitis B vaccination were offered three 10-[mu]g doses of MSD plasma-based HBV vaccine, administered intramuscularly in the deltoid region, at the start of the study and I month and 6 months later. Blood samples were obtained 1, 2, 3, and 5 years after completion of the immunization schedule from the students and after 1, 2, 4, and 6 years from the national service recruits. Since it would not have been ethical to obtain periodic blood samples from those who refused immunization, we tested the sera of unvaccinated national service recruits for HBV markers only once at the end of the 6-year follow-up period. Sera were tested for HbsAg, anti-HBc and anti-HBs. The reciprocal anti-HBs, titres were expressed in mIU/ml, based on a WHO reference standard. …