Long-term immunogenicity and efficacy of a reduced dose of plasma-based hepatitis B vaccine in young adults.

Long-term immunogenicity and efficacy of a reduced dose of plasma-based hepatitis B vaccine in young adults K.T. Goh,1 C.J. Oon,2 B.H. Heng,3 & G.K. Lim4 A cohort of seronegative preclinical medical and dental students and another cohort of seronegative national service recruits who were immunized intramuscularly with a reduced dose (10 pg) of plasma- based hepatitis B vaccine (Merck, Sharp & Dohme) at the start of the study and at 1 month and 6 months thereafter were followed up for 5 years and 6 years, respectively. Among the medical and dental students, antibody to hepatitis B surface antigen (anti-HBs) (> 10 mIUlml) was detected in 81% of the vaccinees at the end of the 5-year follow-up and the geometric mean titre (GMT) had dropped from 412.6 mIU/ml one year after completion of vaccination to 174.9 mIUlml after 5 years. Antibody to hepa- titis B core antigen (anti-HBc) was detected in 0.4-1.0% of the vaccinees but none was positive for hepatitis B surface antigen (HBsAg) during the follow-up period. Among the national servicemen, the anti-HBs seroconversion rate and GMT were considerably lower than those of the preclinical medical and dental students. At the end of the 6-year follow-up, 55% of the vaccinees were positive for anti-HBs (> 10 mlU/ml) and the GMT had dropped from 80.7 mIUlml one year after completion of vaccination to 30.4 mlU/ml after 6 years. Anti-HBc was detected in 8 (2.7%) and transient HBs antigenaemia in 2 (0.7%) of 293 vaccinees after 4 years. However, none of the 196 vaccinees followed up at 6 years was HBsAg-positive compared with 8 (4.2%) of 191 sero- negative national service recruits who were not vaccinated, giving a vaccine efficacy of 100% in the prevention of the chronic HBsAg carrier state. Introduction When hepatitis B vaccine first became commercially available, it was extremely expensive. Clinical trials on the immunogenicity of reduced doses of the vac- cine were therefore conducted in several centres. These studies showed that the dose of vaccines from some manufacturers could be reduced without affec- ting the vaccine-induced antibody response in new- boms, infants and adults (1-7). A reduced dose of 5 pg of Merck, Sharp & Dohme (MSD) plasma-based vaccine is as immunogenic and efficacious as the standard dose of 10 pg in preventing perinatal trans- ' Head, Quarantine and Epidemiology Department, Ministry of the Environment, Environment Building 22-00, 40 Scotts Road, Singapore 0922. Requests for reprints should be sent to Dr Goh at this address. 2 Principal Investigator, Hepatitis and Liver Cancer Research Unit, Department of Clinical Research, Singapore General Hos- pital, Singapore. 3 Senior Registrar, Quarantine and Epidemiology Department, Ministry of the Environment, Singapore. 4 Laboratory Technologist, Department of Clinical Research, Singapore General Hospital, Singapore. Reprint No. 5628 mission of hepatitis B virus (HBV) (8). In healthy adults immunized with the MSD plasma-based vaccine, no significant difference in the immune res- ponse was observed between groups receiving doses of 40 pg, 20 pg, and 10 pg, and there was 92% sero- conversion with the 5-pg dose (9-12). Similarly, cli- nical 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 pg, 1.25 pg, and 2.5 pg) was as good as that of the recommended dose (5 pg) (13). The immunogenicity of reduced doses was also demons- trated for teenagers (2.5 pg) (14-15) and adults (16) (5.0 pg). 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 immuno- genicity 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 pg of MSD plasma-based HBV vaccine. To the best of our knowledge this is the first long-term follow-up of vaccinees adminis- tered such a dose. Bulletin of the World Health Organization, 1995, 73 (4): 523-527 © World Health Organization 1995 523 K.T. Goh et al. Materials and methods The population groups followed up consisted of pre- clinical medical and dental students and national ser- vice recruits. The Singapore Expert Committee on the Immunisation Programme has recommended that these groups should be protected against viral hepati- tis B on a voluntary basis (17). The purpose of the study was carefully explained to the volunteers and their informed con- sent 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 °C before being analysed in batches. The samples were tested for hepatitis B surface antigen (HBsAg), anti- body 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, CORZYME and AUSAB, resp.).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-pg doses of MSD plasma-based HBV vaccine, adminis- tered intramuscularly in the deltoid region, at the start of the study and 1 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.b Seroconversion was defined as an increase in anti-HBs titre to .2.1 mIU/ml. The difference in HBV markers between vaccinated and unvaccinated groups was examined using Fisher's exact test; a a Abbott Laboratories, North Chicago, IL, USA. b Supplied by: International Laboratory for Biological Standards, Central Laboratory of the Netherlands Red Cross Blood Transfu- sion Service, Amsterdam, Netherlands. P-value of <0.05 was considered to be statistically significant. The efficacy of the vaccine (VE) in pre- venting HBV infection at the end of the follow-up period was calculated using the following equation: VE(%)=U-V x100 U where U = % of unvaccinated individuals posi- tive for HBV markers and V = % of vaccinees positive for HBV mark- ers. Results A total of 240 seronegative preclinical medical and dental students aged 19-21 years and 293 seronega- tive national service recruits aged 18-21 years were immunized and followed up for 5 years and 6 years, respectively. At the end of the 6-year follow-up period, 191 unvaccinated national service recruits were also tested for various HBV markers. All the vaccinees remained well and no clinical hepatitis was reported during the follow-up period. Preclinical medical and dental students One year after completion of vaccination, 98.3% of the students had seroconverted, 93.6% having anti- HBs titres 210mIU/ml and 32.3% 21000mIU/ml. The geometric mean titre (GMT) dropped from 412.6 mIU/ml after 1 year to 133.6 mIU/ml after 3 years, and then increased again to 174.9 mIU/ml after 5 years. At the end of the follow-up period, more than 80% of the students continued to have protective levels of anti-HBs (.10 mIU/ml) with about 60% having levels >100 mIU/ml (Table 1). Anti-HBc, a marker of HBV infection, was detected Table 1: Anti-HBs titres of medical and dental students immunized with three 10-pg doses of MSD plasma- based hepatitis B vaccine and followed up for 5 yearsa Anti-HBs % at post-vaccination follow-up after: titre 1 year 2 years 3 years 5 years (mlU/ml) (n= 240) (n= 219) (n= 191) (n= 100) 0 2.2 5.9 2.6 7 <10 4.2 9.6 11.0 12 10 to <100 20.0 33.8 28.8 22 100 to <1000 41.3 31.1 42.4 40 .1000 32.3 19.6 15.2 19 Geometric 412.6 145.8 133.6 174.9 mean titre a Each vaccinee received 1 dose of vaccine at the beginning of the study and 1 month and 6 months later. 524 WHO Bulletin OMS. Vol 73 1995 Long-term immunogenicity of reduced-dose hepatitis B vaccine in 0.4-1.0% of the vaccinees, but none was positive for HBsAg (Table 2). National service recruits For the national service recruits the seroconversion rate 1 year after completion of vaccination was 70.4%; after 2 years, 83.1%; after 4 years, 79.6%; and after 6 years, 76.3%. The GMT dropped from 80.7 mIU/ml after 1 year to 30.4 mIU/ml after 6 years. More than half (55%) of the vaccinees con- tinued to have anti-HBs levels .10 mIU/ml at the end of the follow-up period, with 22.5% having levels .100 mIU/ml (Table 3). The prevalence of anti-HBc increased from 0.9% after 1 year to 2.7% after 4 years and 2.0% after 6 years. HBsAg was weakly positive in two vaccinees after 4 years, but disappeared completely on repeat testing of subsequent samples. When a cohort of 191 unvaccinated seronegative national service recruits from the same army camp was tested for HBV markers at the 6-year follow-up, 8 (4.2%) were HBsAg-positive, 40 (20.9%) were anti-HBc-positive, and 57 (29.8%) were anti-HBs- positive (Table 4). The difference in the prevalence of HBsAg and anti-HBc between the vaccinated and unvaccinated national servicemen at the 6-year follow-up was statistically significant (P <0.01 and P <10-8, resp.), and the vaccine efficacy in preventing the HBsAg carrier state and HBV infection (anti- HBc positivity) in vaccinees was 100% and 90.4%, respectively. Discussion The study was based on the follow-up of two cohorts of seronegative young adults immunized in a normal setting rather than under strict clinical trial condi- tions. The dose of vaccine administered to young national servicemen was highly effective in preven- Table 2: Hepatitis B virus markers in medical and den- tal students Immunized with three 10-pg doses of MSD plasma-based hepatitis B vaccine and followed up for 5 yearsa % +ve at post-vaccination follow-up after: 1 year 2 years 3 years 5 years Markerb (n = 240) (n = 219) (n = 191) (n = 100) HBsAg 0 0 0 0 Anti-HBc 0.4 0.5 0.5 1.0 Anti-HBs 98.3 94.1 97.4 93.0 a See footnote a, Table 1. b HBsAg = hepatitis B surface antigen; anti-HBc = antibody to hepatitis B core antigen; anti-HBs = antibody to hepatitis B sur- face antigen. Table 3: Anti-HBs titres of national servicemen immu- nized with three 10-pg doses of MSD plasma-based hepatitis B vaccine and followed up for 6 yearsa Anti-HBs % +ve at post-vaccination follow-up after: titre 1 year 2 years 4 years 6 years (mlU/ml) (n = 226) (n = 278) (n = 284) (n = 190) 0 29.6 16.9 20.4 23.6 <10 9.0 18.3 21.5 21.5 10 to <100 30.5 35.3 29.9 32.5 100 to <1000 26.9 23.7 23.2 20.9 .1000 4.0 5.8 4.9 1.6 Geometric 80.7 55.2 39.1 30.4 mean titre a See footnote a, Table 1. ting HBV infection (as indicated by anti-HBc positi- vity) and the HBsAg carrier state. Although there was no control group to assess the efficacy of the vaccine among the medical and dental students, the protection appeared to be equally good. In Singapore dental surgeons have a significantly higher prevalen- ce of HBsAg and anti-HBc (11.4% and 45.6%, resp.) than that of the general population (4.2% and 29.7%, resp.) (18). The immune response of the medical and dental students was better than that of the national service- men, with their GMT being 3-5 times higher after 1-2 years' follow-up. This difference could be due to variations in immunogenicity between individual Table 4: Prevalence of hepatitis B virus markers among immunized and non-immunized national servicemen followed up for 6 years Prevalence among: Follow-up: Markera Vaccinated Unvaccinated 1 year HBsAg 0/228b (O)C Anti-HBc 2/226 (0.9) Not tested Anti-HBs 159/226 (70.4) 2 years HBsAg 0/284 (0) Anti-HBc 4/284 (1.4) Not tested Anti-HBs 231/278 (83.1) 4 years HBsAg 2/293d (0.7) Anti-HBc 8/293 (2.7) Not tested Anti-HBs 226/284 (79.6) 6 years HBsAg 0/196 (0) 8/191 (4.2) Anti-HBc 4/196 (2.0) 40/191 (20.9) Anti-HBs 145/190 (76.3) 57/191 (29.8) a See footnote b, Table 2. b No. positive/No. tested. c Figures in parentheses are percentages. d Weakly positive; negative on repeat testing of a subsequent sample. WHO Bulletin OMS. Vol 73 1995 525 K.T. Goh et al. vaccine lots (19), or possibly to loss of potency, especially with a reduced dose, if the vaccine was not properly handled or was stored under suboptimal conditions. The higher immunogenicity of the vac- cine could have accounted for the lower prevalence of anti-HBc (0.4-1.0%) and complete absence of HBsAg among the medical and dental students, com- pared with the national servicemen (0.9-2.7% anti- HBc positivity and 0.7% transient HBs antigen- aemia). A total of 45.6% of the national servicemen and 19% of the medical and dental students had no detectable anti-HBs or exhibited titres that were below the level considered to be protective (.10 mIU/ml) (20) 5-6 years after vaccination. The relation between persistence of anti-HBs and dura- tion of protection against HBV infection is still unclear. Low or undetectable levels of circulating anti-HBs may not necessarily indicate loss of protec- tion. In high-risk adults, protection persists even when humoral antibody is no longer detectable (20). Moreover, when a booster dose was administered to healthy adults with undetectable anti-HBs 5-7 years after vaccination, an anamnestic response was elicit- ed (21), implying that immunological memory per- sists. Thus, once an immune response has been induced by vaccination, it can be stimulated by expo- sure to the wild virus, with an active increase in anti- HBs during the early phase of the incubation period of the disease, thereby protecting against clinical ill- ness or development of the carrier state. In an endemic setting repeated exposures to hepatitis B carriers could sustain or even boost the anti-HBs response without any serological evidence of infec- tion (22). In Singapore, a reduced dose (5 pg) of MSD plasma-based HBV vaccine is as effective as the standard child's dose (10 jig) in preventing the HBsAg carrier state in children up to 6 years of age (23). The present study also demonstrated the long- term efficacy of a reduced adult dose of MSD plasma-based vaccine (10 pg) in preventing the HBV carrier state in healthy young adults for at least 6 years after vaccination. The duration of protection conferred by this dose was no different from that of the standard 20-jig dose (24). Based on the results of local studies, reduced vaccine doses have been recommended for immunization of children and young adults in Singapore (17). In a recent national serological survey on vaccine-prevent4ble diseases involving 878 healthy children and adults aged from 6 months to more than 45 years, HBsAg and anti-HBc were detected in 17 (5.6%) and 105 (34.8%), respectively, of 303 unvaccinated persons and in 2 (0.3%) and 41 (7.1%), respectively, of 575 vaccina- ted persons. However, both the HBsAg-positive vaccinees were over 45 years of age and not serologi- cally tested prior to vaccination. This confirmed the adequacy of the reduced dose administered in a norm- al clinical setting in the prevention of the HBsAg carrier state in both children and adults in Singapore (25). Acknowledgements We thank Colonel (Dr) Lim Meng Kin, Chief, Medical Corps, Ministry of Defence, Singapore; Dr Eileen Aw, Senior Health Physician, Student Health Service, National University of Singapore; and the medical and nursing staff of the Quarantine and Epidemiology Department, Ministry of the Environment, for their assistance and cooperation. Resume Immunog6nicit6 et efficacit6 A long terme d'une dose r6duite de vaccin antih6patite B d6rive de plasma chez le jeune adulte Pour d6terminer l'immunog6nicite et l'efficacit6 d'une dose reduite (10 pg) de vaccin antih6patite B (Merck Sharp & Dohme) administree a de jeunes adultes seron6gatifs au debut de l'6tude puis au bout de 1 et 6 mois, nous avons suivi pendant 5 ans une cohorte d'6tudiants en m6decine et en art dentaire vaccines et pendant 6 ans une cohorte de recrues du service national non vaccin6es et vaccin6es. Parmi les etudiants, 81% avaient toujours des anticorps dirig6s contre l'antigene de surface de l'hepatite B (anti-HBs) a un titre .10 mUl/mi au bout de 5 ans, a la fin de la p6riode de suivi. Le titre moyen g6om6trique 6tait pass6 de 412,6 mUl/ml au bout d'un an a 174,9 mUl/mi au bout de 5 ans. Bien que 0,4 a 1,0% des sujets aient ete infectes, comme l'indique la pr6sence d'anticorps diriges contre l'antigene central de l'h6patite B (anti-HBc), aucun d'entre eux n'a 6t6 positif pour l'antigene de surface de l'hepatite B (HBsAg) pendant la p6riode de suivi. Dans le cas des recrues, les sujets vaccin6s ont ete revus au bout de 1, 2, 4 et 6 ans et les sujets non vaccin6s seulement au bout de 6 ans. Les taux de seroconversion anti-HBs et les titres moyens g6om6triques des sujets vaccines etaient beaucoup plus faibles que chez les 6tudiants, avec un taux de s6roconversion anti-HBs de 55% (.10 mUl/ml) et un titre moyen g6om6trique de 30,4 mUl/ml a la fin de la p6riode de suivi. Une antig6n6mie HBsAg transitoire a 6t6 d6tect6e chez 2 (0,7%) des 293 vaccin6s au bout de 4 ans. WHO Bulletin OMS. Vol 73 1995526 Long-term immunogenicity of reduced-dose hepatitis B vaccine Au bout de 6 ans, I'HBsAg etait absent chez 1'ensemble des 196 vaccines revus, mais a ete detect6 chez 8 (4,2%) des 191 recrues non vacci- nees. Les taux correspondants pour I'anti-HBc etaient respectivement de 2,0% et 20,9%. Ces r6sultats confirment 1'efficacite a long terme du vaccin antih6patite B dans la pr6vention du porta- ge chronique de l'HBsAg chez le jeune adulte, meme lorsqu'une dose reduite est administree. References 1. Yvonnet B et al. Low-dose hepatitis B vaccine immunization in children. Lancet, 1987,1:1:69. 2. Matsaniotis N et al. Immunogenicity of low doses of hepatitis B vaccine in normal children. Vaccine, 1985, 3: 297-299. 3. Milne A et al. Low dose hepatitis B vaccination in children. New Zealand journal of medicine, 1988, 99: 47-49. 4. Lelie PN et al. 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