EPIDEMIOLOGY AND CONTROL OF HEPATITIS B VIRUS INFECTION IN SINGAPORE

Abstract

The proiect was undertaken with the objectives to define the epidemiology of hepatitis B virus (HBV) infection in Singapore to formulate strategies for the prevention and control of hepatitis B by vaccination and to evaluate the efficacy of the hepatitis B vaccination programme. Epidemiological investigations into each notified case of acute viral hepatitis, seropidemiological surveys on the prevalence of HBV markers in various population groups and specific studies on the modes of transmission were carried out. Clinical trials with Merck plasma-based and yeast-derived hepatitis B vaccine were conducted to determine the optimal schedule and dosage for the hepatitis B vaccination programme and vaccines followed up to monitor the duration of immunity. Surveilance data showed that hepatitis B accounted for up to 74% of the reported acute viral hepatitis cases. The mean annual incidence rate of acute hepatitis B was 8.2 per 100,000 population and the case fatality 2.2%. There was a male predominance with high mortality rate of in young adults between 25 and 34 years of age. The mortality rate of Chinese and of Indians was significantly higher than that of Malay 11.2% of the reported cases were imported. Transmission was significantly associated with exposure to various parenteral procedures and outbreak had been traced to contaminated tattoo needles and syringes. However, more than 70% of the infection were acquired non parenterally from asymptomatic carriers. 10% of the acute cases became carriers. The stereo prevalence of HBV infection of the genetic population increased from 9.3% in children below 7 years of age to 34.6% in adults above 55 years. There was a marked increase during adolescence. The overall hepatitis B surface antigen (HBsAg) prevalence among males and females was 3.7% and 3.4% respectively and Chinese had the highest prevalence followed by Malays and Indians. There were an estimated 120,000 carriers with 1.9 million susceptible to HBV infection. Children born to carrier mothers and family contacts if chronic carriers had the highest HBsAg prevalence. The primary mode of transmission during the first year of life was perinatal with 43% of the babies born to carrier mothers developing the carrier state. Within the infected household horizontal transmission occurred through intimate contact and by sharing various personal and household articles. The main determinant in HBV transmission was the presence of hepatitis B e antigen 1 (HBsAg). Mosquitoes are not considered to be of epidemiological importance in the spread of infection. Immunoprophylaxix with a reduced dose (5 ug) of Merck plasma-based vaccine given at birth together with hepatitis B immunoglobulin (HBIG) one month and two months was as immunogenic and efficacious as the recommended dose (10 ug) in the prevention of perinatal transmission in babies born to HBeAg-carrier mothers. The overall vaccine efficacy was 80% but if intrauterine infection was excluded it increased to 88%. Even without HBIG, protection was 100% in babies of HBeAg-negative carrier mothers. Reduced doses of Merck yeast-derived vaccine were as immunogenic as the recommended dose in seronegative children. Based on the results of seroepidemiological surveys and clinical trials, a national hepatitis B immunization programme was formulated and successfully implemented in phases, beginning with babies born to carrier mothers in October 1985 and then extending to include all newborns in September 1987. The percentage of children who had completed the full course of immunization below one year of age increased from 49% in 1988. To 71% in 1989. Follow-up studies up to four years for babies immunisation with the reduced dose of Merck yeast-derived vaccine of immunological memory with methods detected is 87% - 100% of the vaccines. Its efficacy in the prevention of carriers state is maintained. A booster dose administered four years after the former course resulted in 16 to / 19 fold increase in geometric meantime while all the non-respondents stereotyped when to immunized with another kind of vaccine. Although they induced antibody at low level has not prevent HBV infection in children and adult would occur. If continued to prevent clinical hepatitis B and HBsAg carrier state up to 4 years follow-up. Prevention and control measures implemented so far not resulted in reduction to the overall incidence of acute hepatitis B but the incidence of transfusion and other parenterally recorded hepatitis has dropped significantly. There are also indications that the prevalence of HBsAg in various population groups has been declining especially high-risk children vaccinated against hepatitis B at birth. Recommendations for further epidemiological surveillance and research in the prevention and control of hepatitis B are made.