The average estimated carrier rate of hepatitis B virus (HBV) in India is 4%, with a total pool of approximately 36 million carriers. Wide variations in social, economic, and health factors in different regions may explain variations in carrier rates from one part of the country to another. Professional blood donors constitute the major high risk group for HBV infection in India, with a hepatitis B surface antigen positivity rate of 14%. Blood transfusions represent the most important route of HBV transmission among adults. However, most of India's carrier pool is established in early childhood, predominantly by horizontal spread due to crowded living conditions and poor hygiene. Acute and subacute liver failure are common complications of viral hepatitis in India and HBV is reckoned to be the aetiological agent in 42% and 45% of adult cases, respectively. HBV is reported to be responsible for 70% of cases of chronic hepatitis and 80% of cases of cirrhosis of the liver. About 60% of patients with hepatocellular carcinoma are HBV marker positive. Small numbers of patients have been reported to be infected with the pre-core mutant virus but none with the S mutant. Coinfection with hepatitis C virus or hepatitis delta virus is comparatively uncommon. In conclusion, hepatitis B is a major public health problem in India and will continue to be until appropriate nationwide vaccination programmes and other control measures are established.

There are approximately 50 million chronic carriers of hepatitis B virus (HBV) in Africa, with a 25% mortality risk. In sub-Saharan Africa, carrier rates range from 9-20%. Many studies have suggested that HBV transmission in Africa occurs predominantly in childhood, by the horizontal rather than the perinatal route. The exact mode of transmission is uncertain but probably involves percutaneous infection through saliva or traces of blood, as well through unsterile needles, tribal scarification, and other possible vehicles. Compared with adult HBsAg carriers in the Far East, those in Africa have a low rate of HBeAg positivity, which may account for the relatively low rates of perinatal infection. It is also possible that African infants are less susceptible to perinatal HBV infection compared with their Asian counterparts. Alternatively, it may be that African infants are indeed infected with HBV at birth but, for genetically determined reasons, have persistently negative tests for a number of years until the virus is reactivated. In view of the high HBV carrier rates in the general population, universal immunisation of all infants is recommended. Ways of incorporating the hepatitis B vaccine into the Expanded Programme on Immunisation in each country are being evaluated.

About 100,000 cases of acute hepatitis B virus (HBV) infection occur annually in South America. The overall prevalence of HBV infection in low risk populations ranges from 6.7% to 41%, while hepatitis B surface antigen (HBsAg) rates range from 0.4% to 13%. In high endemicity aboriginal or rural populations, perinatal transmission may play a major part in the spread of HBV. In urban populations, however, horizontal transmission, probably by sexual contact, is the predominant mode of spread, with higher rates of HBV positivity in lower socioeconomic groups. High risk populations such as health care workers and haemodialysis patients show higher rates of HBV infection than comparable populations elsewhere. The risk of posttransfusion hepatitis B remains high in some areas. Concomitant HBV infection may accelerate the chronic liver disease seen in decompensated hepatosplenic schistosomiasis. In the north, the prevalence of hepatitis delta virus (HDV) infection ranks among the highest in the world. In the south, the problem appears negligible although it is increasing within high risk urban communities. HDV superinfection has been the cause of large outbreaks of fulminant hepatitis. The cost of comprehensive or mass vaccination programmes remains unaffordable for most South American countries. Less expensive alternatives such as low dose intradermal schedules of immunisation have been used with success in selected adult subjects.

The prevalence of chronic hepatitis B virus (HBV) infection in the Philippines, as indicated by hepatitis B surface antigen (HBsAg) positivity, ranges from 2% to 16.5%, with an average of 12% in a study of rural villagers. Although mother to child transmission is a major route of HBV infection, other routes (particularly child to child transmission) play an important part after the first year of life. In a study assessing the feasibility and effectiveness of incorporating hepatitis B vaccine into the national Expanded Programme on Immunisation, the coverage rate for fully immunised 1 year olds ranged from 80.9-84% and anti-HBs seroconversion rates ranged from 72-88%. In countries where HBV is not endemic, high risk groups include commercial sex workers (CSWs) and intravenous drug users (IVDUs), who generally have higher HBsAg positivity rates than the general population. In countries with a high HBV endemicity, carrier rates may be only slightly higher among CSWs, suggesting that other modes of transmission are more important in those regions. CSWs who are also IVDUs are at even greater risk. If HBV infection is to be controlled, innovative education and screening programmes are needed, together with the mass immunisation of neonates now started in many countries around the world.

China has one of the highest rates of hepatitis B virus (HBV) endemicity in the world. In a survey of five provinces, the overall HBV infection rate in the general population was found to be 42.6%, with 10.3% testing positive for hepatitis B surface antigen (HBsAg). Higher rates were found in rural than in urban areas. The prevalence of HBsAg among children under 1 year of age is quite low but increases rapidly thereafter, reaching a peak among 5 to 9 year olds. The pattern of age distribution suggests that horizontal transmission is an important route of HBV infection during early childhood, and the proportion of chronic HBsAg carriage attributable to perinatal transmission has been estimated at only 13-20%. Contact with infected family members probably accounts for much of the horizontal transmission in children. In a nationwide survey, 27.2% of families were found to have one or more HBsAg positive members and a strong tendency for family clustering has been identified. The strategy for prevention of HBV infection includes vaccination of all newborns, whether their mothers are HBsAg positive or negative, together with vaccination of high risk populations, and improved control measures in clinics and blood transfusion centres.

Hepatitis B remains one of the most important infectious diseases in China. In 1980, an overall hepatitis B virus (HBV) infection rate of 42.6% was reported and a hepatitis B surface antigen (HBsAg) carrier rate of 10.3%. HBsAg positivity among children under 1 year of age ranged from 5.1% in Beijing to 7% in Guangdong. A peak in carrier rate was observed in 7 to 14 year olds, reaching 24% in Guangdong. During the past decade, there has been no significant change in overall HBV carrier rates. However, in areas where hepatitis B vaccination for all neonates has been introduced, a decline in HBsAg positivity in lower age groups has been observed. Perinatal transmission is believed to account for 35-50% of carriers although horizontal transmission is also important, particularly within families. Infants born to HBeAg positive carrier mothers are at even greater risk of infection. HBV infection during childhood leads to an increased risk of serious longterm sequelae, including hepatocellular carcinoma (HCC). It is hoped that universal childhood immunisation will allow control of HBV infections in China within a few generations.

The carrier rate of hepatitis B virus (HBV) in black Africans averages 10.4% throughout the continent. Even within each country, however, there may be wide variations. HBV carriage in black Africans is largely established in early childhood, mostly through horizontal transmission. Perinatal transmission also occurs but to a much lesser extent than in the Far East. For unknown reasons, HBeAg positivity rates are much lower in black Africans of childbearing age than in women in the Far East. Universal HBV vaccination of infants in South Africa started in April 1995. Effective integration into the Expanded Programme on Vaccination will probably be necessary if administration of second and third doses is to be ensured. Fortunately, a natural process of urbanisation is also having a beneficial effect on the black carrier rate in South Africa, as urban carrier rates tend to be much lower than those in rural area. Within the next 20 years, therefore, there should be a great reduction in the numbers of acute HBV infections and new carriers. However, it will take much longer before there is any substantial impact on the burden of the longterm sequelae of HBV infection--that is, cirrhosis and hepatocellular carcinoma.

In the United States, the reported rate of hepatitis B has declined by over 50% since 1987, probably as a result of vaccination programmes, behavioural changes, refinements in blood screening procedures, and the availability of virus inactivated blood components. The majority of new hepatitis B infections occur in 20-39 year olds, and perinatal transmission is uncommon except in certain at risk groups. Initial efforts to control hepatitis B in the US were targeted at high risk groups, including health care personnel. Then, in 1988, the Centers for Disease Control and Prevention (CDC) recommended screening of all pregnant females for hepatitis B surface antigen and full immunisation of infants born to those testing positive. A recommendation for universal immunisation of infants was endorsed in 1991. Compliance has been slow but progressive. The CDC also has recommended 'catch up' immunisation of adolescents and high risk children and adults. Demonstration projects suggest that these can be successful, given the provision of free or low cost vaccine and appropriate support. Hepatitis B vaccination has been shown to be cost effective and should be integrated into the routine childhood immunisation schedule. Responses to hepatitis B vaccine have largely been shown to be durable, although at least one booster dose after five to 10 years seems prudent, especially if a low dose, yeast derived vaccine has been used.

Postnatal horizontal transmission of hepatitis B virus (HBV) in early childhood seems to be the predominant method by which high hepatitis B carrier rates in the Middle East are maintained. The prevalence of hepatitis B surface antigen (HBsAg) positive status among siblings of HBV carriers is similar during childhood and adulthood, suggesting that childhood intrafamilial transmission patterns persist into adult life. There is a tendency for asymptomatic HBV carriers to have higher alanine aminotransferase (ALT) values, a feature that also tends to cluster in families. Infection in the first five years of life contributes most to the case load of chronic liver disease and thus to mortality from HBV. Mass hepatitis B immunisation programmes have been started, and while they may eventually reduce the HBV carrier state and liver disease loads significantly, prospects for total eradication of HBV in the near future are not good.

The Western Pacific and South East Asia regions are the largest and most populous of the six World Health Organisation regions and include more than 40 countries. More than 75% of the world's estimated 350 million carriers are located here. The region has therefore provided many insights into the epidemiology, natural history, and control of hepatitis B infection and has been home to the first national control programmes. Hepatitis B is hyperendemic in most countries of the region, with carrier rates ranging from 5-35% except in Australia, New Zealand, and Japan, where the mean carrier rate is less than 2%. Patterns of infection vary considerably from country to country, city to city, and even village to village, and can change with time. Most infections are acquired early in childhood or in early adult life. A variety of control measures are in place and many countries in the region have introduced widespread or universal childhood immunisation policies with significant success. While it is theoretically possible that hepatitis B infection could be eradicated by universal childhood immunisation, there are several biological and practical issues that make this extremely difficult, suggesting that, for the foreseeable future, control may be a more realisable goal.

Although Singapore is in an endemic region for hepatitis B infection, the hepatitis B carriage rate of 5-6% is relatively low. The highest positivity rates for hepatitis B surface antigen (HBsAg) are found in the paediatric age group, with another peak in 40-49 year olds. Studies suggest that, although perinatal transmission is an important route of infection, most children acquire the virus through horizontal transmission between family members. Viral replication continues at a high rate in young carriers and tends to slow down with increasing age. Up to 50% of hepatitis B carriers in Singapore have chronic hepatitis, shown by raised serum ALT values and liver histology, and about 10% are infected with the precore mutant virus. About 20% of carriers have cirrhosis. Among patients with HCC, up to 75% are HBsAg positive, of whom 45% are still viraemic. Mass vaccination against hepatitis B was introduced into Singapore on a voluntary basis in 1983, with compulsory vaccination of babies born to HBeAg positive mothers since 1985. The number of cases of acute hepatitis B has fallen by 60% between 1989 and 1995 although the problems of the longterm complications of chronic hepatitis B still need to be tackled.

Both hepatocellular carcinoma (HCC) and iron overload are important health problems in Africa. Chronic hepatitis B virus (HBV) infection is recognised as a major risk factor for HCC, but iron overload in Africans has not been considered in pathogenesis. Up to half the patients with HCC in Africa do not have any recognised risk factors such as preceding chronic HBV infection, and other risk factors remain unidentified. HCC is an important complication of HLA-linked haemochromatosis, an iron loading disorder found in Europeans. It is proposed that African iron overload might also be a risk factor for HCC.

Numerous epidemiological studies have been performed to determine factors that might contribute to the development of inflammatory bowel disease. Although the role of oral contraceptive agents in Crohn's disease (CD) and ulcerative colitis (UC) have been assessed, most studies were of small sample size and characterised by low statistical precision. A meta-analysis was performed to increase the statistical power and to investigate the association between the use of oral contraceptives and the development of CD and UC. The study was based on a search of a Medline database from 1975 to October 1993 and a review of reference lists from published articles, reviews, symposia proceedings, and abstracts from major gastrointestinal meetings. All studies specifically designed to evaluate this association were selected. The combined results of nine studies--two cohort studies (30,379 unexposed and 30,673 exposed patients) and seven case-control studies (482 CD, 237 UC, and 3198 controls)--which satisfied our selection criteria were evaluated. The pooled relative risk (adjusted for smoking) associated with oral contraceptive use was 1.44 (1.12, 1.86) for CD and 1.29 (0.94, 1.77) for UC. These results suggest modest associations between the use of oral contraceptives and the development of CD and UC. As these associations are weak, non-causal explanations for the findings cannot be eliminated.