8 Hepatitis B

8.3 Epidemiology

8.3.1 Burden of chronic disease and geography

Almost half the world’s population have been exposed to the hepatitis B virus (HBV), and an estimated 400 million people have chronic infection and remain at risk of developing hepatocellular carcinoma and cirrhosis. More than 90 percent of individuals with chronic HBV reside in the Asia–Pacific region, where most countries have high prevalence rates of HBV infection (between 5 and 20 percent). More than 99 percent of HBV-infected people in this region acquired infection through vertical transmission from their mother (usually at the time of delivery) or in early childhood. Acquisition of HBV during adulthood (usually via sexual transmission or injecting drug use) is associated with a high rate of symptomatic hepatitis but a low rate of chronic infection.

The introduction of universal childhood HBV immunisation has changed the epidemiology of chronic infection in many countries, but it will be several decades (one to two human generations) before the full benefits are realised. The world can be divided into regions with high (8 percent and over), high-intermediate (5–7 percent), low-intermediate (2–4 percent) and low (less than 2 percent) prevalence of chronic infection, defined as the presence of HBsAg in serum.5, 6

In regions with a high prevalence of chronic infection, the lifetime risk of exposure to HBV is almost 80 percent, with most infections occurring in the first decade of life. The Pacific Islands and most of Asia (except Japan and India) are high prevalence regions. Other high prevalence regions include Sub-Saharan Africa and Andean Latin America.5 In contrast, in countries with a low HBsAg prevalence, the lifetime risk of HBV exposure is less than 20 percent, with most infections acquired in adulthood.

New Zealand has a low overall prevalence of hepatitis B carriage but contains certain populations with high prevalence (see section 8.3.2 below).

8.3.2 New Zealand epidemiologyTop

Before the introduction of HBV immunisation in New Zealand, HBV transmission was common among preschool and school-aged children. The exact mode of transmission is uncertain but is thought to be related to close contact. In the eastern Bay of Plenty region almost half of the population had been infected by age 15 years.7, 8 Even after the introduction of universal hepatitis B vaccine in 1988 (see Appendix 1), there were regions in New Zealand where children were still at risk of HBV infection due to poor immunisation coverage rates.9–11

Acute HBV infection

Only acute hepatitis B is a notifiable disease in New Zealand. Therefore notification rates do not describe the burden of chronic hepatitis B infections.

The HBV notification rate in 2012 was 0.9 per 100,000 population, which is a small decrease from the 2011 rate (1.2 per 100,000).12 The highest notification rate was in the 40–49 years age group (2.4 per 100,000, 15 cases), followed by the 50–59 years age group (1.4 per 100,000, 8 cases). There were no notified cases in children aged under 15 years. The notification rate was higher for males (1.1 per 100,000 population, 25 cases) than for females (0.6 per 100,000, 14 cases).

Ethnicity was recorded for 38 (97.4 percent) cases.12 Of the ethnic groups with more than five cases reported in 2012, the highest notification rate was among the Asian (1.7 per 100,000, 7 cases) ethnic group, followed by the Māori (1.2 per 100,000, 8 cases) and European or Other (0.7 per 100,000, 20 cases) ethnic groups. The most common risk factors reported by hepatitis B cases in 2012 were:

Hepatitis B notifications have declined from 609 cases in 1984 to 28 cases in 2013 (see Figure 8.1). While difficult to quantify accurately, the introduction of universal infant immunisation in 1988 has contributed to the dramatic decline in the number of newly notified cases of HBV infection.

Figure 8.1: Notifications of hepatitis B, 1971–2013

Figure 8.1: Notifications of hepatitis B, 1971–2013

Source: Ministry of Health and the Institute of Environmental Science and Research

Chronic HBV infection

Approximately 100,000 people in New Zealand are chronically infected with HBV. The National Hepatitis B Screening Programme was a three-year programme that started in 1999 and targeted at-risk populations in the North Island (Māori, Pacific peoples and Asian New Zealanders aged over 15 years). The programme also enrolled people from other ethnic groups and included follow-up of individuals aged under 15 years with chronic HBV.

Approximately one-third of the at-risk populations were screened. Of these, the highest rates were among Chinese (9.1 percent), Pacific peoples (8.5 percent) and Māori (5.8 percent). Although Europeans were not specifically targeted in this screening programme, they have an estimated prevalence rate of 1 percent (higher than in Australia, North America and Europe), reflecting the risk of early horizontal transmission.13

A New Zealand-based modelling study estimated that until the year 2100, people with chronic HBV infection will continue to provide a source of infection to susceptible people.14 Increased immigration from high-prevalence countries in the Asia–Pacific region is also likely to influence HBV prevalence in New Zealand.

Because people who acquire chronic HBV infection in childhood usually do not develop hepatocellular carcinoma (HCC) until aged 40 years or older, the introduction of a universal HBV vaccination in 1988 is unlikely to have a significant effect on the incidence of HCC until approximately 2030.

A retrospective laboratory data study of antenatal HBsAg tests from the Midlands region (Bay of Plenty, Eastern Bay of Plenty, Waikato and Rotorua) between 1997 and 2009 found a declining prevalence of HBV infection. This decrease was seen across all age groups, but was most marked in the antenatal tests of women aged under 20 years, who would have been eligible to receive funded hepatitis B vaccine in childhood.15

Strategy for prevention

In 1988 New Zealand was one of the first countries to introduce universal infant hepatitis B immunisation. At the end of 2013 approximately 93 percent of New Zealand children aged 2 years had completed a primary course of hepatitis B vaccine, which confers lifelong immunity in approximately 95 percent of vaccinees.

The Hepatitis Foundation of New Zealand16 recommend that the following individuals be screened for HBV – people who:

Screening for HBsAg is also part of routine antenatal care.