The WHO estimated that in 2015, 257 million persons or 3.5% of the population were living with chronic HBV infection. Among HBV-infected patients, approximately 27 million (10.5%) were aware of their infection and an estimated 4.5 million (16.7%) of them were on treatment. The prevalence of HBV infection differs geographically: 6% in Africa, 2% in Southeast Asia, and 1% in the Americas [4].

HBV endemicity differs according to the mode of infection (Table 1) [5]. Countries with a high prevalence have perinatal transmission as the major mode of infection; while low prevalence was due to horizontal transmission. Transmission by both modes is important in countries with an intermediate prevalence, such as South Korea, due to vaccination, perinatal care, and public health. The HBV disease burden in Korea results mainly from infections acquired through perinatal transmission. However, after implementation of a vaccination program in 1983, Korea was classified as an area of intermediate endemicity. The prevalence of hepatitis B surface antigen (HBsAg) carriage was 8% to 10% in the 1980s, 4.6% in the 1990s, and has remained at 2.9% since 2010. HBV vaccination for all newborns was introduced in 1983 (coverage rate, 79.7%), and the Expanded Program on Immunization (EPI) was introduced in 1995 (coverage rate, 98.9%) [1,6]. The HBsAg-positivity rates of younger age groups markedly declined to 2.2% in 1998, 1.9% in 2001, 1.9% in 2007, and 0.3% in 2016. The prevalence of HBsAg carriage also decreased significantly among those in their 20s and 30s (Fig. 1). The prevalence of CHB in Korean children has met the WHO interim 2020 target, 1% in children aged 5 years, and is projected to decline to 0.1% by 2030.

The overall HBsAg positivity rate was slightly higher in men than in women, but the difference gradually decreased from 1.1% in 1998 to 0.3% in 2016. In 2016, the rate was highest in men and women in their 40s and 50s (5.7% and 4.7%, respectively) (Fig. 2) [1]. HBsAg seropositivity did not differ between low (2.6%) and high income (2.4%) groups or between urban (2.9%) and rural (2.5%) residents [1].

The trends of acute hepatitis B changed in response to implementation of the HBV vaccination program. The incidence of acute viral hepatitis A and B among overall acute hepatitis in the early 1990s was 3.4% and 60.3%, respectively, with HBV infection as the most common cause of acute hepatitis [7]. In 2008, the incidence of acute HBV infection decreased to 5%, while that of acute hepatitis A virus infection increased to 70% [8]. The most frequently infected age group was 10 to 29 years of age in the 1980s, and 30 to 39 years of age in the early 2000s (Fig. 3) [9]. Approximately 5% of all acute HBV infections progress to chronic infection [10].

In patients who recover from acute hepatitis B, HBV replication may persist in some organs, most likely the liver or peripheral blood cells, for a long period despite the presence of serum antibodies to HBsAg (anti-HBs) and HBV-specific cytotoxic T-cells [11,12]. HBV DNA was detected in the liver tissues of 13 of 14 healthy liver transplant donors who were positive for hepatitis B core antibody (anti-HBc) and anti-HBs [13]. Persistent histological abnormalities, including fibrosis and mild inflammation, were present for 10 years in another study, despite complete serological recovery after acute infection [14].

HBsAg-negative but anti-HBc-positive patients are at risk of HBsAg reversion, particularly during rituximab-based chemotherapy. Therefore, HBsAg and anti-HBc should be monitored in patients who are candidates for rituximab-based chemotherapy or bone marrow transplantation [15-17]. Because HBV transmission may still occur in unvaccinated and uninfected adults and can lead to chronicity, “catch-up” HBV vaccination should be given to prevent acute HBV infection in individuals who were not vaccinated in early childhood.

The frequency of complications varies geographically. In low-endemic areas, the incidence of liver-related complications in untreated, asymptomatic patients can be as low as 1.2% after 16 years [18]. By contrast, the 5-year cumulative incidence of cirrhosis in East Asian (Taiwan and Korea) patients with hepatitis B e-antigen (HBeAg)-positive and -negative hepatitis was 8% to 17% and 13% to 38%, respectively. The 5-year cumulative incidence of liver decompensation among patients with cirrhosis was 15%. The incidence of HCC differs according to liver disease status; the 5-year cumulative incidence was 1% in inactive carriage, 3% in CHB patients without cirrhosis, and 17% in those with cirrhosis [19]. The 5-year cumulative incidence of survival in patients with compensated and decompensated cirrhosis was 85% and 14% to 35%, respectively [19-21]. The prevalence was similar in Korea before the era of antiviral agents, and the predominant cause of HCC was HBV infection (62% to 75% of HCC) [22].

After the introduction of oral antiviral therapy in 1999, the epidemiology of liver disease and liver cancer in Korea changed [23]. Real-world population-level data from the national death certificate database of Korea for liver disease and liver cancer showed that the annual number of patients receiving oral antivirals increased from 1,716 to 187,226 from 1999 to 2013. Long-term treatment induced histological improvement, including reversal of cirrhosis and reducing the incidence of HCC [24,25]. The 5-year cumulative incidence of cirrhosis decreased from 23% before the era of antiviral therapy to 5.3% in the era of antiviral therapy, that of hepatic decompensation from 5% to 0.3%, and the incidence of HCC from 3% to 0.8% [26].

There was a dissociation of mortality trends between liver disease and liver cancer despite use of antiviral agents. The crude death rate (CDR) for liver disease decreased from 21.2 to 7.5 per 100,000 persons (64.6% decrease). This reduction is far greater than that reported in a study from Taiwan (8%), also an HBV-endemic area [27]. In contrast, the CDR for liver cancer increased from 20.5% to 22.6% (10.2% increase). The mean age at death from liver disease increased by 8.8 years, significantly greater than the 6.1-year increase for liver cancer and 6.3-year increase for the entire population during the same period. The marked reduction in liver disease mortality due to newborn vaccination and widespread use of antivirals for HBV may have increased the life expectancy and number of patients at risk of developing liver cancer, inadvertently leading to an increased burden of liver cancer in HBV-endemic areas [28,29]. Although the mortality rate of liver disease has decreased, patients with liver cancer have a high mortality rate. This suggests the need for early screening for HCC in patients with HBV infection.

Both virologic and non-virologic factors influence disease progression and survival in chronic HBV-infected patients. The factors associated with disease activity include HBeAg seropositivity, HBV DNA and HBsAg levels, and HBV genotype. HBV variants such as core promoter or pre-S deletion variants are associated with progressive liver disease and HCC [30,31]. Among these factors, the serum HBV DNA level is the cardinal predictor of disease progression in patients with a high viral load (≥ 2,000 IU/mL), and is a determinant of the risk of progression in those with an HBV DNA level < 2,000 IU/mL [32].

In HBeAg-negative patients with an HBV DNA level < 2,000 IU/mL, the serum HBsAg level is predictive of liver disease progression. An HBsAg level > 1,000 IU/mL is associated with an increased risk of disease progression [33,34]. Among HBeAg-positive patients, a high viral load is generally not predictive of complications, but a moderately high HBV DNA level (> 20,000 IU/mL) and advanced age are closely associated with an increased risk of HCC compared to that of immune‑active phase CHB patients treated with nucleos(t)ide analogues. This suggests the need for earlier antiviral treatment in certain immune-tolerant CHB patients [35].

Spontaneous HBsAg clearance occurs infrequently; the annual incidence is 0.5% to 2.3% [36-38]. The annual HBsAg seroclearance rate is lower in Korea (0.76%) than in other countries. The difference may be due to HBV genotype; the majority of Korean patients are infected with genotype C HBV [39]. After HBsAg seroclearance, the annual rate of HCC development was 0.55% and the associated risk factors were liver cirrhosis, male gender, and age ≥ 45 to 50 years at the time of HBsAg seroclearance [40,41]. Therefore, HCC surveillance is required in patients with these risk factors even after HBsAg seroclearance.

In 2005, the societal cost of HBV infection was 1.937 trillion KRW, including direct costs of 474,642 million KRW and indirect costs of 1.463 trillion KRW (Table 2) [42]. This is equivalent to 0.24% of the 2005 Korean gross domestic product (GDP), or 3.98% of Korea’s 2005 national health expenditure (NHE) and 2.01% of annual national health insurance expenditure. However, the economic burden is anticipated to decrease due to vaccination and prevention of perinatal transmission.

The risk of becoming an HBsAg carrier is 90% for perinatal infection, 25% to 30% for infection acquired during the preschool years, and less than 10% for adults [43]. Korea was classified as an area of high endemicity before implementing universal vaccination; in the 1980s, around 11% of neonates in Korea were infected with HBV via vertical transmission [6]. The domestically developed plasma-derived hepatitis B vaccine was approved in Korea in 1983. The first dose of monovalent hepatitis B vaccine is given as soon as possible after birth, preferably within 24 hours, followed by at least two subsequent booster doses at 1 and 6 months. After the introduction of hepatitis B vaccination for all newborns, the coverage rate was 79%; this was further increased to 94% (third dose) through the implementation of the EPI in 1995, higher than the global rate of 84% [4].

The Korea Advisory Committee on Immunization Practices (KACIP) implemented the Hepatitis B Perinatal Transmission Prevention Program (HBPTPP) in July 2002. This program aimed to screen all pregnant women for CHB infection, provide prophylactic treatment (hepatitis B immunoglobulin [HBIG]) to all infants born to HBsAg-positive mothers, and vaccinate all infants against hepatitis B. To ensure compliance among providers and patients in the private sector, the government provided full financial support. A voucher system was implemented to cover the cost of vaccination and to track progress. Voucher information is sent to the Korea CHB database.

Infants born to HBsAg-positive mothers receive HBIG at birth in addition to vaccination. This has reduced the HBsAg prevalence among blood donors to 0% to 0.4% (Fig. 4), and the third-dose coverage rate among infants is 94% (Table 3). In 2008, the WHO certified that Korea has achieved control of vertical transmission of HBV [44].

The HBPTPP was reviewed in 2011 and among 69,999 children that were enrolled in this program with available follow-up serological test results, prophylaxis failure rate was 3.14%. A high viral load in the mother is a major risk factor for post-birth HBV vaccine failure [45]. Immunoprophylaxis may fail in infants born to mothers with an HBV DNA level > 200,000 IU/mL, HBeAg seropositivity, and an HBsAg level < 10,000 IU/mL [46,47]. Recent study from China showed that prophylactic tenofovir in HBsAg-positive mothers with an HBV DNA level > 200,000 IU/mL during the third trimester decreased the rate of mother-to-child transmission [48]. Concerning the safety of tenofovir, no adverse events have been observed in mothers and infants [49]. Our recent meta-analysis also showed that tenofovir prevented vertical transmission and had no significant adverse events [50]. Global guidelines now recommend tenofovir to prevent vertical transmission in the third trimester of pregnancy [16], but health insurance does not cover the cost of antiviral therapy for pregnant women with only a high HBV DNA level. Thus, these data could provide evidence to a policy change for the coverage of antiviral costs in addition to vaccination program.

In the national immunization guidelines for adults, catch-up hepatitis B vaccinations are recommended for adults without a history of immunization or who had past HBV infection, particularly those with chronic liver diseases, hemodialysis, human immunodeficiency virus infection, or who are frequent recipients of blood products, as well as persons at elevated risk of exposure to HBV (residents and staff of facilities for people with developmental disabilities, prisoners, employees of correctional facilities, people with household contact to someone who is HBsAg-positive, injection drug users, or people with sexually transmitted infections) [45].

According to monthly statistics published by the Ministry of Justice and Korea Immigration Service, in 2015 there were 1,774,603 foreigners in Korea, more than half of whom were from HBV-endemic areas. Ongoing immigration from countries with high endemicity such as China (918,951, 51.8%), Vietnam (129,423, 7.3%), the Philippines (51,836, 2.9%), and Indonesia (42,520, 2.4%), as well as the increasing number of North Korean defectors (HBV prevalence, 10.9%), represents a public health threat to South Korea and warrants government intervention.