In previous studies conducted in Taiwan, the seropositivity rate of anti-HBs declined from 99% at 1 year of age to 83% at 5 years of age  and further dropped to 71.1% at 7 years of age, 37.4% at 12 years of age and 37% at 15–17 years of age . The HBsAg seropositivity rate in children less than 12 years of age decreased from 9.8% in 1984 to 1.3% in 1994. The seropositivity rate of anti-HBc also decreased from 26% in 1984 and 15% in 1989 to 4% in 1994 . In a 2004 survey, Ni et al. showed that the anti-HBc seropositivity rate was low (1%) in children less than 15 years of age . In 2004, the seropositivity rates for HBsAg, anti-HBs, and anti-HBc were 1.2%, 50.5%, and 3.7%, respectively, in those born after implementation of the vaccination program (age <20 years) .
In our study, the overall seropositivity rates of HBsAg and anti-HBs were 1.2% and 43.6%, respectively, in those individuals less than 18 years of age. In Table 1, we noted a phenomenon in which the seropositivity rate of anti-HBs decayed from 63.4% (age 6 years) to 31.6% (age 12 years) and then increased from 12 to 18 years of age. To the best of our knowledge, there is no research reporting a phenomenon similar to the data presented in Figure 2. Reactivation effects (HBV vaccine booster), natural boosters (boosting of the anti-HBs titer after a natural exposure), and breakthrough infections (having HBV infection despite receiving three or more doses of HBV vaccine) could all contribute to the pattern of increase in anti-HBs from 12 to 18 years of age as observed in this study. Tracing our subjects by their birth cohort (1992–1994) in the 12-year age group, we found 110 subjects (110/510, 21.6% of total enrolled students of senior high school in the corresponding period) had also enrolled in our senior high school at 15 years of age. Of subjects with seronegative anti-HBs titers at 12 years of age, 78.8% (63/80) were found to be seropositive at age 15 years (data not shown). We were unable to trace their booster records individually, although such seroconversion was highly indicative of booster effects, which may be a factor that interfered with the anti-HBs seropositivity rate in this birth cohort in the 15-year age group. Similarly, booster effects might also contribute to the elevated seropositivity rate and median titers of anti-HBs in the 18-year age group with less amplitude of influence if compared with those values in the 15-year age group.
In previous studies, the anti-HBc positivity rate of elementary school first graders within the 3-year follow-up period (2005–2007, birth cohort 1999) was between 1 and 2% in the North, Central, and South of Taiwan but was higher (3.5–4.8%) in Eastern Taiwan . Another survey conducted in 2004 among children younger than 18 years of age in Taiwan showed breakthrough HBV infection was about 1% in children sampled (136/13765) . Therefore, the unique phenomenon of increasing anti-HBs seropositivity rate after 12 years of age was likely because of booster reactivation effects. Poovorawan et al. reported that breakthrough infections during the first decade after primary vaccination in infancy were only observed in children born to high-risk families (maternal seropositive for both HBsAg and HBeAg). In the second decade after primary vaccination, breakthrough infections were detected in 12.9% of individuals, possibly reflecting increased exposure outside the home, linked to high-risk adolescent behaviors .
The highest rate of HBsAg seropositivity was 1.6% in the 12-year age group of this study, but no statistical significance (p = 0.154) was observed in the pattern of anti-HBs seroprevalence by age. All students in the 12-year age group were born between 1992 and 1994 and their HBsAg seropositivity rate was consistent with other studies within the same time frame . Natural HBV infections (positive for anti-HBc) and HBV carriers (seropositive HBsAg) were found in 4.1% and 1.6% of individuals, respectively, at 15–17 years of age in a post-1986 cohort in Taiwan .
Based on data from previous studies, different types of HB vaccines, doses, and brands as well as the timing of primary vaccination can all influence the persistence of anti-HBs titers . In Taiwan, neonates born before July 1992 received four doses of plasma-derived vaccine (Hevac B; Pasteur-Merieux, or its equivalent derivative); neonates born after July 1992 received recombinant HBV vaccines (5 μg/dose of Recombivax [Merck] or 20 μg/dose of Engerix [GSK]) instead. In Table 2, we demonstrate that individuals in the plasma-derived subgroup had a higher proportion of positivity for anti-HBs than did those of the recombinant subgroup in both the 15- and 18-year age groups (43.2% vs 34.7% at 15 years, p < 0.004; 46.0% vs 40.8% at 18 years, p < 0.003). Our finding is consistent with several other studies [20, 21]. The seropositivity rate of HBsAg in the plasma-derived subgroup is significantly higher than that of the recombinant subgroup in the 18-year age group (p = 0.049) alone, but not in the 15-year age group (p = 0.131). In our subsequent study, only the subjects who received recombinant HBV vaccine were pooled together for a longitudinal study to avoid the interference of different vaccine type and dosage given in infancy.
According to the birth cohort (1987–2003) analysis for long-term immunity following infant HBV vaccination, no increase of the seropositivity rates of HBsAg and anti-HBc was noted after 17 years of age . In contrast, 15–50% of the children who initially responded to the three-dose series of HBV vaccination had low or undetectable anti-HBs levels 5–15 years after primary vaccination . Wu et al. reported that the overall anti-HBs seropositivity rate after a booster dose of HB vaccine was estimated to be 84.3% among 1974 students from senior high schools with negative titers of both anti-HBs and HBsAg before the HB vaccine booster . Long-term protection studies indicated that immunological memory usually persisted even if anti-HBs levels fell below the protective threshold (10 mIU/mL) [20, 21]. However, without protective levels of anti-HBs, memory cells alone are probably unable to protect against acute infection . In our subjects within the 15-year age group from the birth cohort 1992–1997 presented in Table 3, booster response rate was 92.5% (529/570) after one booster dose of recombinant HBV vaccine (20 μg/dose). Anti-HBs seropositivity rate before one booster dose was 37.7% (397/1054) and became 87.7% afterwards. The median titers of anti-HBs before and after booster dose were 1.1 mIU/mL and 545.5 mIU/mL, respectively. All the subjects received recombinant HBV vaccine in their neonatal period, except the individuals from the 1992 birth cohort, who received plasma-derived HBV vaccine if they were born before July 1992. Lu et al. showed the booster response rate to one dose of recombinant HBV vaccine was 71% in 15- to 17-year-old subjects who received plasma-derived HBV vaccine during neonatal immunization . All the subjects from the Wu et al.  study were born between July 1987 and July 1991 and thus belonged to the plasma-derived HBV vaccine era in Taiwan. Therefore, the booster response pointed to the recipients who received the plasma-derived vaccine in infancy.
In Figure 3, we selected and followed 38 subjects who had graduated from our senior high school and consequently studied in our university. We monitored the changes in the seropositivity rates of anti-HBs and HBsAg within a time frame of 3 years. We found that the initial booster response rate at 6 weeks after one booster dose of HBV vaccine was as high as 96% (24/25) in our 15-year-old subjects. Surprisingly, seven subjects (29.2%, 7/24 booster responders) who became seropositive after their booster dose lost their anti-HBs seropositivity again within 3 years after booster. The overall anti-HBs seropositivity rate for all “the same school” subjects at 6 weeks and 3 years after one booster dose was 94.8% and 76.3%, respectively. For the comparison group labeled as “the others”, subjects from the same birth cohort in the 18-year age group, the seropositivity rates of anti-HBs and HBsAg were 39.7% (499/1256) and 0.8% (10/1256), respectively. The median titer of subjects in “the others” group was 4.2 mIU/mL (24.7 mIU/mL in “the same school”). During the 3-year follow-up period, no new HBV carriers were detected in “the same school” group, which included students given a booster dose if their anti-HBs titers were seronegative. However, there was no statistical difference (p = 0.581) in HBV carrier rate between “the same school” subgroup and “the others” subgroup (0% vs 0.8%, respectively). An HBsAg seropositivity rate of 0.8% at 18 years of age in our study group was comparable to the seroprevalence of the same age group in other studies . In our study, booster vaccination did not confer additional protection against HBsAg carriage. High booster response rate (96%) at 6 weeks after one booster dose at 15 years of age also indicated intact long-term protection of HBV infection by immunological memory, as a result of anamnestic response after primary infant HBV vaccination. Immunity against HBV provided protection against infection as well as against disease. Protection against infection is associated with antibody persistence, which is directly related to the peak production of anti-HBs after primary vaccination. Protection against disease (i.e., acute hepatitis, prolonged viraemia, carriership, and chronic infection) is associated with immune memory that persists beyond the time at which anti-HBs disappears . Currently, Middleman et al. show several variables independently associated with higher geometric mean titer response to a challenge dose of vaccine included a higher baseline anti-HBs titer, older age at first dose of primary series (≥4 weeks after birth), higher test dosage, and non-white race . In contrast to the high endemicity of HBV in Taiwan, Middleman et al. collected these new data regarding duration of protection in the setting of low hepatitis B endemicity in the United States, with a likely absence of natural boosting. The response to the challenge dose of HB vaccine was remarkably good, as high as 92% , which is similar to our booster response rate after one booster dose of HBV vaccine at 15 years of age.
Taking Taiwan as an example of an HBV endemic area, the seropositivity rates of HBsAg and HBeAg for pregnant women were still 10.3% and 2.3% in 2009 (data from Taiwan Centre for Disease Control, Department of Health). Additionally, the mean age of mothers giving birth in 2007–2009 was 29.45 years of age, which meant the majority were born after the introduction of infant HBV vaccination. Horizontal and breakthrough infection could also occur after waning or eventual loss of the vaccine protectiveness in older children, especially with changes in lifestyle and sexual activity . A concern exists about sexual exposure to HBsAg carriers in hyperendemia areas such as Taiwan when vaccinated children become adolescents and young adults. Ni et al. showed no increase in seropositivity rates of either HBsAg or anti-HBc when vaccinated individuals progressed to 17 years of age  and the rate of chronicity declined as the age of infection increased: 25% in infected preschool children and 3–10% in adolescents and young adults . Although universal booster dose may not be necessary up to 20 years after the primary vaccination because HBsAg and anti-HBc seropositivity did not increase , the methods for the prevention of horizontal transmission, such as avoidance of skin tattooing, use of disposable needles, and condom use in sexual contact still need continuous implementation in the adolescent group . Based on our data, the current guidelines from Taiwan Advisory Committee on Immunization Practice (ACIP) appear to be adequate, and states that individuals may receive a booster dose if they have negative anti-HBs antibodies and are in high-risk groups (i.e., hemodialytic, organ transplant, and immunocompromised patients; intravenous drug users; participants in high-risk sexual activity; or health care workers).
The major limitations of our study were as follows. First, the retrospective cross-sectional study design and records analysis was conducted without reviewing any vaccination records for all subjects. Second, quantitative data of anti-HBs titers were available for all four age groups in the period of 2007–2012 but anti-HBc was not routinely screened for during entrant health-screening examination owing to a decreased seropositivity rate of this marker [26, 27] and a high proportion of HBV-DNA negativity in anti-HBc positive subjects . As a result, the seropositivity rates of natural infection could not be estimated in our study. Third, despite achievement of high HBV vaccine coverage rates from 88.8% to 96.9% (from 1984 to 2010) according to data from the Centre for Disease Control in Taiwan [11, 12], there was a possible bias in primary versus booster doses in our study subjects. The proportion of booster rate in “the others” subgroup analysis could not be identified and therefore we could only state possible booster dose effects according to the level of anti-HBs titers when comparing these data to the value of anti-HBs in subjects from the same birth cohort.