Three-dose hepatitis A and B vaccination schedules were based on early clinical trials that suggested that 2 doses are needed to prime for a secondary response to the third dose.1 The need to administer 3 doses has an important impact on vaccination completion rates and is resource consuming. Despite the high rates of incomplete vaccination, the number of cases of partially vaccinated individuals remains extremely low.2 Trials have also demonstrated that preadolescents respond more vigorously to vaccination when compared with infants and adults.3
The immunogenicity after 2 pediatric doses of Recombivax-HB (RB) or Twinrix Junior (TX) given to 8 to 10-year-old children were excellent.4 However, it is known that antibodies decrease and eventually disappear in some vaccinees. The response to a challenge dose of vaccine is used to determine the presence of immune memory.1
The aim of this study was to measure anti-HBs and anti-HAV persistence and the effect of a challenge dose given 7 years after vaccination with 2 doses of TX or RB.
MATERIALS AND METHODS
This open-label study was initiated in 2000, when 2 cohorts of 8 to 10-year-old children were vaccinated with 2 pediatric doses of either RB or TX.4 The participants who completed the 2000 phase of the study and agreed to be contacted later, and had not received hepatitis vaccines outside the study protocol were eligible for this phase of the study. Subjects who had <10 mIU/mL of anti-HBs postsecond dose received additional doses in 2001 and were excluded from the follow-up.
The study was approved by the Ethical Review Board of the Laval University Hospital Center. Written informed consents were signed by parents in 2000 and by participants themselves in 2007.
Vaccines, Schedule, and Blood Samples.
Participants in the RB group were administered 2.5 μg of vaccine between 2000 and 2001 and 5 μg in 2008, whereas those in the TX group were administered a dose containing 10 μg of HBs antigen and 360 El.U of inactivated hepatitis A antigen. The first 2 doses were administered at 6 month interval. The third dose was administered 7 years after the second dose. Vaccines were given intramuscularly in the deltoid. Blood samples were collected immediately pre- and 4 to 8 weeks postchallenge dose.
Anti-HBs titers were assessed by AUSAB Quantification Panel and AUSAB EIA (Abbott Laboratories Diagnostic Division, Abbot Park, IL) and anti-HBc by the ORTHO HBc ELISA Test system (Ortho-Clinical Diagnostics, Inc. Rarirtan, NJ). Serum samples of subjects vaccinated with TX were assayed for anti-HAV titers by HAV Total Assay EIA (Bio-Rad, France). Seroprotection was defined as ≥10 mIU/mL of anti-HBs and ≥20 mIU/mL of anti-HAV.
We defined the anamnestic response as an increase of anti-HBs and anti-HAV (TX group only) titers by ≥4-fold postchallenge when compared with prechallenge and at least a seroprotective titer of antibodies.
The exact binomial proportion method with 95% CI was used for the comparison of results. The Fisher exact test was used for GMTs analysis of variance. Statistical tests were 2-tailed. P ≤ 0.05 were considered significant. SAS Institute software version 9.1 (Cary, NC) was used for analysis.
We succeeded to communicate with 588 of the 643 subjects who agreed in 2001 to be contacted later. Thirty subjects had received hepatitis vaccines outside of the study protocol and were excluded. Of the remaining 558 subjects, 465 (83%) agreed to participate. One subject from the RB group refused the postchallenge blood sample. This subject had an anti-HBs titer of 16 mIU/mL prechallenge dose.
Participants did not significantly differ from nonparticipants in their immune response to the first 2 doses.
Seven years postsecond dose, 92% (95% CI, 87.6%–95.0%) of the TX and 75% (95% CI, 68.9%–80.4%) of the RB subjects had detectable anti-HBs (P < 0.0001). The seroprotection rates were 75.1% (95% CI, 69.0%–80.5%) and 55.6% (95% CI, 48.9%–62.1%), respectively (P < 0.0001). The anti-HBs GMT was 60 mIU/mL in the TX and 12 mIU/mL in the RB group (P < 0.0001). In the TX group, 100% of subjects had an anti-HAV ≥1 mIU/mL; 97.8% had a titer ≥20 mIU/mL. The anti-HAV GMT was 275 mIU/mL (Table 1).
Response to the Challenge Dose.
All vaccinees in the TX group and all but 4 in the RB group had anti-HBs titers of ≥10 mIU/mL postchallenge. All 4 nonrespondents were anti-HBs negative prechallenge and their titers postsecond dose varied from 39 to 241 mIU/mL. All other subjects had ≥4-fold anti-HBs titer increase. The anti-HBs GMTs were 39,573 mIU/mL in the TX and 5114 mIU/mL in the RB group. All subjects in the TX group had an anti-HAV titer ≥20 mIU/mL and all but 1 (99.6%) had an anti-HBs ≥100 mIU/mL; 94% had ≥1000 mIU/mL. The anti-HAV GMTs were 8203 mIU/mL (Table 1). No serious adverse events were reported.
The results confirm that 2 pediatric doses of hepatitis B vaccine given at 6 months interval to 8 to 10-year-old children confer long-term immunity. Both vaccines performed well, but higher anti-HBs GMTs and a better persistence of antibodies were observed in the TX group.
The proportion of vaccinees with a seroprotective titer of anti-HBs, 7 years postsecond dose of RB, was comparable to that reported after 3 doses of hepatitis B vaccine given to infants. The latter schedule has been widely used and is commonly recognized as efficacious.
The results obtained in the TX group suggest a better persistence of antibodies when compared with 3 doses of hepatitis B vaccines given to infants, but in the range reported after 3 doses of monovalent vaccines given to preadolescents.5 The vigorous response to the challenge dose confirms that immune memory is present in all primary respondents to 2 doses of TX. These results are different from those reported in individuals vaccinated during infancy. In the latter studies, from 9% to 50% of the individuals did not show evidence of immune memory after a challenge dose of vaccine.6,7 Our results suggest a better persistence of both anti-HBs and immune memory when vaccinating preadolescents.
We compared the results of this study with those obtained in our ongoing long-term study with 3 primary doses of 2 monovalent hepatitis B vaccines.5 Based on the similarity of results observed with 3 doses of RB and 2 doses of TX and the well established high efficacy of 3 doses of RB, we conclude that a 2-dose TX schedule ensures an excellent protection against hepatitis B. All above and recently published data showing that 2 and 3 pediatric doses had equal effectiveness in terms of protection against disease8 strongly suggest that the second dose of the 3 dose schedule when given to 8 to 10-year-old children adds very little, if anything, to long-term protection.
The fact that virtually all subjects, at each study time point, had anti-HAV titers greater than 20 mIU/mL and high GMTs postchallenge indicate that 2 doses of TX ensure a long-term immunity against hepatitis A.
Our study has several limitations. Because primary nonrespondents were excluded, our results might overestimate the response to the challenge by 3.5% in the TX and 5.6% in the RB group. However, we cannot exclude the presence of immune memory in subjects with an initial titer <10 mIU/mL. Nonparticipation selection bias is possible. However, this is unlikely because no important differences were observed between postsecond dose results in participants and nonparticipants in the long-term phase of the study. Another limitation is the time of blood sampling. In some studies the period between the challenge and blood sampling was 1 to 4 weeks and in the present study it was 4 to 8 weeks. This might increase the proportion of subjects who were classified as having an anamnestic response. However, the great majority of subjects showed very high titers, which are uncommon in primary respondents.
An increasing proportion of children are born to vaccinated anti-HBs positive mothers, and they are at a low risk of infection. Recent data also show that infants born to anti-HBs positive mothers respond less well to vaccination.9 This could influence the duration of vaccine-induced immunity and raises the question of what might be the alternative to infants’ vaccination in low endemic areas.
Childhood hepatitis A infection is rare in industrialized countries. This left an increasing proportion of the adult population unprotected against an infection which is more severe in adults. A Canadian review showed an anti-HAV prevalence of only 1% in ages 8 to 13, 1% to 6% in 20 to 24, 10% to 17% in 25 to 39, and 40% to 60% in 40 to 60.10 These data emphasize the need of exploring alternative immunization schedules for both hepatitis A and hepatitis B.
In summary, although there were higher anti-HBs titers after 2-pediatric doses of TX compared with RB, the response to the challenge dose demonstrates the presence of an immune memory in most of vaccinees in both groups. The similarities between the results obtained with 2 doses of TX in our study and those reported in several long-term studies with 3 doses of monovalent hepatitis vaccines suggest that the 2-dose TX schedule might be the alternative way to protect against both hepatitis A and B without additional vaccination-related visits or important financial costs.
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