From 2002 through 2005, the Argentinean Health Department was notified of >120,000 hepatitis A (HA) cases, mainly in 5 to 9 year olds. Children provide a reservoir for hepatitis A virus (HAV) in communities.1 Effective and well-tolerated HA vaccines for children exist1 including a pediatric formulation of the inactivated HA vaccine, Avaxim (Avaxim 80U Pediatric, Sanofi Pasteur, Lyon, France). A 2-dose Avaxim 80U schedule has demonstrated good immunogenicity and safety profiles in children 1 to 15 years of age.2 In one clinical trial in Buenos Aires, Argentina, 2 doses of Avaxim 80U were administered 6 months apart to children aged between 1 and 15 years. In 111 children, those aged between 12 and 47 months were seronegative at inclusion, HAV antibody titers (anti-HAV) geometric mean concentration (GMC) at baseline and weeks 2, 24 (prebooster), and 27 (postbooster) of the initial schedule were respectively 6.25, 98.5, 190, and 6743 mIU/mL.2 Although Dagan et al3 have demonstrated the persistence of anti-HAV antibodies in children after 2 Avaxim 80U doses after 3 years, no longer term immunogenicity data in children have yet been published. We describe the evaluation of long-term anti-HAV antibody persistence 10 years after 2 doses of Avaxim 80U.2
MATERIALS AND METHODS
Trial Design and Study Population
Healthy children who were 12 to 47 months of age at the time of primary vaccination, when they received 2 doses of Avaxim 80U 6 months apart were invited for serum anti-HAV measurements 10 years later. Participants were excluded if they had received an additional dose of HA vaccine after the first booster or if they had moderate or severe illness, immunodeficiency, or ongoing immunosuppressive therapy, if they had previous treatment with growth hormone or human immunoglobulins or if they had received whole blood cells or blood product transfusion during the last 6 months. The protocol was approved by the internal review board and the ethics committee of the Hospital de Niños “Dr. Ricardo Gutiérrez.” Informed, written consent from parents/guardians and the children's agreement was obtained following explanation of the purpose of the trial, before inclusion (specific questions asked at the time of recontact, to determine eligibility were as follows:
- Did your child receive HA vaccine during participation in the initial trial vaccine 10 years ago?
- Were blood samples taken from your child to measure HAV antibodies before and after the vaccination 10 years ago?
- In the last 10 years, has your child received any extra dose of HA vaccine in addition to the 2 doses administered in the trial 10 years ago?
- Has your child received blood or blood products in the last 6 months?
Vaccine and Vaccinations
Inactivated HA vaccine Avaxim 80U Pediatric (Sanofi Pasteur, Lyon, France) was used in the initial study. It is prepared from cultured purified formaldehyde-inactivated GBM strain of HAV and contains 80 HAV antigen units per 0.5 mL dose and reduced aluminum hydroxide content (0.15 mg). Each dose contains 2-phenoxyethanol (2.5 μL), formaldehyde (12.5 μg), and Medium 199, water for injection up to 0.5 mL. It was administered intramuscularly into the deltoid muscle. An additional dose was offered to participants whose anti-HAV titer was less than or equal to the lower limit of detection of the serology assay (≤20 mIU/mL) when measured 10 years after the 2-dose vaccination course. This value is known to protect against HA infection.4
Serum anti-HAV antibody titers were measured by VIDAS Anti-HAV Total (HAVT, BioMerieux, France). The assay combines a 2-step enzyme immunoassay competition method with fluorescent detection (ELFA).5 Results are calculated automatically and the samples' relative fluorescent value is interpreted by the VIDAS system. Results are expressed in mIU/mL (WHO reference standard first Reference Preparation Hepatitis A Immunoglobulin [100 mIU/mL]). Sera with anti-HAV ≥20 mIU/mL were considered seropositive. The test used in the initial study was not available (antibody titers had been assessed using commercial radioimmunoassay [HAVAB, Abbot Laboratories, North Chicago] modified to increase the sensitivity).5
The study was descriptive. GMC with 95% confidence intervals were calculated. χ2 and Mann-Whitney tests were used when appropriate using STATA software (Version 8.0; Stata Corporation, College Station, TX). Participants also completed a questionnaire about potential exposure to natural HAV infection since the initial immunization.
Between May and November 2007, 48 of the 111 children who had been included in the initial trial were enrolled in this 10-year follow-up study; of the 63 children who were not included in this follow-up trial, 30 children had moved to a different neighborhood or city, 26 children could not be contacted since the telephone number had changed, 1 child's family was contacted but did not consent to the child's participation in the follow-up trial, and 6 children were excluded due to being seropositive in the initial trial. Of the 48 children included in the follow-up trial, 26 (54.2%) were female, mean age was 156.06 (±11.7) months; no significant age difference was observed between sexes (P = 0.64). All participants had had anti-HAV titers <20 mIU/mL at inclusion 10 years previously. Fourteen days after the first vaccination, 100% had had anti-HAV ≥20 mIU/mL. GMC increased during the 6 months after the first vaccine dose and all participants had anti-HAV ≥20 mIU/mL at week 24. A strong anamnestic response to the second dose was seen at week 27.2 There was no significant difference between anti-HAV titers of the 48 participants enrolled in the follow-up study and the whole population at week 27 (postbooster) in the initial study (P = 0.25). Ten year persistence of anti-HAV ≥20 mIU/mL was observed in 47/48 (97.9%) participants, with GMC 390.91 (±370.14) mIU/mL; (95% confidence interval, 282.2–499.5 mIU/mL), range, 36–1860 (Table 1). There was no significant difference in titers between sexes (P = 0.18).
Regarding potential exposure to natural HAV infection, 17/48 (35.4%) had had known close contact with an acute HA case since immunization. Contacts were as follows: household, 4/48 (8.33%); school, 11/48 (22.9%); close neighbors, 7/48 (14.6%). Four of 17 (23.5%) had had more than one contact. There was no significant difference between anti-HAV titers in children reporting exposure (445.6 [±464.4 mIU/mL, range: 108–1860 mIU/mL]) and those reporting no contact with HA patients during the last 10 years (359.9 [±309.2 mIU/mL, range: 36–1460 mIU/mL]). Moreover, previous titers at postbooster (week 27) in the initial study for these 2 groups were 7564 (±7004 mIU/mL, range: 902–31,424 mIU/mL) reporting contact versus 7484 (±4942 mIU/mL, range: 1200 – 20,592 mIU/mL) for those reporting no contact (P = 0.78). The one participant with anti-HAV <20 mIU/mL at year 10 was revaccinated and his titer reached 1400 mIU/mL, even higher than after the first booster dose (1200 mIU/mL at week 27).
Inactivated HA vaccines are highly immunogenic although an absolute protective anti-HAV antibody titer threshold has not been defined. Serum antibody concentrations after passive transfer by immune globulin or active induction by vaccination are 10- to 100-fold lower than those produced by natural infection. Concentrations of 10 to 20 mIU/mL, 1 to 2 months after immunoglobulin administration protect against HA infection.1 In vitro studies using cell culture-derived HAV show antibody concentrations <20 mIU/mL can be neutralizing.4 The lower limit of detection of a particular assay is considered as protective. After vaccination, persons who are anti-HAV seronegative by standard assays might nevertheless have protective concentrations of antibody.6
This study presents a 10-year follow-up of an Avaxim 80U Pediatric clinical trial. In the initial study, 99.1% of vaccinees had anti-HAV titers ≥20 mIU/mL 2 weeks after the first vaccine dose, and 100% at week 24 (prebooster) and at week 27 (postbooster).
Serum anti-HAV persistence has been studied in children and in adults1,3,6–10 but, to our knowledge, only 3-year follow-up data in children have been published for Avaxim 80U vaccine and no longer term data are available. This is the first long-term evaluation of humoral immunity showing 10-year persistence of anti-HAV antibodies in 97.9% of healthy children after 2-doses of Avaxim 80U Pediatric. Similar long-term data were previously shown for other inactivated HA vaccines in different age populations11,12 while mathematical models predict that in children anti-HAV will persist for 14 to 30 years.11,13
The results of this study support “The International Consensus Group on Hepatitis A Virus Immunity,” which concluded that after a full vaccination course, protective antibody amounts persist beyond 10 years in healthy individuals, and underlying immune memory provides protection far beyond the duration of anti-HAV antibodies.11 Even in the small population, in our study, no differences were observed related to sex as also was shown in children and adults in other studies.8,13 No difference in anti-HAV titers was observed in children who recorded having had close contact with HA patients versus those with no known potential exposure, although it is known that the natural exposure to HAV produces high anti-HAV titers.11
In 2006, the Argentinean Department of Health, introduced HA vaccine into the national immunization program because of the epidemiologic situation of the preceding 4 years. Only one dose was indicated based on the assumption that natural exposure to HAV would act as a natural booster and provide good protection against disease. Although the number of participants in this study is limited, potentially HAV exposed children did not have higher anti-HAV titers than those reporting no contact. Because all had received a complete 2-dose schedule, it is not known whether natural exposure to HAV acts as a natural booster after only one dose of inactivated HA vaccine.
This study show that Avaxim 80U Pediatric induces high titers of anti-HAV, which persist for over 10 years following 2 doses in children demonstrating long-term protection. Our data support the International Consensus Statement that today no data support the need for booster doses of HA vaccine in immunocompetent individuals who have received a full vaccination course.11
The authors thank Simon M. Jones and Anvar Rasuli for their constructive review of the study report and of this manuscript.
1. Bell B, Feinstone S. Hepatitis A
vaccines. In: Plotkin S, Orenstein W, eds. Vaccines
. Philadelphia, PA: WB Saunders; 2004:269–297.
2. Lopez EL, Del C, X, Torrado LE, et al. Safety and immunogenicity of a pediatric
formulation of inactivated hepatitis A
vaccine in Argentinean children. Pediatr Infect Dis J
3. Dagan R, Greenberg D, Weber F. Immunogenicity of an inactivated hepatitis A pediatric
vaccine: three-year post-booster follow-up. Vaccine
4. Lemon SM, Murphy PC, Provost PJ, et al. Immunoprecipitation and virus neutralization assays demonstrate qualitative differences between protective antibody responses to inactivated hepatitis A
vaccine and passive immunization
with immune globulin. J Infect Dis
5. Miller WJ, Clark W, Hurni W, et al. Sensitive assays for hepatitis A
antibodies. J Med Virol
6. Fiore AE, Wasley A, Bell BP. Prevention of hepatitis A
through active or passive immunization
: recommendations of the Advisory Committee on Immunization
Practices (ACIP). MMWR Recomm Rep
7. Lolekha S, Pratuangtham S, Punpanich W, et al. Immunogenicity and safety of two doses of a paediatric hepatitis A
vaccine in Thai children: comparison of three vaccination schedules. J Trop Pediatr
8. Rendi-Wagner P, Korinek M, Winkler B, et al. Persistence of seroprotection 10 years after primary hepatitis A
vaccination in an unselected study population. Vaccine
9. Van Herck K, Van Damme P. Inactivated hepatitis A
vaccine-induced antibodies: follow-up and estimates of long-term persistence. J Med Virol
10. Van Herck K, Van Damme P, Lievens M, et al. Hepatitis A
vaccine: indirect evidence of immune memory 12 years after the primary course. J Med Virol
11. Van Damme P, Banatvala J, Fay O, et al. Hepatitis A
booster vaccination: is there a need? Lancet
12. Hammitt LL, Bulkow L, Hennessy TW, et al. Persistence of antibody to hepatitis A
virus 10 years after vaccination among children and adults. J Infect Dis
13. Fan PC, Chang MH, Lee PI, et al. Follow-up immunogenicity of an inactivated hepatitis A
virus vaccine in healthy children: results after 5 years. Vaccine