In this study, we report the anti-HAV antibodies at 15 years from the first Indian study of single dose live HAV vaccine in children. Immunogenicity data from the same cohort at 2 months, 30 months and 10 years post-immunization has previously been reported [7-9].

METHODS

The study began in 2004 wherein 143 children were given a single dose of H2 strain of a live attenuated hepatitis A vaccine (Biovac-A, Wockhardt Ltd) and assessed for anti-HAV antibodies 2-months post vaccination [7]. These children were then called for follow-up every year, for clinical assessment and to record history of hepatitis, if any. They were assessed serially for anti-HAV antibodies in 2007, 2010 and 2014 [8,9]. Subjects with low anti-HAV antibody titres (<20 mIU/mL) were given additional doses of vaccine viz., in 2004 they were given two doses of the then licensed HAV vaccine (Havrix Jr GSK Biologicals) and in 2007 they received an additional dose of Biovac-A vaccine. No further vaccines were given in 2010 or 2014.

Contact details of the cohort were maintained by medical social workers. At the yearly visits, participants/parents were asked for history of hepatitis like illness (fever, anorexia, nausea, vomiting and jaundice). Clinical examination included noting for hepatomegaly or splenomegaly, if any. Parents were reminded to report complaints of hepatitis like illness immediately. No diary was given to participants for recording signs and symptoms. In the present study (2019) too, these children were clinically assessed for evidence of hepatitis (if any) and their anti-HAV antibodies were measured.

During the study of 15 years, institutional ethics committee approval and informed consent were obtained three times: Initial study (2004), second phase of study (2007-2015), and third phase of study (2016-2019). During the study, whenever a participant attained the age of 18 years, informed consent was obtained from him/her for continuing in the study.

Of the original 143 children who received a single dose of live attenuated hepatitis A vaccine in 2004, 109 subjects (72 males) came for the fifteen-year follow-up assess-ment in 2019. Mean age was 19.7 years (range 16-26.8 years). The number of subjects who came for follow-up since vaccination is as follows: 2.5 years (n=131), 6 years (n=126), 10 years (n=121) and 15 years (n=109). None of the parents/children retracted consent in writing. Clinical examination of the participants did not reveal any abnormal findings, and none gave any history of hepatitis like illness in the past.

Of the 109 children who came for the present follow-up, 4 had received two doses of licensed inactivated HAV vaccine in 2004, and 7 others had received a second dose of live HAV vaccine in 2007, as their total anti-HAV antibody levels had dropped to <20mIU/mL.

Of the remaining 98 children, 4 had low anti-HAV titres (<20 mIU/mL) giving a seroprotection rate of 95.9%. If the 11 children who were given additional doses of HAV vaccine are also included, the seroprotec-tion rate in all 109 children was 86.2%. The comparison of sero-protection rate in 2019 with previous assessment years is shown in Fig. 1.

Fig. 1 Serial seroprotection rates over 15 years (%).

All children were found to be negative for anti-HAV IgM. The total anti-HAV geometric mean titre (GMT) in ‘seroprotected children’ who received single dose of live attenuated vaccine (n=94) is 79.6 mIU/mL (95% CI 69.2-91.56). The comparison of GMT value at 15 years with previous assessment years is shown in Fig. 2.

Fig. 2 Serum anti-HAV antibody GMTs over 15 years.

In 2010, there were 25 children with anti-HAV titres <20mIU/mL. They were not given any additional dose / doses of live/inactivated HAV vaccine. The serial anti-HAV GMTs of these 25 children as compared to all 98 with single dose of live HAV vaccine is shown in Fig. 3. In 2014 and 2019, 23 of these 25 regained seroprotective levels. In 2019, the anti-HAV antibody titre of two other children (who had seroprotective levels earlier) are now <　20 mIU/mL.

Fig. 3 Serum anti-HAV antibody GMTs of children with low titers in 2010.

Children who received two additional doses of an inactivated HAV vaccine in 2004 (n=4), or an additional dose of live HAV vaccine in 2007 (n=7) have continued to show seroprotective levels since additional vaccination.

This 15-year follow-up of a cohort of children vaccinated with a single dose of live attenuated HAV vaccine shows a seroprotective rate of 86.2% with anti HAV GMT value of 79.6 mIU/mL. The serial seroprotection rates of the cohort are 95.8% at 2 months, 87.8% at 30 months, 77.7% at 6 years, 87.6% at 10 years and 86.2% at 15 years. This evaluation at 15 years confirms the robust long-term immunogenicity of a single dose of live HAV vaccine and compares well with other Indian and Chinese studies [3,10-12]. The comparable long term Chinese seroprotection data (Zhuang, et al.) at 15 years is 81.3% (GMT 128 mIU/mL) [2,3]. The other Indian long term multicentric study reported an immunogenicity of 97.3% at 5 years with GMT of 127.1mIU/mL [11].

In our serial evaluations since 2004, we used two types of immunoassay test kits: Axsym HAVB ELISA (Abbott Labs), (2004, 2007 and 2010). These kits were not available in India in 2014. Hence, we used COBAS kits based on ECLIA technology (Roche Diagnostics) in 2014 and 2019.The higher antibody titres of 2014 and 2019 could be due to differences in kits as ECLIA based reports are known to be of a higher sensitivity as compared to ELISA [13]. Alternatively, higher titres could be due to ‘booster like’ response to exposure to naturally occurring antigens of HAV in the community [14]. As the child grows from a teenager to adulthood, the frequency of consuming food and water outside the home increases, thereby increasing exposure to hepatitis A.

Another limitation of our study was cohort contamination with additional doses of hepatitis A vaccine [9]. In 2004, 6 children were given two doses of inactivated HAV vaccine and in the next evaluation at 30 months, 9 others were given a second dose of the live HAV vaccine. At the 6 yrs follow-up in 2010, 25 children were found to be seronegative. These 25 children were not given any additional doses of vaccine. Interestingly, 23 of these 25 were in the seroprotected range in the present evaluation, further implying the probability of an anamnestic response to natural boosters. Chen, et al. [15] have recently demonstrated that anamnestic responses via memory B and memory T cells may provide long term protection after a single dose of live Hepatitis A vaccine, despite low levels of anti-HAV antibodies.

In conclusion, 15-year follow-up after a single dose of live hepatitis A vaccine (H2 strain) demonstrated robust immunogenicity in Indian children. The continued safety and immunogenicity profile of the vaccine reiterates its value in primary immunization of Indian children. As a policy decision, the single dose schedule cuts costs while providing definitive long-term protection.

1. Mao JS, Dong DX, Zhang HY, et al. Primary study of attenuated live hepatitis A vaccine (H2 strain) in humans. J Infect Dis. 1989;159:621-24.

2. Zhuang FC, Qian W, et al. Persistent efficacy of live attenuated hepatitis A vaccine (H2-strain) after a mass vaccination program. Chin Med J (Engl).　2005;118:1851-856.

3. Zhuang FC, Mao ZA, Jiang LM, et al. Long term immunogenicity and effectiveness of live attenuated hepatitis A vaccine (H2-strain) – A study on the result of 15 years’ follow up. Zhonghua Liu Xing Bing Xue Za Zhi. 2010;31:1332-335.

4. Shah N, Faridi MMA, Mitra M, et al. Review of long term immunogenicity and tolerability of live hepatitis A vaccine. Hum Vaccin Immunother.　2020 Apr 3;1-6

5. WHO position paper on hepatitis A vaccines – June, 2012. Weekly Epidemiological Record. 2012;87:261-76.

6. Pemde H. Hepatitis A vaccines. In: Advisory Committee on Vaccines and Immunization Practices, Indian Academy of Pediatrics. IAP Guidebook on Immunization 2018–2019. 3rd edition. Jaypee Brothers Medical Publishers; 2020. p. 265-78.

7. Bhave S, Bavdekar A, Madan Z, et al. Evaluation of immunogenicity and tolerability of a live attenuated hepatitis A vaccine in Indian children. Indian Pediatr. 2006;43:983-7

8. Bhave S, Sapru A, Bavdekar A, Bawangade S, Pandit A. Immunogenicity of single dose live attenuated hepatitis A vaccine. Indian Pediatr. 2011;48:135-37.

9. Bhave S, Sapru A, Bavdekar A, Kapatkar V, Mane A. Long-term immunogenicity of single dose of live attenuated hepatitis A vaccine in Indian children. Indian Pediatr.　2015; 52:687-90.

10. Faridi MMA, Shah N, Ghosh TK, et al. Immunogenicity and safety of live attenuated hepatitis A vaccine: A multicentric study. Indian Pediatr. 2009;46:29-34.

11. Mitra M, Shah N, Faridi MMA, et al. Long term follow-up study to evaluate immunogenicity and safety of a single dose of live attenuated hepatitis A vaccine in children. Hum Vaccin Immunother. 2015;11:1147-152.

12. Wang XY, Xu ZY, Ma JC, et al. Long term immunogenicity after single and booster dose of a live attenuated hepatitis A vaccine: Results from 8-year follow-up. Vaccine. 2007; 25:446-49.

14. Arankalle V, Mitra M, Bhave S, et al. Changing epidemiology of hepatitis A virus in Indian children. Vaccine: Development and Therapy. 2014;4:7-13.

15. Chen Y, Zhou C-L, Zhang X-J, et al. Immune memory at 17-years of follow up of a single dose of live attenuated hepatitis A vaccine. Vaccine. 2018;36:114-21.