Update on Hepatitis A Vaccines
By William S. Kammerer, MD
Merck & co., inc., has introduced a new hepa-titis A vaccine, VAQTA, which follows upon the introduction of Havrix (SmithKline Beecham). Both are formalin killed, attenuated strains of hepatitis A virus grown in human diploid cell fibroblasts, purified and absorbed onto aluminum hydroxide. Dosing schedules are similar, although VAQTA offers an extended period for the booster dose in children 2-17 years old, providing a little more flexibility. On the other hand, the booster dose for adults is fixed at six months for VAQTA, compared to 6-12 months for Havrix. (See Table.) The average cost to the pharmacist is approximately the same for both products. Efficacy as proven in large-scale field trials appears equivalent, with more than 95% of recipients seroconverting by four weeks after the first dose.1,2
Table
Schedule for Havrix and VAQTA booster doses
| Age 2-17 | Age > 17 | |
| Havrix | 720 E1.U (0.5 mL) at | 1440 E1.U (1.0 mL) |
| 6-12 months | at 6-12 months | |
| VAQTA | 25U of hepatitis A virus | 50U hepatitis A virus |
| antigen (0.5 mL) at | antigen (1.0 mL) at | |
| 6-18 months | 6 months |
Seroconversion after only two weeks is considerably less with both. Adverse reactions are mild in each case, although no data are available regarding safety in pregnancy. There are no data available to indicate that if one starts with one vaccine that the other could be used for the booster dose. Both can be given simultaneously with other vaccines without an increase in adverse reactions or diminished effectiveness.3 When given simultaneously with gamma globulin, a decreased, but still effective, titer of protective antibody results with both products. Thus, at initial marketing, there appears to be no significant difference between the two in terms of safety or efficacy. Havrix offers more flexibility in terms of the timing of the booster dose for adults and VAQTA for the booster dose for children. Time will tell.
Hepatitis A develops in up to 4% of susceptible travelers to developing countries.4 In those on extended hiking trips through high-risk countries (e.g., India, Southeast Asia), the incidence may exceed 20%. Many of these cases may be asymptomatic, but up to 20% could be severe and require hospitalization. In children under the age of 2 years, the infection usually passes unnoticed; in those 3-8 years old, it is usually mild. Fifty percent of North American and European travelers over age 50 (i.e., born before 1945) are found to be immune. Which travelers, then, should be offered hepatitis A vaccine? An analysis by Tormans and colleagues suggests that those under 50 planning two or more trips of 25 days or more to developing countries in the succeeding 10 years would benefit from the hepatitis A vaccine.5 For one trip of less than 25 days, gamma globulin (0.02 mL/kg) would be a more economical, albeit slightly less effective, option. For travelers older than 50, the type of travel and countries to be visited would need to be considered individually. Pretravel serologic testing for hepatitis A IgG antibody could be considered in such cases. (Dr. Kammerer is Associate Professor of Medicine, Mayo School of Medicine; Senior Associate Consultant, Mayo Clinic, Jacksonville, FL.)
References
1. Werzberger A, et al. Anatomy of a trial: A historical view of the Monroe inactivated hepatitis A protective efficacy trial. J Hepatol 1993;18 (Suppl 2):S46-S50.
2. Van Damme P, et al. Safety and immunogenicity of a high-potency inactivated hepatitis A vaccine. J Trav Med 1996;l3:83-90.
3. Krupenbacher J, et al. Co-administration of an inactivated hepatitis A vaccine with other travelers vaccines: Interference with the immune response (Abstracts). In: Proceedings of the 34th Interscience Conference on Antimicrobial Agents and Chemotherapy. Washington, DC: American Society of Microbiologists; 1994:255.
4. Steffen R. Risk of hepatitis A in travelers. Vaccine 1992;10 (Suppl 1):69-72.
5. Tormans G, et al. Recommendations for prevention of hepatitis A based on a cost-effectiveness analysis. J Trav Med 1994;1:127-135.
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