Description: Hepatitis A vaccine is used to confer immunity against hepatitis A in persons at risk. Hepatitis A vaccine is not effective in the prevention of other forms of hepatitis, such as hepatitis B, hepatitis C, or hepatitis E. There are currently two different products available; both require the administration of a series of two doses, although substantial protection is afforded by the initial (i.e., primary) dose. Both products utilize inactivated whole virus propagated in MRC5 human diploid cells and are administered IM. Havrix® contains no human blood or plasma-derived components. Clinical data reveal Havrix® induced an immune response in 97% of those immunized following a single dose of 720 EL.U. After a second dose there was a 100% seroconversion rate. Havrix® received FDA approval on February 22, 1995. VAQTA® was approved March 29, 1996.
Mechanism of Action: Injection of hepatitis A vaccine produces antibodies that confer protection against hepatitis A infection. Stimulation of specific antibodies takes place without producing any disease symptoms. During the course of natural infection with the hepatitis A virus, the initial antibody response is predominantly of the IgM class. This response lasts for several months, but during convalescence antibodies of the IgG class become dominant. Patients with anti-HAV of the IgG class are immune to reinfection. The IgG antibodies remain detectable indefinitely. Two years after immunization with hepatitis A vaccine IgG levels remained relatively high in the serum of immunized patients. The duration of protection from a course of hepatitis A vaccine is as yet unknown. Long term follow-up studies will determine the necessity for booster doses of HAV.
Pharmacokinetics: Both hepatitis A vaccine products (e.g., Havrix®, VAQTA®) are administered by intramuscular injection. Rapid seroconversion from a single-dose can provide protection against hepatitis A for at least 12 months. Increasing the dose of viral antigen directly affects the speed at which seroconversion occurs. However, a primary response to the vaccine can be expected 8?10 days after administration. To maintain the highest antibody titers a booster dose is recommended between 6 and 12 months after the initial dose. The response to this booster dose is vigorous and increases the protection time against hepatitis A. Some investigators have postulated a minimum level of protective antibody concentration at 10 mIU/ml. However, a concentration of 20 mIU/ml of anti-HAV antibodies was recognized as the definition of seroconversion in clinical trials. Repoductive studies have not been undertaken with hepatitis A vaccine. It is not known whether the vaccine will cause any fetal damage or if it is distributed into breast milk.
[ Revised 7/1/2002 ]