hepatitis B recombinant vaccines are given as a three-dose series. This consists of two priming doses given 1 month apart; this is followed by a third dose given 6 months after the first one (Centers for Disease Control, 1990). An alternative schedule, consisting of three priming doses at 1-month intervals and then a fourth dose 12 months after the first one, is approved for one vaccine (SK-RIT). The priming doses induce detectable antibody to HBsAg in 70-85 percent of healthy adults and children, but they are of relatively low titer. The final dose induces adequate high-titer antibody in more than 90 percent of healthy adults under the age of 50 and 95 percent of children and infants (100-3,000 IU/liter in adults and >5,000 IU/liter in children). The immunogenicity and safety of hepatitis B vaccine in premature infants are less well defined (Lau et al., 1992). Studies show seroconversion rates similar to those observed with the plasma-derived vaccine licensed for use in the United States (Andre and Safary, 1989; McLean et al., 1983; Zajac et al., 1986).

Factors affecting the antibody response to recombinant vaccine include vaccine type and handling, timing of doses, and site of injection. Freezing of the vaccine during shipment or excessive heat may reduce its potency. The deltoid muscle is the preferred site for vaccination, and it is now clear that gluteal injection may decrease the response to the vaccine by as much as 50 percent (Shaw et al, 1989). The anterolateral thigh is the preferred site of vaccine injection in infants. Recombinant vaccine has decreased immunogenicity compared with that of plasma-derived vaccine when the vaccine is administered by the intradermal route, so this route of administration is not recommended by the Centers for Disease Control and Prevention. Factors that do not affect the response include simultaneous administration with hepatitis B immune globulin and with other vaccines, including diphtheria and tetanus toxoids and pertussis vaccine (DPT) (Coursaget et al., 1986).

Age is an important factor affecting the immune response (Andre, 1989; Shaw et al., 1989). The maximal response is in children (ages 2-19 years); this is followed by equivalent responses in young adults and infants (West et al, 1990). The poorest response is in older adults, beginning in the sixth decade of life, and only 50 to 70 percent of adults over age 60 have satisfactory antibody responses. The age-related decrease in immune response is significantly greater in men than in women. The response is diminished in persons with immunosuppressive illnesses, including renal failure and HIV infection. Both higher-titer vaccine and increased numbers of doses are required to achieve a 70 percent response in patients who are on hemodialysis (Centers for Disease Control, 1990).

More than 50 trials of plasma-derived vaccine are reported in the literature. These trials were conducted in nearly half that number of countries and have included the vaccination of more than 100,000 individuals (Beasley



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