Clinical Pharmacology for ActHIB
Mechanism Of Action
Haemophilus influenzae (H. influenzae) is a gram-negative coccobacillus. Most strains of H. influenzae that cause invasive disease (e.g., sepsis and meningitis) are H. influenzae type b.
The response to ActHIB vaccine is typical of a T-dependent immune response to antigens. The prominent isotype of anti-capsular PRP antibody induced by ActHIB vaccine is IgG. (6) A booster response for IgG has been demonstrated in children 12 months of age or older who previously received two or three doses of ActHIB vaccine. Bactericidal activity against H. influenzae type b was demonstrated in serum after immunization and correlated with the anti-PRP antibody response induced by ActHIB vaccine. (1)
Antibody titers to H. influenzae capsular polysaccharide (anti-PRP) of >1.0 mcg/mL following vaccination with unconjugated PRP vaccine correlated with long-term protection against invasive H. influenzae type b disease in children older than 24 months of age. (7) Although the relevance of this threshold to clinical protection after immunization with conjugate vaccines is not known, particularly in light of the induced, immunologic memory, this level continues to be considered as indicative of long-term protection. (8) In clinical studies, ActHIB vaccine induced, on average, anti-PRP levels ≥1.0 mcg/mL in 90% of infants after the primary series (2, 4, and 6 months) and in more than 98% of infants following a booster dose given at 15 to 19 months of age. (1)
Clinical Studies
Immunogenicity Of ActHIB Vaccine In Children 2, 4, And 6 Months Of Age
Two clinical trials supported by the National Institutes of Health (NIH) have compared the anti-PRP antibody responses to three Haemophilus influenzae type b conjugate vaccines in racially mixed populations of children. These studies were done in Tennessee (9) (Table 3) and in Minnesota, Missouri, and Texas (10) (Table 4) in infants immunized with ActHIB vaccine and other Haemophilus influenzae type b conjugate vaccines at 2, 4, and 6 months of age. All Haemophilus influenzae type b conjugate vaccines were administered concomitantly with OPV and whole-cell DTP vaccines at separate sites. Neither OPV nor whole-cell DTP vaccines are licensed or distributed in the US currently.
Table 3: Anti-PRP Antibody Responses Following a Two or Three Dose Series of a Haemophilus influenza type b Vaccine at 2, 4, and 6 Months of Age – Tennessee (9)
| Vaccine | N* | Geometric Mean Concentration (GMC) (mcg/mL) | Post Third Immunization % ≥1.0 mcg/mL |
| PreImmunization at 2 months | Post Second Immunization at 6 months | Post Third Immunization at 7 months |
| PRP-T†(ActHIB vaccine) | 65 | 0.10 | 0.30 | 3.64 | 83% |
| PRP-OMP‡ (PedvaxHIB®) | 64 | 0.11 | 0.84 | N/A | 50%§ |
| HbOC¶ (HibTITER®) | 61 | 0.07 | 0.13 | 3.08 | 75% |
N/A = Not applicable in this comparison trial although third dose data have been published *N = Number of children †Haemophilus influenzae type b Conjugate Vaccine (Tetanus Toxoid Conjugate) ‡Haemophilus influenzae type b Conjugate Vaccine (Meningococcal Protein Conjugate) §Seroconversion after the recommended 2-dose primary immunization series is shown ¶Haemophilus influenzae type b Conjugate Vaccine (Diphtheria CRM 197 Protein Conjugate) |
Table 4: Anti-PRP Antibody Responses Following a Two or Three Dose Series of a Haemophilus influenza type b Vaccine at 2, 4, and 6 Months of Age - Minnesota, Missouri, and Texas (10)
| Vaccine | N* | Geometric Mean Concentration (GMC) (mcg/mL) | Post Third† Immunization % ≥1.0 mcg/mL |
| PreImmunization at 2 months | Post Second Immunization at 6 months | Post Third† Immunization at 7 months |
| PRP-T‡ (ActHIB vaccine) | 142 | 0.25 | 1.25 | 6.37 | 97% |
| PRP-OMP§ (PedvaxHIB) | 149 | 0.18 | 4.00 | N/A | 85%¶ |
| HbOC# (HibTITER) | 167 | 0.17 | 0.45 | 6.31 | 90% |
N/A = Not applicable in this comparison trial although third dose data have been published (10) *N = Number of children †Sera were obtained after the third dose from 86 and 110 infants, in PRP-T and HbOC vaccine groups, respectively ‡Haemophilus influenzae type b Conjugate Vaccine (Tetanus Toxoid Conjugate) §Haemophilus influenzae type b Conjugate Vaccine (Meningococcal Protein Conjugate) ¶Seroconversion after the recommended 2-dose primary immunization series is shown #Haemophilus influenzae type b Conjugate Vaccine (Diphtheria CRM 197 Protein Conjugate) |
Native American populations have had high rates of H. influenzae type b disease and have been observed to have low immune responses to Haemophilus influenzae type b conjugate vaccines. In a clinical study enrolling Alaskan Native Americans, following the administration of a three-dose series of ActHIB vaccine at 6 weeks, 4 months, and 6 months of age, 75% of subjects achieved an anti-PRP antibody titer of ≥1.0 mcg/mL at 7 months of age (1 month after the last vaccination). (11)
Immunogenicity Of ActHIB Vaccine In Children 12 To 24 Months Of Age
In four separate studies, children 12 to 24 months of age who had not previously received Haemophilus influenzae type b conjugate vaccination were immunized with a single dose of ActHIB vaccine (Table 5). Geometric Mean Concentration (GMC) of anti-PRP antibody responses were 5.12 mcg/mL (90% responding with ≥1.0 mcg/mL) for children 12 to 15 months of age and 4.4 mcg/mL (82% responding with ≥1.0 mcg/mL) for children 17 to 24 months of age. (2)
Table 5: Anti-PRP Antibody Responses in 12- to 24-month-old Children Immunized with a Single Dose of ActHIB
| Age Group | N* | Geometric Mean Concentration (GMC) (mcg/mL) | % Subjects With >1.0 mcg/mL |
| Pre-Immunization | Post-Immunization' | Pre-Immunization | Post-Immunization† |
| 12 to 15 months | 256 | 0.06 | 5.12 | 1.6 | 90.2 |
| 17 to 24 months | 81 | 0.10 | 4.40 | 3.7 | 81.5 |
* N = Number of children †Post immunization responses measured at approximately 1 month after vaccination |
ActHIB vaccine has been found to be immunogenic in children with sickle cell anemia, a condition that may cause increased susceptibility to Haemophilus influenzae type b disease. Following two doses of ActHIB vaccine given at two-month intervals, 89% of these children (mean age 11 months) had anti-PRP antibody titers of ≥1.0 mcg/mL. This is comparable to anti-PRP antibody levels demonstrated in children without sickle-cell anemia of similar age following two doses of ActHIB vaccine. (12)
REFERENCES
1. Data on file, Sanofi Pasteur SA.
2. Data on file, Sanofi Pasteur Inc.
6. Holmes SJ, et al. Immunogenicity of four Haemophilus influenzae type b conjugate vaccines in 17- to 19-month-old children. J Pediatr 118:364-371, 1991.
7. Peltola H, et al. Prevention of Haemophilus influenzae type b bacteremic infections with the capsular polysaccharide vaccine. N Engl J Med 310:1561-1566, 1984.
8. Recommendations of the Immunization Practices Advisory Committee (ACIP). Haemophilus b conjugate vaccines for prevention of Haemophilus influenzae type b disease among infants and children two months of age and older. MMWR 40:No. RR-1, 1991.
9. Decker MD, et al. Comparative trial in infants of four conjugate Haemophilus influenzae type b vaccines. J Pediatr 120:184- 189, 1992.
10. Granoff DM, et al. Differences in the immunogenicity of three Haemophilus influenzae type b conjugate vaccines in infants. J Pediatr 121:187-194, 1992.
11. Bulkow LR, et al. Comparative immunogenicity of four Haemophilus influenzae type b conjugate vaccines in Alaska Native infants. Pediatr Infect Dis J 12:484-92, 993.
12. Kaplan SL, et al. Immunogenicity of Haemophilus influenzae type b polysaccharide-tetanus protein conjugate vaccine in children with sickle hemoglobinopathy or malignancies, and after systemic Haemophilus influenzae type b infection. J Pediatr 120:367-370, 1992.