CLINICAL PHARMACOLOGY
Haemophilus influenzae type b (Haemophilus b) was a leading cause of
serious systemic bacterial disease in the United States. Prior to licensure
of Haemophilus b Conjugate Vaccines, it was the most common cause of bacterial
meningitis, accounting for an estimated 12,000 cases annually, primarily among
children under five years of age. The mortality rate was 5%, and neurologic
sequelae were observed in as many as 25%-35% of survivors1. Most
cases of Haemophilus influenzae meningitis among children are caused
by capsular strains of type b, although this capsular type represents only one
of the six types known for this species. In addition to bacterial meningitis,
Haemophilus b is responsible for other invasive diseases, including epiglottitis,
sepsis, cellulitis, septic arthritis, osteomyelitis, pericarditis, and pneumonia.1
In the United States prior to licensure of Haemophilus b Conjugate Vaccines,
approximately one of every 1000 children under five years of age developed systemic
Haemophilus b disease each year, and a child's cumulative risk of developing
systemic Haemophilus b disease at some time during the first five years of life
was approximately one in 200. Attack rates peaked between six months and one
year of age and declined thereafter.1
Approximately 75%-85% of Haemophilus b disease occurs among children less than
24 months of age.2,3,4 Incidence rates of Haemophilus b disease are
increased in certain high-risk groups, such as native Americans (both American
Indian and Eskimos), blacks, individuals of lower socioeconomic status, and
patients with asplenia, sickle cell disease, Hodgkin's disease, and antibody
deficiency syndromes.1,4 Recent studies also have suggested that
the risk of acquiring primary Haemophilus b disease for children under five
years of age appears to be greater for those who attend day-care facilities.5,6,7,8
The potential for person-to-person transmission of the organism among susceptible
individuals has been recognized. Studies of secondary spread of disease in household
contacts of index patients have shown a substantially increased risk among exposed
household contacts under four years of age.9 Adults can be colonized
with Haemophilus influenzaetype b from children infected with 10 the
organism.
In 1974, a randomized controlled trial was conducted in Finland, which allowed
the evaluation of clinical efficacy of a non-conjugated Haemophilus type b polysaccharide
vaccine in children 3 to 71 months of age.11 Approximately 98,000
children, half of whom received the Haemophilus b vaccine, were enrolled in
the field trial and followed for a four-year period for the occurrence of Haemophilus
b disease. Among children 18 to 71 months of age, 90% protective efficacy (95%
confidence limits, 55%-98%) was demonstrated for the four-year follow-up period
in prevention of all forms of invasive Haemophilus b disease.
Based on evidence from this 1974 Finnish efficacy trial, from passive protection
in the infant rat model, and from experience with agammaglobulinemic children,
an antibody concentration of ≥ 0.15 µg/mL has been correlated with protection.11,12,13,14
Antibody levels of ≥ 1 µg/mL were correlated with long-term protection in
three-week post-vaccination serum. Anti-capsular antibodies induced by ProHIBiT (haemophilus b conjugate vaccine) ®
in children 18 months of age and older had bactericidal activity, opsonic activity
and were also active in passive protection assays.15,16,17
The development of stable humoral immunity requires the recognition of foreign
material by at least two separate sets of lymphocytes. These sets are the B-lymphocytes
which are precursors of antibody forming cells, and the T-lymphocytes which
modulate the function of B-cells. Some antigens such as polysaccharides are
capable of stimulating B-cells directly to produce antibody (T-independent).
The responses to many other antigens are augmented by helper T-lymphocytes (T-dependent).18
The manufacturing process utilizes a technology of covalent bonding the capsular
polysaccharide of Haemophilus influenzae type b to diphtheria toxoid,
to produce an antigen which is postulated to convert a T-independent antigen
into a T-dependent antigen.19,20 The protein carries both its own
antigenic determinants and those of the covalently bound polysaccharide. As
a result of the conjugation to protein, the polysaccharide is presented as a
T-dependent antigen resulting in both an enhanced antibody response and an immunologic
memory.
In studies conducted with ProHIBiT (haemophilus b conjugate vaccine) ® in several locations throughout the
US, the antibody responses of 18- to 26-month-old children were measured (Table
1).15 In other studies, the antibody responses to licensed Haemophilus
b polysaccharide vaccines were measured in a comparable age group (Table 1).15
The data shown in Table 1 were obtained from sera tested in one laboratory using
a single radioimmunoassay (RIA). Mean antibody levels induced by ProHIBiT (haemophilus b conjugate vaccine) ®
in children 18 to 20 months of age are 30-fold higher than those induced by
polysaccharide vaccines in the same age group.15
The RIA procedure used by Connaught Laboratories, Inc. to estimate antibody
responses to the Haemophilus b vaccines has been shown to correlate with the
assay used by the Finland National Public Health Institute.21 Antibody
levels ( ≥ 1.0 µg/mL) estimated by the Finnish assay were correlated with protection.11
TABLE 115: Immunogenicity Studies of ProHIBiT (haemophilus b conjugate vaccine) ®
and Polysaccharide Vaccines*
Vaccine |
Age Group |
No. of Subjects |
Anti-Polysaccharide GMT (µg/mL) |
% Subjects Responding with ≥
1.0 µg/mL** |
Pre |
Post |
ProHIBiT® |
15 to 17 Mo. |
43 |
0.017 |
1.12 |
53% |
18 to 21 Mo. |
173 |
0.025 |
2.85 |
75% |
22 to 26 Mo. |
37 |
0.021 |
2.96 |
73% |
POLYSACCHARIDE |
18 to 20 Mo. |
51 |
0.021 |
0.100 |
24% |
24 to 27 Mo. |
84 |
0.035 |
0.520 |
43% |
* Only subjects whose sera had preimmunization
levels ≤ 0.60 µg/mL were included in this analysis.
** A subset of these data was obtained from a randomized comparison of
the two vaccines, in which the percentage of children 18 to 20 months
of age responding with ≥ 1.0 µg/mL was 75% for ProHIBiT (haemophilus b conjugate vaccine) ® (n=12)
and 27% for the polysaccharide (n=11). |
Following immunization of 16 to 24-month-old children with a single dose of
ProHIBiT (haemophilus b conjugate vaccine) ®, 89% (109/123) had antibody levels ≥ 0.15 µg/mL 12 months post-immunization,
compared to 93% one month post-immunization.15
The immunogenicity of ProHIBiT (haemophilus b conjugate vaccine) ® as a booster vaccination administered to
children 12 months of age has been studied in the United States, Finland and
Canada.15
Based on the study conducted by Drs. Edwards and Decker at Vanderbilt University,
it was demonstrated that ProHIBiT (haemophilus b conjugate vaccine) ® induce booster responses in children
immunized with any of four different Hib conjugate vaccines as well as or better
than the homologous vaccine.15
No impairment of the immune response to ProHIBiT (haemophilus b conjugate vaccine) ® was observed in a group
of 36 patients with sickle cell disease (SS, SC, S-thalassemia), aged 1.5 to
5.0 years (mean 3.3 years).15,22,23 Satisfactory immune responses
were obtained following administration of ProHIBiT (haemophilus b conjugate vaccine) ® in children 2 to 6 years
of age with acute leukemia who had been on chemotherapy < 1 year.24
However, similar children with chemotherapy > 1 year frequently failed to
respond to the vaccine.
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months of age. J Pediatr 106:185-189,1985
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with sickle cell diseases. Pediatr 82:571-575,1988
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