CLINICAL PHARMACOLOGY
Prior to the introduction of Haemophilus b Conjugate Vaccines, Haemophilus
influenzae type b (Hib) was the most frequent cause of bacterial meningitis
and a leading cause of serious, systemic bacterial disease in young children
worldwide.1,2,3,4
Hib disease occurred primarily in children under 5 years of age in the United
States prior to the initiation of a vaccine program and was estimated to account
for nearly 20,000 cases of invasive infections annually, approximately 12,000
of which were meningitis. The mortality rate from Hib meningitis is about 5%.
In addition, up to 35% of survivors develop neurologic sequelae including seizures,
deafness, and mental retardation.5,6 Other invasive diseases caused
by this bacterium include cellulitis, epiglottitis, sepsis, pneumonia, septic
arthritis, osteomyelitis and pericarditis.
Prior to the introduction of the vaccine, it was estimated that 17% of all
cases of Hib disease occurred in infants less than 6 months of age.7
The peak incidence of Hib meningitis occurs between 6 to 11 months of age. Forty-seven
percent of all cases occur by one year of age with the remaining 53% of cases
occurring over the next four years.2,20
Among children under 5 years of age, the risk of invasive Hib disease is increased in certain populations including the following:
- Daycare attendees8,9
- Lower socio-economic groups10
- Blacks11 (especially those who lack the Km(1) immunoglobulin
allotype)12
- Caucasians who lack the G2m(n or 23) immunoglobulin allotype13
- Native Americans14,15,16
- Household contacts of cases17
- Individuals with asplenia, sickle cell disease, or antibody deficiency
syndromes18,19
An important virulence factor of the Hib bacterium is its polysaccharide capsule
(PRP). Antibody to PRP (anti-PRP) has been shown to correlate with protection
against Hib disease.3,21 While the anti-PRP level associated with
protection using conjugated vaccines has not yet been determined, the level
of anti-PRP associated with protection in studies using bacterial polysaccharide
immune globulin or nonconjugated PRP vaccines ranged from > 0.15 to > 1.0
mcg/mL.22-28
Nonconjugated PRP vaccines are capable of stimulating B-lymphocytes to produce
antibody without the help of T-lymphocytes (T-independent). The responses to
many other antigens are augmented by helper T-lymphocytes (T-dependent). PedvaxHIB
is a PRP-conjugate vaccine in which the PRP is covalently bound to the OMPC
carrier29 producing an antigen which is postulated to convert the
T-independent antigen (PRP alone) into a T-dependent antigen resulting in both
an enhanced antibody response and immunologic memory.
Clinical Evaluation of PedvaxHIB
PedvaxHIB, in a lyophilized formulation (lyophilized PedvaxHIB), was initially
evaluated in 3,486 Native American (Navajo) infants, who completed the primary
two-dose regimen in a randomized, double-blind, placebo-controlled study (The
Protective Efficacy Study). At the time of the study, this population had a
much higher incidence of Hib disease than the United States population as a
whole and also had a lower antibody response to Haemophilus b Conjugate Vaccines,
including PedvaxHIB.14,15,16,30,33
Each infant in this study received two doses of either placebo or lyophilized PedvaxHIB with the first dose administered at a mean of 8 weeks of age and the second administered approximately two months later; DTP and OPV were administered concomitantly. Antibody levels were measured in a subset of each group (TABLE 1).
TABLE 1
Antibody Responses in Navajo Infants
Vaccine |
No. of
Subjects |
Time |
% Subjects with |
Anti-PRP GMT
(mcg/mL) |
> 0.15 mcg/mL |
> 1.0 mcg/mL |
Lyophilized
PedvaxHIB* |
416** |
Pre-Vaccination |
44 |
10 |
0.16 |
416 |
Post-Dose 1 |
88 |
52 |
0.95 |
416 |
Post-Dose 2 |
91 |
60 |
1.43 |
Placebo* |
461** |
Pre-Vaccination |
44 |
9 |
0.16 |
461 |
Post-Dose 1 |
21 |
2 |
0.09 |
461 |
Post-Dose 2 |
14 |
1 |
0.08 |
Lyophilized
PedvaxHIB |
27† |
Prebooster |
70 |
33 |
0.51 |
27 |
Postbooster‡ |
100 |
89 |
8.39 |
* Post-Vaccination values obtained approximately 1-3 months
after each dose.
** The Protective Efficacy Study
† Immunogenicity Trial34
‡ Booster given at 12 months of age; Post-Vaccination values obtained 1
month after administration of booster dose. |
Most subjects were initially followed until 15 to 18 months of age. During this time, 22 cases of invasive Hib disease occurred in the placebo group (8 cases after the first dose and 14 cases after the second dose) and only 1 case in the vaccine group (none after the first dose and 1 after the second dose). Following the primary two-dose regimen, the protective efficacy of lyophilized PedvaxHIB was calculated to be 93% with a 95% confidence interval of 57%-98% (p=0.001, two- tailed). In the two months between the first and second doses, the difference in number of cases of disease between placebo and vaccine recipients (8 vs. 0 cases, respectively) was statistically significant (p=0.008, two-tailed); however, a primary two-dose regimen is required for infants 2-14 months of age.
At termination of the study, placebo recipients were offered vaccine. All original
participants were then followed two years and nine months from termination of
the study. During this extended follow-up, invasive Hib disease occurred in
an additional seven of the original placebo recipients prior to receiving vaccine
and in one of the original vaccine recipients (who had received only one dose
of vaccine). No cases of invasive Hib disease were observed in placebo recipients
after they received at least one dose of vaccine. Efficacy for this follow-up
period, estimated from person- days at risk, was 96.6% (95 C.I., 72.2-99.9%)
in children under 18 months of age and 100% (95 C.I., 23.5-100%) in children
over 18 months of age.33
Since protective efficacy with lyophilized PedvaxHIB was demonstrated in such a high risk population, it would be expected to be predictive of efficacy in other populations.
The safety and immunogenicity of lyophilized PedvaxHIB were evaluated in infants
and children in other clinical studies that were conducted in various locations
throughout the United States. PedvaxHIB was highly immunogenic in all age groups
studied.31,32
Lyophilized PedvaxHIB induced antibody levels greater than 1.0 mcg/mL in children
who were poor responders to nonconjugated PRP vaccines. In a study involving
such a subpopulation,33,34 34 children ranging in age from 27 to
61 months who developed invasive Hib disease despite previous vaccination with
nonconjugated PRP vaccines were randomly assigned to 2 groups. One group (n=14)
was vaccinated with lyophilized PedvaxHIB and the other group (n=20) with a
nonconjugated PRP vaccine at a mean interval of approximately 12 months after
recovery from disease. All 14 children vaccinated with lyophilized PedvaxHIB
but only 6 of 20 children re- vaccinated with a nonconjugated PRP vaccine achieved
an antibody level of > 1.0 mcg/mL. The 14 children who had not responded to
revaccination with the nonconjugated PRP vaccine were then vaccinated with a
single dose of lyophilized PedvaxHIB; following this vaccination, all achieved
antibody levels of > 1.0 mcg/mL.
In addition, lyophilized PedvaxHIB has been studied in children at high risk of Hib disease because of genetically-related deficiencies [Blacks who were Km(1) allotype negative and Caucasians who were G2m(23) allotype negative] and are considered hyporesponsive to nonconjugated PRP vaccines on this basis.35 The hyporesponsive children had anti-PRP responses comparable to those of allotype positive children of similar age range when vaccinated with lyophilized PedvaxHIB. All children achieved anti-PRP levels of > 1.0 mcg/mL.
The safety and immunogenicity of Liquid PedvaxHIB were compared with those of lyophilized PedvaxHIB in a randomized clinical study involving 903 infants 2 to 6 months of age from the general U.S. population. DTP and OPV were administered concomitantly to most subjects. The antibody responses induced by each formulation of PedvaxHIB were similar. TABLE 2 shows antibody responses from this clinical study in subjects who received their first dose at 2 to 3 months of age.
TABLE 2
Antibody Responses to Liquid and Lyophilized PedvaxHIB in Infants From the General
U.S. Population
Formulation |
Age
(Months) |
Time |
No. of
Subjects |
% Subjects with anti-PRP |
Anti-PRP GMT
(mcg/mL) |
> 0.15 mcg/mL |
> 1.0 mcg/mL |
|
|
Pre-Vaccination |
487 |
32 |
7 |
0.12 |
Liquid |
2-3 |
Post-Dose 1* |
480 |
94 |
64 |
1.55 |
PedvaxHIB |
|
Post-Dose 2** |
393 |
97 |
80 |
3.22 |
(7.5 mcg PRP) |
12-15 |
Prebooster |
284 |
80 |
30 |
0.49 |
|
|
Postbooster** |
284 |
99 |
95 |
10.23 |
|
24† |
Persistence |
94 |
97 |
55 |
1.29 |
|
|
Pre-Vaccination |
171 |
37 |
6 |
0.13 |
Lyophilized |
2-3 |
Post-Dose 1* |
169 |
97 |
72 |
1.88 |
PedvaxHIB |
|
Post-Dose 2** |
133 |
99 |
81 |
2.69 |
(15 mcg PRP) |
12-15 |
Prebooster |
87 |
71 |
28 |
0.39 |
|
|
Postbooster** |
87 |
99 |
91 |
7.64 |
|
24† |
Persistence |
37 |
97 |
54 |
1.10 |
* Approximately two months Post-Vaccination
** Approximately one month Post-Vaccination
† Approximately |
A booster dose of PedvaxHIB is required in infants who complete the primary
two-dose regimen before 12 months of age. This booster dose will help maintain
antibody levels during the first two years of life when children are at highest
risk for invasive Hib disease. (See TABLE 2 and DOSAGE
AND ADMINISTRATION.)
In four United States studies, antibody responses to lyophilized PedvaxHIB
were evaluated in several subpopulations of infants initially vaccinated between
2 to 3 months of age. (See TABLE 3.)
TABLE 3
Antibody Responses* After Two Doses of Lyophilized PedvaxHIB Among Infants Initially
Vaccinated at 2-3 Months of Age By Racial/Ethnic Group
Racial/Ethnic
Groups |
No. of Subjects |
LYOPHILIZED |
Anti-PRP GMT (mcg/mL) |
% Subjects With Anti-PRP |
> 0.15 mcg/mL |
> 1.0 mcg/mL |
Native American† |
54 |
96 |
70 |
2.47 |
Caucasian |
201 |
99 |
82 |
3.52 |
Hispanic |
76 |
99 |
88 |
3.54 |
Black |
23 |
100 |
96 |
5.40 |
* One month after the second dose
† Apache and Navajo |
In two United States studies, antibody responses to Liquid PedvaxHIB were evaluated
in several subpopulations of infants initially vaccinated between 2 to 3 months
of age. (See TABLE 4.)
TABLE 4
Antibody Responses* After Two Doses of Liquid PedvaxHIB Among Infants Initially
Vaccinated at 2-3 Months of Age By Racial/Ethnic Group
Racial/Ethnic Groups |
No. of
Subjects |
LIQUID |
Anti-PRP GMT
(mcg/mL) |
% Subjects With Anti-PRP |
> 0.15 mcg/mL |
> 1.0 mcg/mL |
Native American** |
90 |
97 |
78 |
2.76 |
Caucasian |
143 |
94 |
72 |
2.16 |
Hispanic |
184 |
98 |
85 |
4.34 |
Black |
18 |
100 |
94 |
7.58 |
* One month after the second dose
** Apache and Navajo |
Antibodies to the OMPC of N. meningitidis have been demonstrated in
vaccinee sera, but the clinical relevance of these antibodies has not been established.33
Interchangeability of Licensed Haemophilus b Conjugate Vaccines and PedvaxHIB
Published studies have examined the interchangeability of other licensed Haemophilus
b Conjugate Vaccines and PedvaxHIB.42,43,44,45,52 According to the
American Academy of Pediatrics, excellent immune responses have been achieved
when different vaccines have been interchanged in the primary series. If PedvaxHIB
is given in a series with one of the other products licensed for infants, the
recommended number of doses to complete the series is determined by the other
product and not by PedvaxHIB. PedvaxHIB may be interchanged with other licensed
Haemophilus b Conjugate Vaccines for the booster dose.52
Use with Other Vaccines
Results from clinical studies indicate that Liquid PedvaxHIB can be administered
concomitantly with DTP, OPV, eIPV (enhanced inactivated poliovirus vaccine),
VARIVAX* [Varicella Virus Vaccine Live (Oka/Merck)], M-M-R* II (Measles, Mumps,
and Rubella Virus Vaccine Live) or RECOMBIVAX HB* [Hepatitis B Vaccine (Recombinant)].33
No impairment of immune response to individual tested vaccine antigens was demonstrated.
The type, frequency and severity of adverse experiences observed in these studies with PedvaxHIB were similar to those seen when the other vaccines were given alone.
In addition, a PRP-OMPC-containing product, COMVAX* [Haemophilus b Conjugate
(Meningococcal Protein Conjugate) and Hepatitis B (Recombinant) Vaccine], was
given concomitantly with a booster dose of DTaP [diphtheria, tetanus, acellular
pertussis] at approximately 15 months of age, using separate sites and syringes
for injectable vaccines. No impairment of immune response to these individually
tested vaccine antigens was demonstrated. COMVAX has also been administered
concomitantly with the primary series of DTaP to a limited number of infants.
PRP antibody responses are satisfactory for COMVAX, but immune responses are
currently unavailable for DTaP (see Manufacturer's Product Circular for COMVAX).
No serious vaccine-related adverse events were reported.33
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