IMMUNOGENICITY
SUPPLEMENT ARTICLE
Bridging Phase 2 and Phase 3 Pneumococcal
Immunologic Data for Future Combination
Vaccines
Helena Käyhty and Heidi Åhman
National Public Health Institute, Helsinki, Finland
Pneumococcal conjugate vaccines (PncCs) will be introduced into childhood vaccination programs now that
the first PncC has been licensed for use. The next generation of PncCs and possible combination vaccines
containing PncC will most probably be approved on the basis of phase 2 immunogenicity and safety data.
PncCs are combination vaccines that include, at present, 7–11 components. Immune response to different
components may vary. Furthermore, there seem to be population-based differences in immune response.
Whether these differences are due to the other vaccines that are given simultaneously or due to the genetic
background remains to be seen. Immune response can be evaluated by determining both the quantity and
the quality of antibodies after vaccination. However, data are still missing on the minimal protective immune
response and serologic correlates or surrogates of protection.
The first pneumococcal conjugate vaccine (PncC) has
just been licensed [1] and will be gradually introduced
into the infant vaccination programs of industrialized
countries. Among the alternatives for decreasing the
number of visits for vaccinations and the number of
injections are simultaneous administration of PncCs
with other childhood vaccines or combination vaccines
containing PncC and variable numbers of other routinely-given vaccines for infant immunization. However, PncCs are themselves combination vaccines containing 7–11 pnemococcal capsular polysaccharides
conjugated to 1 or 2 carrier proteins. Knowledge about
the characteristics of protective immune response after
PncC vaccination and about how other childhood vaccines influence the response to the PncCs, and vice
versa, is urgently needed but is still sparse.
PncCs
There are at present published data on clinical phase 2
and phase 3 trials of PncCs from Wyeth Lederle Vaccines and Pediatrics (the vaccines PncCRM and Prevnar), Merck (PncOMPC), and Aventis Pasteur (PncT,
PncD, and PncD/T; table 1). The conjugates have been
made on the basis of the experience gained from Haemophilus influenzae type b (Hib) conjugate vaccines [2].
In addition, the Rijksinstitut voor Volksgezondheit en
Milieu (Bilthoven, The Netherlands) has prepared a 4valent PncC for the Dutch Nordic Consortium (M.
Beurret and P. Hoogerhout, personal communication),
which has been tested in phase 1 studies in Finland (S.
Rapola, personal communication), and a PncC prepared by Glaxo SmithKline, which uses an alternative
carrier, has been tested in clinical trials [3].
CLINICAL EFFICACY OF PncCs
Reprints or correspondence: Dr. Helena Käyhty, Vaccine Immunology Laboratory,
National Public Health Institute, Mannerheinintie 166, 00300 Helsinki, Finland
([email protected]).
Clinical Infectious Diseases 2001; 33(Suppl 4):S292–8
2001 by the Infectious Diseases Society of America. All rights reserved.
1058-4838/2001/3312S4-0006$03.00
S292 • CID 2001:33 (Suppl 4) • Käyhty and Åhman
PncCs have proven immunogenic in infants [4–23].
The first clinical efficacy trials with PncCRM have been
successful, and 7-valent PncCRM prevented 97.4%
(95% CI, 82.7%–99.9%) of invasive vaccine–type pneu-
Table 1.
Pneumococcal conjugate vaccines used in phase 2 and phase 3 clinical trials.
Vaccine
Carrier
Serotypes included
Manufacturer
PncOMPC
Outer membrane protein complex of
Neisseria meningitidis
6B, 14, 19F, 23F; or 4, 6B, 9V, 14,
18C, 19F, 23F
Merck
PncCRM
CRM197, nontoxic mutant variant of
diphtheria toxin
4, 6B, 9V, 14, 18C, 19F, 23F; or 1, 4, 5,
6B, 9V, 14, 18C, 19F, 23F
Wyeth Lederle Vaccines
and Pediatrics
PncT
Tetanus toxoid
6B, 14, 19F, 23F; or 3, 4, 6B, 9V, 14,
18C, 19F, 23F
Aventis Pasteur
PncD
Diphtheria toxoid
6B, 14, 19F, 23F; or 3, 4, 6B, 9V, 14,
18C, 19F, 23F
Aventis Pasteur
PncD/T
Tetanus and diphtheria toxoids
1, 3, 4, 5, 6B, 7F, 9V, 14, 18C, 19F, 23F
Aventis Pasteur
mococcal infections in the United States [23, 24]. In the Finnish
study, the same vaccine prevented 57% (95% CI, 44%–67%)
of acute otitis media (AOM) episodes caused by the vaccine
serotypes; 34% of all pneumococcal AOM episodes; and 6%
of all AOM episodes, regardless of etiology [25]. The efficacy
varied by serotype [26]. In the US study, 33% of radiologically
confirmed cases of pneumonia and 11% of clinically diagnosed
cases of pneumonia were prevented. The efficacy of the vaccine
for cases of pneumonia with concurrent bacteremia was 90%
[24].
Studies with Hib conjugate vaccines suggest that the prevention of nasopharyngeal carriage is important for induction
of herd immunity; therefore, carriage of pneumococci after
PncC vaccination has already been studied in many different
settings [14, 27–30]. A decrease in the carriage has been reported, but the effect of PncCs has been much less prominent
that the effect of Hib conjugate vaccines [31].
EVALUATION OF THE IMMUNOGENICITY OF
PncCs
The same parameters that have been listed as markers of satisfactory immunogenicity of Hib conjugate vaccines [32] can
be applied to PncCs. They are as follows:
1. The overall immunogenicity of the vaccines is judged
on the basis of the IgG antibody response, as measured by EIA,
during and after the primary course in infancy.
2. Persistence of antibodies until the booster dose is given.
3. The induction of immunologic memory.
4. Isotype or IgG subclass distribution.
5. Functional activity of antibodies.
IgG ANTIPNEMOCOCCAL POLYSACCHARIDE
CONCENTRATIONS IN INFANCY
IgG antibody response induced by different conjugates. Published data that can be used to compare immune response to
different PncCs in infants come mainly from the Finnish trials.
These separate, consecutive phase 2 studies were not originally
planned to be comparative, but they used the same serologic
methodology, population, and schedule, which allows comparisons on the basis of separately published data. The vaccines
used in Finland have ranged from 4- valent to 11-valent [4–8,
12, 20]. Different vaccines induce very consistent IgG response
to some of the serotypes (e.g., for type 6B, there are no marked
differences in either the kinetics or the magnitude of the response; figure 1). However, for some serotypes, quite marked
vaccine-specific differences have been found (e.g., different experimental lots of PncCs induced a large variation in the response to types 14 and 19F; figure 1).
IgG antibody response to different serotypes. PncCs are
combination vaccines that contain 7–11 pneumococcal polysaccharide–carrier protein conjugates. Generally, a patient’s response to any of the serotypes included in the PncCs has Tcell dependent characteristics that are higher than the response
of a patient of the same age to pneumococcal polysaccharide
vaccine, and the PncC can be boosted after the primary course.
However, there are clear type-specific characteristics. Figure 2
and figure 3 show examples of type-specific responses to
PncCRM [8]. A significant increase in anti-6B IgG can be seen
only after the third dose, which is given at 6 months; however,
the booster response is fairly high. For type 4, there is a marked
response already after the first dose, and further increase after
the second dose, but not after the third dose. There is a booster
response but the fold increase in IgG is lower than for type 6B.
Type 19F induces antibody response already after the second
dose, and the third dose does not increase the mean antibody
concentrations. The concentrations decline sharply in months
7-15 after vaccination. Again, the booster induces higher concentrations than does the primary immunization.
Population-based differences in the antibody response to
PncCs. PncCRM vaccine containing 7 or 9 serotypes has been
used now in the United States [9, 11, 23], Finland [8, 20], the
United Kingdom [15], and South Africa [14]. In the United
States and Finland, the vaccine was given to infants at ages 2,
4, and 6 months, simultaneously with combined diphtheria–tetanus–whole cell pertussis and Hib oligosaccharide
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(!2-fold difference in the geometric mean concentrations) in
both groups.
INDUCTION OF IMMUNOLOGIC MEMORY
Figure 1. Concentrations of antibodies to pneumococcal polysaccharide (Pnc PS) types 6B, 14, and 19F in Finnish infants aged 7 months
who were immunized at 2, 4, and 6 months with different pneumococcal
conjugate vaccines. The vaccines are as mentioned in table 1; the numbers
in the parentheses indicate the valences of the vaccines [4, 6–8, 12].
CRM197 (DTwP-HbOC) and polio vaccines. In the South African study, the Expanded Program for Immunization schedule
of vaccination at ages 6, 10, and 14 weeks was used, and the
simultaneously given vaccines were DTwP-HbOC, oral poliovirus vaccine, and hepatitis B vaccine (Hep B). In the United
Kingdom, the schedule of vaccinations at ages 2, 3, and 4
months was used, and DTwP-HbOC was given simultaneously.
In the United States, the United Kingdom, and in the South
African study, the antipnemococcal polysaccharide concentrations were measured in the vaccine manufacturer’s laboratory
(Wyeth Lederle Vaccines and Pediatrics), whereas the measurements for the Finnish study were performed at the National
Public Health Institute (KTL), Helsinki, Finland. For some serotypes, the variation in response between trials was quite large
(figure 3). The most marked variation was noted for serotypes
4, 6B, 18C, and 19F.
The 11-valent PncD/T vaccine has now been used in 2 studies, and the preliminary data are available [12, 21]. In the first
study, the aluminum adjuvanted PncD/T was given to Finnish
and Israeli infants at ages 2, 4, and 6 months, together with
DTwP–polyribosyl ribitol phosphate tetanus (PRP-T) and polio
vaccines. In the second study, Filipino infants received the same
PncD/T at the EPI schedule of ages 6, 10, and 14 weeks, simultaneously with other EPI vaccines and PRP-T. In both studies, the antibody response was measured at KTL, Finland. The
data from Finland and Israel were combined; there were no
significant differences between the 2 populations. After the primary course, the IgG concentrations to serotypes 1, 4, 5, and
18C were markedly higher (14-fold difference in the geometric
mean concentrations) in the Filipino infants [21] than in the
Finnish or Israeli infants [12]. However, the mean concentrations to serotypes 3, 6B, 14, 19F and 23F were fairly similar
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The experience with the Hib conjugate vaccines suggests that
the development of immunologic memory is important for
protection. The same is believed to be true for the PncCs.
However, the recent appearance of Hib conjugate vaccine failures in Alaska suggest that sustaining antibody levels might also
be needed for protection in high-risk populations [33].
There are no direct, easy-to-perform tests to measure the
induction of memory cells. Instead, antibody concentration and
avidity measurements have been used as indirect indicators of
memory induction. In many studies, the test booster vaccination has been done with a pneumococcal polysaccharide vaccine [6–8, 10, 17–19], which is believed to best mimic the
response upon contact with pneumococcus. The response of
primed and unprimed children of the same age can be compared. This kind of comparison (e.g., in the studies of Anderson
et al. [17] and Obaro et al. [18]) show clearly that primed
children respond to the pneumococcal polysaccharide booster
with a high antibody response, whereas unprimed children have
low or no response. This speaks for existence of memory B
cells that can be triggered for antibody production by pneumococcal polysaccharide (e.g., upon nasopharyngeal acquisition of pneumococci).
One feature of a memory response is the maturation of antibody avidity. There are differences in the avidity maturation
after vaccination with Hib conjugate vaccines, PRP–Neisseria
Figure 2. Kinetics and magnitude of antibody response to pneumococcal polysaccharide (Pnc PS) types 4, 6B, and 19F during a course of
the pneumococcal conjugate vaccine PncCRM in Finnish infants. The
PncCRM vaccine was given at 2, 4, 6, and 14 months [8].
FUNCTIONAL ACTIVITY OF PNEUMOCOCCAL
POLYSACCHARIDE ANTIBODIES
Figure 3. Concentrations of antibody to pneumococcal polysaccharide
(Pnc PS) types 4, 6B, 9V, 14, 18C, 19F, and 23F after primary course of
the 7-valent or 9-valent pneumococcal conjugate vaccine PncCRM in
infants in the United States [9, 11], Finland [8], South Africa [14], and
the United Kingdom [15].
meningitidis outer membrane protein, HbOC, and PRP-T [34].
These differences are related to the functional activity of antibodies, but they can also indicate differences in the priming
capacity of these vaccines. Later, Goldblatt et al. [35] suggested
that increase in avidity of anti-Hib polysaccharide IgG could
be used as an indicator of memory response. Studies with pneumococcal conjugates partly confirmed the previous findings
with Hib conjugates: the avidity of IgG anti-6B antibodies was
highest after the course of PncCRM vaccine and lowest after
the course of PncOMPC vaccine [36, 37]. Furthermore, receiving a PncC booster but not pneumococcal polysaccharide
vaccine booster increased the avidity of antipnemococcal polysaccharide [36, 37], which suggests that the response to a PncC
was T cell dependent, but the T cell–independent pneumococcal polysaccharide vaccine only triggered the existing memory B cells.
IgG SUBCLASSES
The IgG antibody response to PncCs in infants is mainly of
the subclass IgG1. After the booster at the second year of life,
IgG2 also starts to appear, and the response in adults to both
pneumococcal polysaccharide and conjugate vaccines is mainly
of IgG2 subclass [18, 19, 22, 38–40]. The response to T
cell–dependent protein antigens is different; it consists mainly
of IgG1 subclass, both in adults and in children. In adults, some
differences have been found in the IgG1:IgG2 ratio after vaccination with pneumococcal polysaccharide and conjugate vaccines or between different conjugate vaccines [22, 38–40], but
the relevance of these findings for estimating the T cell dependence of the response in infants to the same vaccines and the
functional activity of antibodies remains to be studied.
Antibodies to pneumococcal polysaccharide protect against infection by helping the phagocytosis of pneumococci; they do
this because they are opsonic themselves, activate complement,
and create opsonizing 3b complement components. Therefore,
the opsonic activity of antipnemococcal polysaccharide antibodies is believed to mirror their protective activity. Several
methods for measuring opsonic activity have been applied [38,
41–44], and they seem to give similar results [45]. The correlation between IgG antipnemococcal polysaccharide antibody
concentration and opsonic activity seems to be rather high in
postvaccination serum samples from infants [38, 44, 45]. However, in serum samples from adults, the correlation is usually
lower, although it exists [38, 46, 47]. The reason for this is that
the serum samples from adults seem to contain antibodies that
are detected by standard EIAs but are not functionally active
[46–48]. These antibodies are most probably directed to contaminants in the pneumococcal polysaccharide preparations
that are used as antigens in EIA or directed to polyreactive
epitopes present in the polysaccharide antigens. Thus, in connection of clinical trials, it seems important to confirm the
functional activity of antibodies detected by EIA; this should
be done at least in a subcohort of representative serum samples.
The functionality of antibodies can be confirmed also in several
animal models that have been established for different types of
pneumococcal infections—for example, for sepsis, pneumonia,
AOM, or colonization [49–53].
EFFECT OF CONCURRENT VACCINATION ON
THE RESPONSE TO PncCs
There are few data on how concurrent vaccinations with other
childhood vaccines influence the response to PncCs. The clinical relevance of these findings remains to be seen. In the study
of Shinefield et al. [9], the primary series of PncCRM was given
with or without Hep B vaccine; Hep B had no significant effect
on the antipnemococcal polysaccharide concentrations that were
measured after the third dose of PncCRM was given. In the same
study, PncCRM booster was given with and without concurrent
HbOC–acellular pertussis–diphtheria–tetanus (DTaP). The responses to pnemococcal polysaccharides were lower in the
group that had received HbOC-DTaP, but the differences were
not statistically significant [9]. In the study of Choo et al. [15],
PncCRM was given mixed or as a separate injection with HbOC
simultaneously with DTwP. The mixed administration decreased the response to pnemococcal polysaccharides
significantly.
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EFFECT OF PncCs ON THE RESPONSE TO
CONCURRENTLY ADMINISTERED VACCINES
Data are also sparse on how PncCs interfere with the antibody
response to other childhood vaccines that are given simultaneously. Two studies [14, 16] have shown that the response to
HbOC (Hib conjugate vaccine with the same CRM197 carrier
as in the PncCRM vaccine), diphtheria toxoid (protein which
is similar to CRM carrier), but not to tetanus toxoid, is higher
when PncCRM is administered concurrently with HbOC-DTP
than when it is administered without concurrent PncCRM.
Furthermore, the study of Daum et al. [16] showed that the
increase in anti-Hib polysaccharide was dependent on the dose
of PncCRM, so that a dose of 5 mg of pnemococcal polysaccharide or oligosaccharide in the PncC induced a smaller difference than did doses of 2 or 0.5 mg.
For pneumococcal polysaccharide–tetanus toxoid protein
conjugate (PncT), the opposite is true: the response to PRP-T
(Hib conjugate) and tetanus was lower among the receivers of
concurrent PncT and PRP-T–DTP than in those who did not
receive PncT [54]. Furthermore, the decrease in anti-Hib polysaccharide response was dose dependent: the higher the PncT
dose, the lower the anti-Hib and anti-tetanus response. Interestingly, in that same study, IgG response to primary course of
4-valent PncT was not dependent on the dose of PncT. However, the response to pnemococcal polysaccharide booster at
14 months of age was dependent on the dose used in the
primary course; the booster response was lowest among those
who had received the highest dose of 10 mg of each pnemococcal
polysaccharide per dose in infancy [7].
ANTIBODY DATA FROM EFFICACY TRIALS
WITH PncCRM
Preliminary antibody data from the completed efficacy trials
are already available [9, 20, 23]. The PncCRM was able to
prevent 97.4% (82.7–99.9%) of invasive infections caused by
vaccine serotypes [23]. Thus it can be concluded that an antibody response similar to that reported by Shinefield et al. [9],
Black et al. [23], or Rennels et al. [11] is high enough for
protection. Because of the high protection rate, at least in the
short term, this study does not give a direct answer on characteristics of minimal antibody response needed for protection
against invasive pneumococcal infection
In the Finnish trial, the primary end point was culture-confirmed AOM caused by vaccine serotypes. Etiology was determined by cultures of middle ear fluid samples. The efficacy of
PncCRM against AOM caused by vaccine serotypes was 57%
(95% CI, 44%–67%) [25, 26]. However, there were serotypespecific differences in efficacy [26]. Interestingly, efficacy against
type 19F was only 25% (95% CI, ⫺14% to 51%), whereas
efficacy against type 6B was 84% (95% CI, 61%–93%). SeroS296 • CID 2001:33 (Suppl 4) • Käyhty and Åhman
types 19F and 6B induced similar IgG antibody concentrations
(as measured by use of EIA) [20]. This suggests that more
antibodies are needed for protection against type 19F than for
protection against type 6B. This has indeed been shown in
studies for opsonic activity against these serotypes [38] and also
in animal models [50]. An alternative explanation is that EIAs
for type 19F partly measure nonprotective, polyreactive antibodies, and that the concentration of specific antibodies needed
for protection is in fact similar for 6B and 19F. Performance
of animal studies and development of improved, specific EIAs
will tell us which alternative is true in the future.
In conclusion, there are at present no established surrogates
or correlates of protection after vaccination with PncCs. On
the basis of the existing data, we know that the immune response evoked by PncCRM is high enough for protection
against invasive infections, but protection against a local infection, AOM, is partial. It remains to be seen whether this is
due to insufficient systemic immune response or whether better
local immunity is also needed for protection against mucosal
infections. PncCs also seem to induce local antibody responses,
but their prevalence is not always high [8, 55–57]. Data are
still sparse on how PncCs affect antibody response to routine
childhood vaccines given as mixed or as separate simultaneous
injections. The serotype-specific, vaccine-specific, and population-specific differences in immune response to PncCs make
the comparative immunogenicity evaluations complex. Several
ongoing efficacy trials that have immunogenicity trials associated with them will, we hope, soon give us more data regarding the characteristics of the immune response that provides protection against different pneumococcal infections.
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