Efficacy studies to evaluate new Hib conjugate vaccines are no longer tenable now that highly effective Hib vaccines are widely available. The understanding and the use of surrogate markers of protection have thus become key factors in supporting the licensure of new combination Hib vaccines. Measurement of antibody concentrations against PRP remains the cornerstone for the putative demonstration of protection. However, it is generally accepted that antibody quality and functional activity are equally important as or possibly more important than serum IgG concentrations alone. Antibody functional capacity depends on concentration, avidity, and isotype, including the IgG subclass, which is also possibly modulated by the age of the subject and the form of antigen stimulation (conjugate/nonconjugate/naturally acquired) (21
). In this study, we have characterized the anti-PRP antibody response induced by a PRP-TT conjugate vaccine administered concomitantly or mixed with various DTPa-based and DTPw-based vaccines.
Higher postprimary anti-PRP antibody GMCs were observed after vaccination with a Hib vaccine administered separately than with mixed vaccination with a DTPa-based vaccine. This corroborates the observation that DTPa/Hib combinations typically induce lower postprimary anti-PRP responses (6
). It was also observed, in the Myanmar study, that anti-PRP antibody GMCs were considerably higher following DTPw-based Hib vaccinations, which we may attribute to the use of whole-cell pertussis, which was already known to elicit higher antibody titers than acellular pertussis (11
). However, after the booster dose, these differences between vaccines were no longer present, in line with a recent study in which there was no association between postbooster anti-PRP antibody concentrations and the vaccine (DTPw/Hib and/or DTPa/Hib) used for primary vaccination (15
). On the other hand, the DTPa/Hib Germany A and B studies demonstrated lower postbooster antibody levels for the DTPa/Hib combination than with separate administration (Table ).
Antibody avidity is a measurement of the binding strength of the antibodies to the polysaccharide antigen. Booster responses are characterized by the production of high-avidity antibodies, and antibody avidity has been proposed as a surrogate marker for the induction of successful priming (8
). There is evidence that avidity may develop in the absence of a detectable primary response to DTPa-based Hib vaccines (9
). A similar range of avidity indices was observed in subjects vaccinated with Hib, whether administered mixed with or separate from DTPa-based vaccines, and in subjects who received DTPw-based Hib vaccines, with most maturation evident by the time of the booster dose. However, there appeared to be differences between the maturation of avidity following primary vaccination with DTPa/Hib and that with DTPa-HBV-IPV/Hib or DTPw-based Hib vaccination. The avidity index was statistically significantly lower prior to and after the booster dose of DTPa/Hib than that of coadministered DTPa and Hib, whereas no such difference was observed between the mixed and separately administered DTPa-HBV-IPV and Hib vaccines. The lower booster response in the Germany A DTPa/Hib study is in line with the lower avidity maturation and confirms that antibody avidity is a surrogate marker for immune memory for conjugate vaccines (8
). This observation is in accordance with a recent report by Johnson et al. (15
), who noted a reduced Hib conjugate postbooster antibody avidity when the primary vaccination was done with DTPa/Hib compared to primary vaccination with DTPw/Hib. We have previously reported the absence of any difference in avidity maturation following primary vaccination with the larger combined DTPa-HBV-IPV/Hib vaccine (27
). Taken together, our data, and those described previously by Johnson et al., support the notion that the DTPa/Hib vaccine may have a reduced ability to induce avidity maturation compared with those of DTPa-HBV-IPV/Hib and DTPw-based Hib vaccines. We have evidence that IPV acts as an immunostimulant, which might explain the differences between DTPa/Hib and DTPa-HBV-IPV/Hib (data not shown).
Despite this reduced antibody avidity after primary vaccination with DTPa/Hib versus DTPa-HBV-IPV/Hib or DTPw-based Hib vaccinations, we observed no difference between vaccines with regard to antibody quality and function, as measured by bactericidal activity, opsonophagocytic activity (GMR), and the ability to protect in a passive rat protection assay. Significantly lower postprimary opsonophagocytic and bactericidal GMTs in DTPa-based Hib vaccine recipients and lower postbooster GMTs in DTPa/Hib recipients in the Germany A study reflect the lower antibody levels achieved at these time points. There were no observed differences in GMR, indicating that the antibodies had similar functional activities regardless of their concentrations. Maturation of the bactericidal and opsonic responses evolved over time in a similar fashion in all groups. The reduced avidity maturation and booster responses following DTPa/Hib vaccination versus those of the other vaccines tested contrasts with the maturation of the bactericidal and opsonic responses for all DTPa-based Hib combinations, including DTPa/Hib. The reduced avidity and reduced booster responses with DTPa/Hib raise the question of whether the protective efficacy of DTPa/Hib may also be reduced. This seems unlikely, given the absence of any differences in bactericidal activity, opsonophagocytosis, and protection in the infant rat model. Furthermore, similar observations (lower avidity with comparable bactericidal and opsonic activity as well as reduced booster responses than other Hib vaccines) were also made for infants vaccinated with the licensed PRP-OMP vaccine. The protective efficacy of PRP-OMP was 95% in a placebo-controlled trial conducted with high-risk children in the United States that included a booster immunization (6
), suggesting no impact of the reduced avidity maturation on efficacy.
There is a great deal of uncertainty about the importance of direct bactericidal activity or opsonophagocytosis in the human anti-Hib defense mechanism. In a study involving young adults, virtually all samples displayed opsonophagocytic activity, while only half of them had demonstrable bactericidal activity (23
). In mice, C5-deficient but not C3-depleted mice showed normal clearance of Hib from the bloodstream, indicating the critical role of opsonophagocytosis in Hib clearance (24
). Due to the observation that individuals with late complement pathway deficiencies are not at increased risk of clinical Hib disease, it has been suggested that opsonic activity alone is sufficient for protection (7
The clinical effectiveness of DTPa-based Hib combinations in preventing Hib disease has been conclusively demonstrated in Germany, where the vaccine effectiveness was 96.7% after primary immunization and 98.5% after the booster dose in the second year of life (16
). Prior to the introduction of DTPa-based Hib combinations in Germany, it was recognized that the absence of a booster dose was associated with an increase in Hib disease (34
) and with a reduction in the prevention of Hib colonization (18
). The importance of the booster dose in consolidating immunity to Hib has been more recently demonstrated in the United Kingdom, where vaccine effectiveness of DTPw/Hib administered by a 2-, 3-, and 4-month schedule without a booster was rather low (67 to 88%) within the first 2 years and absent (no effectiveness) after 2 years (28
). The rather low effectiveness was initially obscured due to the installation of a catch-up Hib conjugate campaign in children less than 4 years of age in 1992 (12
). No booster dose was administered at that time in the United Kingdom, and an increase in Hib disease was observed starting in 1999 and exacerbated after the introduction of the DTPa/Hib vaccine in 2000 (28
). The absence of a Hib booster has been identified as the single most important factor leading to the increased incidence of Hib disease in the United Kingdom (32
). Within the United Kingdom context of suboptimal control of Hib disease, vaccination with a combined DTPa/Hib vaccine was associated with an increased risk for Hib disease (22
). Although the absence of a Hib booster is a critical factor, anti-PRP antibody GMCs and the proportion of vaccinated United Kingdom subjects with anti-PRP antibodies of ≥0.15 μg/ml were, nevertheless, atypically low for DTPa/Hib combinations (10
Of note, a strikingly similar observation was recently made in the United Kingdom with respect to meningococcal C conjugate immunization. Indeed, a primary immunization according to the 2-, 3-, and 4-month schedule without booster resulted in deficient protection against serogroup C meningococcal disease in children >1 year of age (33
). Primary immunization according to the less immunologically challenging 2-, 4-, and 6-month schedule without booster in Spain was associated with a better persistence of protection (19
Although the mixing of DTPa and Hib conjugate vaccine resulted in a reduced magnitude of the anti-PRP antibody response, combined vaccines demonstrated anti-PRP antibody levels, seroprotection rates, and antibody avidity that were equal to or higher than those observed after primary vaccination with the licensed and efficacious PRP-OMP vaccine (29
). In the present study, we have also demonstrated that antibody quality, maturation, and protective capacity are equivalent in subjects who received primary and booster vaccination with Hib vaccine, independently of the mode of administration with DTPa-based vaccine. Furthermore, these responses were also similar to those induced by the DTPw-based Hib vaccine. These data, along with postmarketing confirmation of ongoing vaccine effectiveness in Germany (30
), attest for the excellent clinical effectiveness of DTPa-based Hib combination vaccines in preventing Hib disease in infants and children.