It has been well described that nasopharyngeal colonization precedes disease [6
]. The effect of pneumococcal conjugate vaccination, in particular on prevention of mucosal diseases like AOM and pneumonia, is presumed to be associated with the reduction of nasopharyngeal colonization and density of vaccine pneumococcal serotypes [23
]. As mentioned, subsequent group protection has contributed greatly to the success of pneumococcal conjugate vaccination. In our study, we found no differential effect of PHiD-CV immunization on NTHi colonization, acquisition, or density of H. influenzae
compared with 7vCRM in healthy Dutch children up to 2 years of age. Therefore, we expect no indirect effects of PHiD-CV on NTHi through prevention of transmission.
The unlicensed 11Pn-PD vaccine reduced 35% of NTHi-caused AOM episodes in the Pneumococcal Otitis Efficacy Trial (POET) with a trend of reduced NTHi colonization [12
]. However, the group difference was only observed approximately 3 months postbooster [12
], and disappeared at 24 months of age [14
]. A study investigating the effects of the licensed PHiD-CV, using a naive, age-matched nonrandomized control group, showed no consistent effect on NTHi colonization [15
]. Importantly, carriage rates were low in both studies [14
]. In the present study, we found high colonization rates and applied molecular methods to quantify the presence of H. influenzae
. We observed no efficacy of PHiD-CV immunization against NTHi colonization or acquisition, at 3 months postbooster or at any other time point. It is unknown if anti-PD antibodies correlate with protection against NTHi-AOM or colonization. Anti-PD antibody concentrations in the present study [19
] were lower as those measured in POET [12
], but comparable with other PHiD-CV trials. Our use of 7vCRM as nonactive control contrasts with previous studies in which controls did not receive a pneumococcal conjugate vaccine [12
]. It has been shown that multiple strains of S. pneumoniae
and H. influenzae
can coexist in the upper respiratory tract [24
]. Therefore, immunization with a pneumococcal conjugate vaccine could exert a bystander effect on the presence of H. influenzae
. However, overall and serotype-specific pneumococcal colonization patterns were similar in both vaccine groups. Additionally, no effect on H. influenzae
colonization was observed in a recent study evaluating reduced-dose schedules with 7vCRM [21
]. Therefore, any possible indirect effects of PHiD-CV on NTHi through effects on S. pneumoniae
are unlikely to have influenced our results.
Our study raises the question of how a PD conjugate vaccine induces protection against NTHi-caused AOM, while not affecting presence of NTHi in the nasopharynx—generally regarded to be the point from which respiratory tract infections originate. We propose several possible explanations.
First, although PD may play a role in pathogenesis of NTHi-caused respiratory infections in animal models [25
], intervening with its function may not fully prevent a complex biological process such as colonization [26
]. Virulence factors other than PD may contribute to colonization of this unencapsulated pathogen. For instance, H. influenzae
is highly adaptive and selection of certain phase variants occurs during colonization [27
]. Also, specific properties could differ between colonizing H. influenzae
strains and those found in disease, such as previously shown for the expression of immunoglobulin A protease [28
Second, in a chinchilla model for AOM, Johnson and colleagues found that abrogation of protein D's activity reduced NTHi adherence in the middle ear, but not in the nasopharynx—suggesting a compartment-specific effect [26
]. Third, the efficacy of the 11Pn-PD vaccine against NTHi AOM could merely reflect prevention of the first pneumococcal AOM. In general, the first otitis episode is more frequently caused by pneumococcus, whereas recurrent episodes are more frequently associated with NTHi etiology [4
]. Therefore, preventing the first pneumococcal AOM episode can be hypothesized to prevent sequelae involving NTHi. This hypothesis seems to contradict results of the Finnish otitis media trial (FinOM), in which a statistically nonsignificant increase in H. influenzae
AOM was observed after a 3 + 1-dose schedule of 7vCRM [29
]. However, although differences in case definitions did not explain the different study results [30
], comparing POET with FinOM is fraught with difficulties, owing to dissimilarities in study design and case ascertainment—further complicated by differences in the distribution of AOM-causing pathogens. Nonetheless, surveillance is warranted to monitor bacterial colonization patterns and pathogens involved in AOM following nationwide initiation of pneumococcal conjugate vaccination. Clinical trials assessing efficacy of PHiD-CV against AOM (NCT00839254 and NCT00466947) are currently ongoing.
It is important to emphasize that this study was undertaken at the time of herd immunity with respect to vaccine pneumococcal serotypes [16
]. As expected, no major differences were observed between groups with respect to pneumococcal acquisition, colonization, and density. Consistent with previous reports [31
], serotypes 1, 5, and 7F, known for their high invasiveness or case-to-carrier ratio, were found to be rarely carried. Also, colonization with potentially cross-reactive serotypes like 6A and 19A and other, non–PHiD-CV pneumococcal serotypes was similar in both groups. Regardless of the administered vaccine and detection method, serotype 19A was the predominant colonizer throughout follow-up. Finally, no major differences were observed between groups in the prevalence of M. catarrhalis
and S. aureus
in nasopharyngeal samples.
Some limitations need to be addressed. First, we have focused in this study on NTHi colonization in healthy children. Therefore, we cannot speculate on effects of PHiD-CV during NTHi disease or, for example, viral infections, when overgrowth of NTHi could be somewhat contained in children immunized with PHiD-CV. In addition, this study was not powered to detect pneumococcal serotype-specific differences.
Strengths of our study include the longitudinal randomized controlled study design with an adequate sample size to assess the efficacy of PHiD-CV on NTHi colonization, virtual absence of loss to follow-up, and a low rate of protocol deviations. In addition, we had relatively high nasopharyngeal bacterial colonization rates compared with other countries. Finally, we applied molecular methods to measure the densities of H. influenzae and S. pneumoniae in the nasopharynx.
In conclusion, PHiD-CV immunization had no differential effect on nasopharyngeal NTHi colonization, acquisition, or density compared with 7vCRM in healthy children up to 2 years of age in the Netherlands. This implies that herd effects on NTHi are not to be expected following introduction of PHiD-CV. Similar nasopharyngeal pneumococcal, M. catarrhalis, and S. aureus colonization rates were also observed in children vaccinated with either PHiD-CV or 7vCRM.