Daniel M. Weinberger

Serotype replacement in disease after pneumococcal vaccination

Vaccination with heptavalent pneumococcal conjugate vaccine (PCV7) has significantly reduced the burden of pneumococcal disease and has had an important public health benefit. Because this vaccine targets only seven of the more than 92 pneumococcal serotypes, concerns have been raised that non-vaccine serotypes (NVTs) could increase in prevalence and reduce the benefits of vaccination. Indeed, among asymptomatic carriers, the prevalence of NVTs has increased substantially, and consequently, there has been little or no net change in the bacterial carriage prevalence. In many populations, pneumococcal disease caused by NVT has increased, but in most cases this increase has been less than the increase in NVT carriage. We review the evidence for serotype replacement in carriage and disease, and address the surveillance biases that might affect these findings. We then discuss possible reasons for the discrepancy between near-complete replacement in carriage and partial replacement for disease, including differences in invasiveness between vaccine serotypes. We contend that the magnitude of serotype replacement in disease can be attributed, in part, to a combination of lower invasiveness of the replacing serotypes, biases in the pre-vaccine carriage data (unmasking), and biases in the disease surveillance systems that could underestimate the true amount of replacement. We conclude by discussing the future potential for serotype replacement in disease and the need for continuing surveillance.

Association of serotype with risk of death due to pneumococcal pneumonia: a meta-analysis.

BACKGROUND The 92 capsular serotypes of Streptococcus pneumoniae differ greatly in nasopharyngeal carriage prevalence, invasiveness, and disease incidence. There has been some debate, though, regarding whether serotype independently affects the outcome of invasive pneumococcal disease (IPD). Published studies have shown variable results with regard to case-fatality ratios for specific serotypes and the role of host factors in affecting these relationships. We evaluated whether risk of death due to IPD is a stable serotype-associated property across studies and then compared the pooled effect estimates with epidemiologic and biological correlates. METHODS We performed a systematic review and meta-analysis of serotype-specific disease outcomes for patients with pneumonia and meningitis. Study-specific estimates of risk of death (risk ratio [RR]) were pooled from 9 studies that provided serotype-specific data on pneumonia and meningitis using a random-effects method with serotype 14 as the reference. Pooled RRs were compared with RRs from adults with low comorbidity scores to evaluate potential confounding by host factors. RESULTS Significant differences were found in the RR estimates among serotypes in patients with bacteremic pneumonia. Overall, serotypes 1, 7F, and 8 were associated with decreased RRs, and serotypes 3, 6A, 6B, 9N, and 19F were associated with increased RRs. Outcomes among meningitis patients did not differ significantly among serotypes. Serotypes with increased RRs had a high carriage prevalence, had low invasiveness, and were more heavily encapsulated in vitro. CONCLUSIONS These results suggest that IPD outcome, like other epidemiologic measures, is a stable serotype-associated property.

Pneumococcal Capsular Polysaccharide Structure Predicts Serotype Prevalence

There are 91 known capsular serotypes of Streptococcus pneumoniae. The nasopharyngeal carriage prevalence of particular serotypes is relatively stable worldwide, but the host and bacterial factors that maintain these patterns are poorly understood. Given the possibility of serotype replacement following vaccination against seven clinically important serotypes, it is increasingly important to understand these factors. We hypothesized that the biochemical structure of the capsular polysaccharides could influence the degree of encapsulation of different serotypes, their susceptibility to killing by neutrophils, and ultimately their success during nasopharyngeal carriage. We sought to measure biological differences among capsular serotypes that may account for epidemiological patterns. Using an in vitro assay with both isogenic capsule-switch variants and clinical carriage isolates, we found an association between increased carriage prevalence and resistance to non-opsonic neutrophil-mediated killing, and serotypes that were resistant to neutrophil-mediated killing tended to be more heavily encapsulated, as determined by FITC-dextran exclusion. Next, we identified a link between polysaccharide structure and carriage prevalence. Significantly, non-vaccine serotypes that have become common in vaccinated populations tend to be those with fewer carbons per repeat unit and low energy expended per repeat unit, suggesting a novel biological principle to explain patterns of serotype replacement. More prevalent serotypes are more heavily encapsulated and more resistant to neutrophil-mediated killing, and these phenotypes are associated with the structure of the capsular polysaccharide, suggesting a direct relationship between polysaccharide biochemistry and the success of a serotype during nasopharyngeal carriage and potentially providing a method for predicting serotype replacement.

Impact of 13-Valent Pneumococcal Conjugate Vaccination in Invasive Pneumococcal Disease Incidence and Mortality

BACKGROUND The impact of the 13-valent pneumococcal conjugate vaccine (PCV13) at the population level is unclear. We explored PCV13s effect in reducing invasive pneumococcal disease (IPD)-related morbidity and mortality, and whether serotype-specific changes were attributable to vaccination or expected as a part of natural, cyclical variations. METHODS This was a Danish nationwide population-based cohort study based on the linkage of laboratory surveillance data and the Danish Civil Registration System. Changes in IPD incidence and mortality during baseline (2000-2007), 7-valent pneumococcal conjugate vaccine (PCV7) (2008-2010), and PCV13 (2011-2013) periods were estimated. Predicted incidences of serotypes were estimated controlling for cyclical trends from historical patterns observed during the past 20 years. RESULTS We observed a 21% reduction (95% confidence interval [CI], 17%-25%) in IPD incidence in the total population after PCV13s introduction, and a 71% reduction (95% CI, 62%-79%) in children aged <2 years, considered as the vaccine effectiveness. We estimated a 28% reduction (95% CI, 18%-37%) in IPD-related 30-day mortality, from 3.4 deaths (95% CI, 3.2-3.6) per 100 000 population in the pre-PCV period to 2.4 (95% CI, 2.2-2.7) in the PCV13 period. The decline in mortality was observed across all age groups but was mainly related to mortality reductions in the nonvaccinated population. For serotypes 1 and 3, there were no significant changes in incidence beyond what would be expected from natural cyclical patterns. Serotype 19A significantly increased following PCV7s introduction, but the incidence declined toward baseline in 2012. CONCLUSIONS PCV13 has brought greater benefits than we had expected in our setting. We observed a further decline on IPD incidence shortly after the shift from PCV7 to PCV13 in the national immunization program. This decline was accompanied by a substantial population-level decline in pneumococcal-related mortality of nearly 30% among nonvaccinated persons.

Epidemiologic Evidence for Serotype-Specific Acquired Immunity to Pneumococcal Carriage

Nasopharyngeal carriage of Streptococcus pneumoniae is required for transmission of the bacteria and for invasive disease. There have been conflicting reports as to whether protection against carriage is serotype specific and which immune mechanisms drive carriage. Analyzing longitudinal carriage data from Israeli toddlers in day care, we found a lower risk of colonization with types 6A, 14, and 23F after previous exposure to the homologous type. Nonsignificant trends suggesting possible protection derived from prior exposure were found for types 19A and 23A. Furthermore, we found that, for types 14 and 23F, this specific protection correlated with increased serotype-specific antibody concentration. We found no evidence of specific protection for type 6B, group 15, or type 19F. Our findings imply that at least some serotypes generate anti-capsular antibodies that can reduce the risk of carriage in unimmunized toddlers.

Impact of the 2009 Influenza Pandemic on Pneumococcal Pneumonia Hospitalizations in the United States

BACKGROUND Infection with influenza virus increases the risk for developing pneumococcal disease. The A/H1N1 influenza pandemic in autumn 2009 provided a unique opportunity to evaluate this relationship. METHODS Using weekly age-, state-, and cause-specific hospitalizations from the US State Inpatient Databases of the Healthcare Cost and Utilization Project 2003-2009, we quantified the increase in pneumococcal pneumonia hospitalization rates above a seasonal baseline during the pandemic period. RESULTS We found a significant increase in pneumococcal hospitalizations from late August to mid-December 2009, which corresponded to the timing of highest pandemic influenza activity. Individuals aged 5-19 years, who have a low baseline level of pneumococcal disease, experienced the largest relative increase in pneumococcal hospitalizations (ratio, 1.6 [95% confidence interval {CI}, 1.4-1.7]), whereas the largest absolute increase was observed among individuals aged 40-64 years. In contrast, there was no excess disease in the elderly. Geographical variation in the timing of excess pneumococcal hospitalizations matched geographical patterns for the fall pandemic influenza wave. CONCLUSIONS The 2009 influenza pandemic had a significant impact on the rate of pneumococcal pneumonia hospitalizations, with the magnitude of this effect varying between age groups and states, mirroring observed variations in influenza activity.

Identifying the interaction between influenza and pneumococcal pneumonia using incidence data.

Influenza virus increases susceptibility to pneumococcal pneumonia in individuals but has only a modest impact on the total population. Tipping the Scales for Pathogen Interaction As if it weren’t bad enough to be infected by one pathogen, sometimes a primary infection can pave the way for a secondary infection as well. This has been thought to be the case with influenza virus and Streptococcus pneumoniae. Indeed, there are a lot of data in both humans and animal models that this does happen at the individual level. However, epidemiological studies measuring population-level effects do not reflect this observation. Now, Shrestha et al. use mathematical modeling of weekly incidence reports to reconcile these two seemingly disparate observations. The authors constructed a mechanistic transmission model within a likelihood-based inference framework and characterized the timing, nature, and magnitude of the interaction between influenza virus and pneumococcal pneumonia. In individuals, they see that flu increases the susceptibility to subsequent pneumococcal pneumonia more than 100 times. However, at the population level, there is only a modest effect. These data suggest that influenza virus may contribute to pneumococcal pneumonia pathogenesis, but that these effects may be masked in the total population. The association between influenza virus and the bacterium Streptococcus pneumoniae (pneumococcus) has been proposed as a polymicrobial system, whereby transmission and pathogenicity of one pathogen (the bacterium) are affected by interactions with the other (the virus). However, studies focusing on different scales of resolution have painted an inconsistent picture: Individual-scale animal experiments have unequivocally demonstrated an association, whereas epidemiological support in human populations is, at best, inconclusive. We integrate weekly incidence reports and a mechanistic transmission model within a likelihood-based inference framework to characterize the nature, timing, and magnitude of this interaction. We find support for a strong but short-lived interaction, with influenza infection increasing susceptibility to pneumococcal pneumonia ~100-fold. We infer modest population-level impacts arising from strong processes at the level of an individual, thereby resolving the dichotomy in seemingly inconsistent observations across scales. An accurate characterization of the influenza-pneumococcal interaction can form a basis for more effective clinical care and public health measures for pneumococcal pneumonia.

Impaired Innate and Adaptive Immunity to Streptococcus pneumoniae and Its Effect on Colonization in an Infant Mouse Model

ABSTRACT Streptococcus pneumoniae colonization and invasive disease peak around the third and first birthdays, respectively, and decline thereafter. While these declines are attributable in part to immunity acquired via natural exposure, maturation of innate immune responses may also be involved. A mucosally administered candidate whole-cell pneumococcal vaccine (WCV) containing killed pneumococcal antigen (WCA) plus a cholera toxin adjuvant protects against intranasal carriage of pneumococci by a mechanism that is antibody independent and CD4+ TH17 cell dependent. Because infants and children are a key target population for this vaccine, we sought to evaluate the immune responses of neonatal and infant mice to S. pneumoniae and to assess whether the WCV would be effective in these mice. Like human infants, infant mice showed impaired clearance of nasopharyngeal colonization with S. pneumoniae. Macrophages from neonatal and infant mice stimulated with killed pneumococci in vitro showed significantly reduced cytokine production, including that of KC, granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, macrophage chemoattractant protein 1, interleukin-6 (IL-6), IL-1α, tumor necrosis factor alpha, and gamma interferon, whereas IL-10 expression was significantly increased compared to that in macrophages from adult mice. IL-17A production from adult immune CD4+ T cells was significantly delayed when neonatal macrophages instead of adult macrophages were used as antigen-presenting cells. Moreover, whole blood from mice immunized as neonates with WCV produced significantly less IL-17A after stimulation with WCA than did blood from mice immunized as adults. Nonetheless, a single immunization of neonatal mice with WCV significantly reduced colonization density. Overall, our data suggest an impairment of both innate and acquired cellular immune responses in neonatal and infant mice. However, WCV confers a significant reduction in colonization following pneumococcal challenge, suggesting that it may still be effective in the setting of immature immune responses.

Estimating rates of carriage acquisition and clearance and competitive ability for pneumococcal serotypes in Kenya with a Markov transition model.

Background: There are more than 90 serotypes of Streptococcus pneumoniae, with varying biologic and epidemiologic properties. Animal studies suggest that carriage induces an acquired immune response that reduces duration of colonization in a nonserotype-specific fashion. Methods: We studied pneumococcal nasopharyngeal carriage longitudinally in Kenyan children 3–59 months of age, following up positive swabs at days 2, 4, 8, 16, and 32 and then monthly thereafter until 2 swabs were negative for the original serotype. As previously reported, 1868/2840 (66%) of children swabbed at baseline were positive. We estimated acquisition, clearance, and competition parameters for 27 serotypes using a Markov transition model. Results: Point estimates of type-specific acquisition rates ranged from 0.00025/d (type 1) to 0.0031/d (type 19F). Point estimates of time to clearance (inverse of type-specific immune clearance rate) ranged from 28 days (type 20) to 124 days (type 6A). For the serotype most resistant to competition (type 19F), acquisition of other serotypes was 52% less likely (95% confidence interval = 37%–63%) than in an uncolonized host. Fitness components (carriage duration, acquisition rate, lack of susceptibility to competition) were positively correlated with each other and with baseline prevalence, and were associated with biologic properties previously shown to associate with serotype. Duration of carriage declined with age for most serotypes. Conclusions: Common S. pneumoniae serotypes appear superior in many dimensions of fitness. Differences in rate of immune clearance are attenuated as children age and become capable of more rapid clearance of the longest-lived serotypes. These findings provide information for comparison after introduction of pneumococcal conjugate vaccine.

Expression of the Helicobacter pylori adhesin SabA is controlled via phase variation and the ArsRS signal transduction system

Adaptation to the acidic microenvironment, and adherence to mucosal epithelium, are essential for persistent colonization of the human stomach by Helicobacter pylori. The expression of SabA, an adhesin implicated in the ability of H. pylori to adhere to the host gastric epithelium, can be modulated by phase variation via slipped-strand mispairing in repetitive nucleotide tracts located in both the promoter region and the coding region. This study demonstrates the occurrence of phase variation at the sabA locus within individual strains of H. pylori, and among multiple isolates from a single patient. In addition, transcription of sabA is repressed by the acid-responsive ArsRS two-component signal transduction system in vitro. Our results demonstrate that isogenic inactivation of the arsS (jhp0151/HP0165) histidine kinase locus results in a 10-fold SabA-dependent increase in adherence to gastric epithelial cells in strain J99 (contains an in-frame sabA allele), but not in strain 26695 (out-of-frame sabA allele). The combination of transcriptional regulation of the sabA locus by the ArsRS two-component signal-transduction system and the generation of subpopulations harbouring alternate sabA alleles by slipped-strand mispairing during chromosomal replication could permit H. pylori to rapidly adapt to varying microenvironments or host immune responses. As a pathogen with a paucity of regulatory proteins, this dual regulation indicates that SabA expression is a tightly regulated process in H. pylori infection.