Boosting understanding of pertussis outbreaks

NOTE ON THE UNIQUENESS OF AN ENDEMIC EQUILIBRIUM OF AN EPIDEMIC MODEL WITH BOOSTING OF IMMUNITY

For some diseases, it is recognized that immunity acquired by natural infection and vaccination subsequently wanes. As such, immunity provides temporal protection to recovered individuals from an infection. An immune period is extended owing to boosting of immunity by asymptomatic re-exposure to an infection. An individual’s immune status plays an important role in the spread of infectious diseases at the population level. We study an age-dependent epidemic model formulated as a nonlinear version of the Aron epidemic model, which incorporates boosting of immunity by a system of delay equations and study the existence of an endemic equilibrium to observe whether boosting of immunity changes the qualitative property of the existence of the equilibrium. We establish a sufficient condition related to the strength of disease transmission from subclinical and clinical infective populations, for the unique existence of an endemic equilibrium.

Enemy attraction: bacterial agonists for leukocyte chemotaxis receptors

The innate immune system recognizes conserved microorganism-associated molecular patterns (MAMPs), some of which are sensed by G protein-coupled receptors (GPCRs), and this leads to chemotactic leukocyte influx. Recent studies have indicated that these processes are crucial for host defence and rely on a larger set of chemotactic MAMPs and corresponding GPCRs than was previously thought. Agonists, such as bacterial formyl peptides, enterococcal pheromone peptides, staphylococcal peptide toxins, bacterial fermentation products and the Helicobacter pylori peptide HP(2-20), stimulate specific GPCRs. The importance of leukocyte chemotaxis in host defence is highlighted by the fact that some bacterial pathogens produce chemotaxis inhibitors. How the various chemoattractants, receptors and antagonists shape antibacterial host defence represents an important topic for future research.

Cost-effectiveness of Tdap vaccination of adults aged ≥65 years in the prevention of pertussis in the US: a dynamic model of disease transmission

OBJECTIVES

In February 2012, the Advisory Committee on Immunization Practices (ACIP) advised that all adults aged ≥65 years receive a single dose of reduced-antigen-content tetanus, diphtheria, and acellular pertussis (Tdap), expanding on a 2010 recommendation for adults >65 that was limited to those with close contact with infants. We evaluated clinical and economic outcomes of adding Tdap booster of adults aged ≥65 to "baseline" practice [full-strength DTaP administered from 2 months to 4-6 years, and one dose of Tdap at 11-64 years replacing decennial Td booster], using a dynamic model.

METHODS

We constructed a population-level disease transmission model to evaluate the cost-effectiveness of supplementing baseline practice by vaccinating 10% of eligible adults aged ≥65 with Tdap replacing the decennial Td booster. US population effects, including indirect benefits accrued by unvaccinated persons, were estimated during a 1-year period after disease incidence reached a new steady state, with consequences of deaths and long-term pertussis sequelae projected over remaining lifetimes. Model outputs include: cases by severity, encephalopathy, deaths, costs (of vaccination and pertussis care) and quality-adjusted life-years (QALYs) associated with each strategy. Results in terms of incremental cost/QALY gained are presented from payer and societal perspectives. Sensitivity analyses vary key parameters within plausible ranges.

RESULTS

For the US population, the intervention is expected to prevent >97,000 cases (>4,000 severe and >5,000 among infants) of pertussis annually at steady state. Additional vaccination costs are $4.7 million. Net cost savings, including vaccination costs, are $47.7 million (societal perspective) and $44.8 million (payer perspective). From both perspectives, the intervention strategy is dominant (less costly, and more effective by >3,000 QALYs) versus baseline. Results are robust to sensitivity analyses and alternative scenarios.

CONCLUSIONS

Immunization of eligible adults aged ≥65, consistent with the current ACIP recommendation, is cost saving from both payer and societal perspectives.

Cost-effectiveness analysis of Tdap in the prevention of pertussis in the elderly

OBJECTIVES

Health benefits and costs of combined reduced-antigen-content tetanus, diphtheria, and pertussis (Tdap) immunization among adults ≥65 years have not been evaluated. In February 2012, the Advisory Committee on Immunization Practices (ACIP) recommended expanding Tdap vaccination (one single dose) to include adults ≥65 years not previously vaccinated with Tdap. Our study estimated the health and economic outcomes of one-time replacement of the decennial tetanus and diphtheria (Td) booster with Tdap in the 10% of individuals aged 65 years assumed eligible each year compared with a baseline scenario of continued Td vaccination.

METHODS

We constructed a model evaluating the cost-effectiveness of vaccinating a cohort of adults aged 65 with Tdap, by calculating pertussis cases averted due to direct vaccine effects only. Results are presented from societal and payer perspectives for a range of pertussis incidences (25-200 cases per 100,000), due to the uncertainty in estimating true annual incidence. Cases averted were accrued throughout the patient 's lifetime, and a probability tree used to estimate the clinical outcomes and costs (US$ 2010) for each case. Quality-adjusted life-years (QALYs) lost to acute disease were calculated by multiplying cases of mild/moderate/severe pertussis by the associated health-state disutility; QALY losses due to death and long-term sequelae were also considered. Incremental costs and QALYs were summed over the cohort to derive incremental cost-effectiveness ratios. Scenario analyses evaluated the effect of alternative plausible parameter estimates on results.

RESULTS

At incidence levels of 25, 100, 200 cases/100,000, vaccinating adults aged 65 years costs an additional $336,000, $63,000 and $17,000/QALY gained, respectively. Vaccination has a cost-effectiveness ratio less than $50,000/QALY if pertussis incidence is >116 cases/100,000 from societal and payer perspectives. Results were robust to scenario analyses.

CONCLUSIONS

Tdap immunization of adults aged 65 years according to current ACIP recommendations is a cost-effective health-care intervention at plausible incidence assumptions.

Calpain-Mediated Processing of Adenylate Cyclase Toxin Generates a Cytosolic Soluble Catalytically Active N-Terminal Domain

Bordetella pertussis, the whooping cough pathogen, secretes several virulence factors among which adenylate cyclase toxin (ACT) is essential for establishment of the disease in the respiratory tract. ACT weakens host defenses by suppressing important bactericidal activities of the phagocytic cells. Up to now, it was believed that cell intoxication by ACT was a consequence of the accumulation of abnormally high levels of cAMP, generated exclusively beneath the host plasma membrane by the toxin N-terminal catalytic adenylate cyclase (AC) domain, upon its direct translocation across the lipid bilayer. Here we show that host calpain, a calcium-dependent Cys-protease, is activated into the phagocytes by a toxin-triggered calcium rise, resulting in the proteolytic cleavage of the toxin N-terminal domain that releases a catalytically active "soluble AC". The calpain-mediated ACT processing allows trafficking of the "soluble AC" domain into subcellular organella. At least two strategic advantages arise from this singular toxin cleavage, enhancing the specificity of action, and simultaneously preventing an indiscriminate activation of cAMP effectors throughout the cell. The present study provides novel insights into the toxin mechanism of action, as the calpain-mediated toxin processing would confer ACT the capacity for a space- and time-coordinated production of different cAMP "pools", which would play different roles in the cell pathophysiology.

Towards selective inhibitors of adenylyl cyclase toxin from Bordetella pertussis

Whooping cough is a very important medical problem that requires novel approaches for treatment. The disease is caused by Bordetella pertussis, with the calmodulin (CaM)-activated adenylyl cyclase (AC) toxin (also known as CyaA) being a major virulence factor. Hence, CyaA inhibitors could constitute novel therapeutics, but it has been difficult to develop potent drugs with high selectivity over mammalian membranous ACs (mACs). Recent studies have shown that bis-anthraniloyl-substituted nucleoside 5'-triphosphates are potent and selective CyaA inhibitors. In addition, the interaction of CyaA with CaM is very different from the interaction of membranous mAC1 with CaM. Accordingly, compounds that interfere with the CyaA-CaM interaction may constitute a novel class of drugs against whooping cough.

Interactions of Bordetella pertussis adenylyl cyclase toxin CyaA with calmodulin mutants and calmodulin antagonists: comparison with membranous adenylyl cyclase I

The adenylyl cyclase (AC) toxin CyaA from Bordetella pertussis constitutes an important virulence factor for the pathogenesis of whooping cough. CyaA is activated by calmodulin (CaM) and compromises host defense by excessive cAMP production. Hence, pharmacological modulation of the CyaA/CaM interaction could constitute a promising approach to treat whooping cough, provided that interactions of endogenous effector proteins with CaM are not affected. As a first step toward this ambitious goal we examined the interactions of CyaA with wild-type CaM and four CaM mutants in which most methionine residues were replaced by leucine residues and studied the effects of the CaM antagonists calmidazolium, trifluoperazine and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7). CyaA/CaM interaction was monitored by CaM-dependent fluorescence resonance energy transfer (FRET) between tryptophan residues in CyaA and 2'-(N-methylanthraniloyl)-3'-deoxy-adenosine 5'-triphosphate and catalytic activity. Comparison of the concentration/response curves of CaM and CaM mutants for FRET and catalysis revealed differences, suggesting a two-step activation mechanism of CyaA by CaM. Even in the absence of CaM, calmidazolium inhibited catalysis, and it did so according to a biphasic function. Trifluoperazine and W-7 did not inhibit FRET or catalysis. In contrast to CyaA, some CaM mutants were more efficacious than CaM at activating membranous AC isoform 1. The slope of CyaA activation by CaM was much steeper than of AC1 activation. Collectively, the two-step activation mechanism of CyaA by CaM offers opportunities for pharmacological intervention. The failure of classic CaM inhibitors to interfere with CyaA/CaM interactions and the different interactions of CaM mutants with CyaA and AC1 point to unique CyaA/CaM interactions.

Short-lived immunity against pertussis, age-specific routes of transmission, and the utility of a teenage booster vaccine

BACKGROUND

Pertussis incidence has been increasing for the past two decades in Norway, as in much of the highly vaccinated world. The greatest increase is in teenagers, although the most severe cases occur in infants. A teenage booster is recommended globally, largely with the aim of reducing infant incidence. However few countries have implemented the booster, and almost no data have been published on its utility in preventing infant cases. We aim to assess the duration of vaccine-induced immunity, and the possibility for a teenage-booster vaccine to protect infants in Norway.

METHODS AND FINDINGS

We used a unique data set that merged case reports with a national vaccine registry from Norway, 1996-2010, to assess age- and cohort-specific hazards of infection. We also developed and implemented a likelihood-based method for estimating the duration of immunity, taking into account age-contact data relevant for pertussis transmission. The risk of infection in thirteen-year olds increased nearly four-fold, however the hazard in infants did not significantly change. The seasonality of cases in pre-school-aged children differed from that of school-aged children. The introduction of a childhood booster vaccine provided indirect protection for unvaccinated members of the cohort, but little protection to neighboring cohorts. Additionally, we found evidence for increasingly rapid infection after three doses of vaccine, potentially caused by significant and heterogeneous loss of immunity. An estimated 15% of vaccinated individuals lost their immunity within five years after vaccination.

CONCLUSIONS

Immunity induced by the acellular pertussis vaccine prevents both disease and transmission, but is short-lived and heterogeneous. The age-mixing patterns lead to little contact between teenagers and infants. Therefore, while a teenage booster vaccine campaign would likely provide strong protection for cohorts of teenagers, it would provide little protection for infants.