Waning Immunity and Microbial Vaccines-Workshop of the National Institute of Allergy and Infectious Diseases

Since the middle of the 20th century, vaccines have made a significant public health impact by controlling infectious diseases globally. Although long-term protection has been achieved with some vaccines, immunity wanes over time with others, resulting in outbreaks or epidemics of infectious diseases. Long-term protection against infectious agents that have a complex life cycle and antigenic variation remains a key challenge. Novel strategies to characterize the short- and long-term immune responses to vaccines and to induce immune responses that mimic natural infection have recently emerged. New technologies and approaches in vaccinology, such as adjuvants, delivery systems, and antigen formulations, have the potential to elicit more durable protection and fewer adverse reactions; together with in vitro systems, these technologies have the capacity to model and accelerate vaccine development. The National Institute of Allergy and Infectious Diseases (NIAID) held a workshop on 19 September 2016 that focused on waning immunity to selected vaccines (for Bordetella pertussis, Salmonella enterica serovar Typhi, Neisseria meningitidis, influenza, mumps, and malaria), with an emphasis on identifying knowledge gaps, future research needs, and how this information can inform development of more effective vaccines for infectious diseases.

Cost-effectiveness analysis of typhoid conjugate vaccines in five endemic low- and middle-income settings

BACKGROUND

Typhoid fever remains endemic in low- and middle-income countries. Programmatic use of existing vaccines is limited, but upcoming typhoid conjugate vaccines (TCVs) could warrant wider use. We evaluated the cost-effectiveness of five TCV delivery strategies in three urban areas (Delhi and Kolkata, India and Nairobi, Kenya) and two rural settings (Lwak, Kenya and Dong Thap, Vietnam) with varying incidence.

METHODS AND FINDINGS

We evaluated routine infant vaccination with and without catch-up campaigns among older individuals. We used a dynamic model of typhoid transmission to simulate cases, hospitalizations, deaths, disability-adjusted life-years (DALY) lost, treatment and intervention costs. We estimated cost-effectiveness (in terms of cost in international dollars (I$) per DALY averted) from the healthcare payer perspective, and assessed how it was influenced by uncertain model parameters. Compared to no vaccination, routine infant vaccination at I$1/dose was cost-saving in Delhi and Dong Thap, "very cost-effective" in Kolkata and Nairobi, and "cost-effective" in Lwak according to World Health Organization thresholds. However, routine vaccination was not the optimal strategy compared to strategies that included a catch-up campaign, which yielded the highest probability of being cost-saving in Delhi and Dong Thap and were most likely to provide a return on investment above a willingness-to-pay threshold of I$1440 in Kolkata, I$2300 in Nairobi, and I$5360 in Lwak. Vaccine price impacted the optimal strategy, and the number of doses required and rate of hospitalization were the primary sources of uncertainty.

CONCLUSION

Routine vaccination with TCV would be cost-effective in most settings, and additional one-time catch-up campaigns would also be economically justified.

A forecast of typhoid conjugate vaccine introduction and demand in typhoid endemic low- and middle-income countries to support vaccine introduction policy and decisions

A Typhoid Conjugate Vaccine (TCV) is expected to acquire WHO prequalification soon, which will pave the way for its use in many low- and middle-income countries where typhoid fever is endemic. Thus it is critical to forecast future vaccine demand to ensure supply meets demand, and to facilitate vaccine policy and introduction planning. We forecasted introduction dates for countries based on specific criteria and estimated vaccine demand by year for defined vaccination strategies in 2 scenarios: rapid vaccine introduction and slow vaccine introduction. In the rapid introduction scenario, we forecasted 17 countries and India introducing TCV in the first 5 y of the vaccine's availability while in the slow introduction scenario we forecasted 4 countries and India introducing TCV in the same time period. If the vaccine is targeting infants in high-risk populations as a routine single dose, the vaccine demand peaks around 40 million doses per year under the rapid introduction scenario. Similarly, if the vaccine is targeting infants in the general population as a routine single dose, the vaccine demand increases to 160 million doses per year under the rapid introduction scenario. The demand forecast projected here is an upper bound estimate of vaccine demand, where actual demand depends on various factors such as country priorities, actual vaccine introduction, vaccination strategies, Gavi financing, costs, and overall product profile. Considering the potential role of TCV in typhoid control globally; manufacturers, policymakers, donors and financing bodies should work together to ensure vaccine access through sufficient production capacity, early WHO prequalification of the vaccine, continued Gavi financing and supportive policy.

An Overview of Structural Features of Antibacterial Glycoconjugate Vaccines That Influence Their Immunogenicity

Bacterial cell-surface-derived or mimicked carbohydrate moieties that act as protective antigens are used in the development of antibacterial glycoconjugate vaccines. The carbohydrate antigen must have a minimum length or size to maintain the conformational structure of the antigenic epitope(s). The presence or absence of O-acetate, phosphate, glycerol phosphate and pyruvate ketal plays a vital role in defining the immunogenicity of the carbohydrate antigen. The nature of the carrier protein, spacer and conjugation pattern used to develop the glycoconjugate vaccine also defines its overall spatial orientation which in turn affects its avidity and selectivity of interaction with the desired target(s). In addition, the ratio of carbohydrate to protein in glycoconjugate vaccines also makes an important contribution in determining the optimum immunological response. This Review article presents the importance of these variables in the development of antibacterial glycoconjugate vaccines and their effects on immune efficacy.

Challenges and Opportunities for Typhoid Fever Control: A Call for Coordinated Action

The burden of enteric fever caused by Salmonella enterica serovars Typhi and Paratyphi is substantial and has high impact in toddlers and young children. This burden is relatively well documented in Asia, and this supplement provides new data on the substantial burden in several sub-Saharan African countries. Challenges in standardized surveillance and imperfect diagnostic tools have resulted in patchy local disease data, which are not well acknowledged or integrated into local country evidence and health awareness for decision making. There is a need to strengthen diagnostics for the generation of burden data in country. Furthermore, the guidelines and training for treatment of enteric fever cases in Africa are sorely needed to help mitigate the inappropriate use of antimicrobial treatment. Classic water safety and access to sanitation development remain powerful tools for the control of typhoid fever, yet the huge economic costs and long timelines are unlikely to provide a short- to middle-term solution. Emerging threats, including multidrug resistance and increasing urbanization in regions such as sub-Saharan Africa, warrant focused attention to shorter-term interventions including immunization, and must include vaccine strategies with the new typhoid conjugate vaccines.

What Have We Learned From the Typhoid Fever Surveillance in Africa Program?

The Typhoid Fever Surveillance in Africa Program (TSAP) was established in 2009 to fill the data void concerning invasive Salmonella disease in sub-Saharan Africa, and to specifically estimate the burden of bloodstream infections caused by the key pathogen, Salmonella enterica serovar Typhi. TSAP has achieved this ambitious target, finding high incidences of typhoid fever in both rural and urban populations in several countries in sub-Saharan Africa. The results of TSAP will undoubtedly dictate the direction of future typhoid fever research in Africa, and at last provides a key piece of the disease burden jigsaw puzzle. With the dawn of new Vi conjugate vaccines against Salmonella Typhi, the next priority for the typhoid community must be providing the required data on these vaccines so they can be licensed and provided to those in high-risk groups and locations across sub-Saharan Africa.

Serum bactericidal assay for the evaluation of typhoid vaccine using a semi-automated colony-counting method

Typhoid fever, mainly caused by Salmonella enterica serovar Typhi (S. Typhi), is a life-threatening disease, mostly in developing countries. Enzyme-linked immunosorbent assay (ELISA) is widely used to quantify antibodies against S. Typhi in serum but does not provide information about functional antibody titers. Although the serum bactericidal assay (SBA) using an agar plate is often used to measure functional antibody titers against various bacterial pathogens in clinical specimens, it has rarely been used for typhoid vaccines because it is time-consuming and labor-intensive. In the present study, we established an improved SBA against S. Typhi using a semi-automated colony-counting system with a square agar plate harboring 24 samples. The semi-automated SBA efficiently measured bactericidal titers of sera from individuals immunized with S. Typhi Vi polysaccharide vaccines. The assay specifically responded to S. Typhi Ty2 but not to other irrelevant enteric bacteria including Vibrio cholerae and Shigella flexneri. Baby rabbit complement was more appropriate source for the SBA against S. Typhi than complements from adult rabbit, guinea pig, and human. We also examined the correlation between SBA and ELISA for measuring antibody responses against S. Typhi using pre- and post-vaccination sera from 18 human volunteers. The SBA titer showed a good correlation with anti-Vi IgG quantity in the serum as determined by Spearman correlation coefficient of 0.737 (P < 0.001). Taken together, the semi-automated SBA might be efficient, accurate, sensitive, and specific enough to measure functional antibody titers against S. Typhi in sera from human subjects immunized with typhoid vaccines.

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Improved SBA against S. Typhi was developed using a colony counting system.

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The improved SBA was specific to S. Typhi but not to other gram-negative bacteria.

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There was a good correlation between SBA and anti-Vi IgG titers in vaccinee’s sera.

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This SBA would be useful for the clinical immuno-monitoring of typhoid vaccines.

Preventing invasive salmonellosis in children through vaccination

INTRODUCTION

Vaccination is an important strategy to control endemic enteric fever (EF) and to interrupt transmission during outbreaks. The main aim of this paper is to discuss the efficacy of available EF vaccines in children and to highlight novel vaccination possibilities against EF and non-typhoid invasive salmonelloses.

AREAS COVERED

Two types of typhoid vaccines are presently available in the industrialized world. One of these vaccines is administered parenterally and is based on the virulence-associated (Vi) capsular polysaccaride of Salmonella typhi. The second vaccine is based on a live attenuated strain of the pathogen and is given orally. In addition, a Vi-tetanus toxoid conjugated vaccine is currently licensed in India; however, it is not available anywhere else. Expert commentary: Unfortunately, only typhoid fever is addressed by the currently licensed typhoid vaccines. Moreover, they are unsuitable for infants and remain a possible aid for reducing the risk of EF only in older subjects. They should be used in developing countries with endemic EF. New vaccines able to confer long-term protection to subjects in the first years of life and those with immature immune systems could significantly reduce incidence rates of EF in younger children. Vi-conjugate preparations are promising solutions in this regard.

Changing Patterns in Enteric Fever Incidence and Increasing Antibiotic Resistance of Enteric Fever Isolates in the United States, 2008-2012

BACKGROUND

Enteric fever in the United States has been primarily associated with travel and with worrisome changes in global patterns of antimicrobial resistance. We present the first comprehensive report of National Typhoid and Paratyphoid Fever Surveillance System (NTPFS) data for a 5-year period (2008-2012).

METHODS

We reviewed data on laboratory-confirmed cases reported to NTPFS, and related antimicrobial susceptibility results of Salmonella Typhi and Paratyphi A isolates sent for testing by participating public health laboratories to the Centers for Disease Control and Prevention's National Antimicrobial Resistance Monitoring System laboratory.

RESULTS

During 2008-2012, 2341 enteric fever cases were reported, 80% typhoid and 20% paratyphoid A. The proportion caused by paratyphoid A increased from 16% (2008) to 22% (2012). Foreign travel within 30 days preceding illness onset was reported by 1961 (86%) patients (86% typhoid and 92% paratyphoid A). Travel to southern Asia was common (82% for typhoid, 97% for paratyphoid A). Among 1091 (58%) typhoid and 262 (56%) paratyphoid A isolates tested for antimicrobial susceptibility, the proportion resistant to nalidixic acid (NAL-R) increased from 2008 to 2012 (Typhi, 60% to 68%; Paratyphi A, 91% to 94%). Almost all NAL-R isolates were resistant or showed decreased susceptibility to ciprofloxacin. Resistance to at least ampicillin, chloramphenicol, and trimethoprim-sulfamethoxazole (multidrug resistant [MDR]) was limited to Typhi isolates, primarily acquired in southern Asia (13%). Most MDR isolates were also NAL-R.

CONCLUSIONS

Enteric fever in the United States is primarily associated with travel to southern Asia, and increasing resistance is adding to treatment challenges. A bivalent typhoid and paratyphoid vaccine is needed.

The Burden of Typhoid and Paratyphoid in India: Systematic Review and Meta-analysis

BACKGROUND

Typhoid is an important public health challenge for India, especially with the spread of antimicrobial resistance. The decision about whether to introduce a public vaccination programme needs to be based on an understanding of disease burden and the age-groups and geographic areas at risk.

METHODS

We searched Medline and Web of Science databases for studies reporting the incidence or prevalence of typhoid and paratyphoid fever confirmed by culture and/or serology, conducted in India and published between 1950 and 2015. We used binomial and Poisson mixed-effects meta-regression models to estimate prevalence and incidence from hospital and community studies, and to identify risk-factors.

RESULTS

We identified 791 titles and abstracts, and included 37 studies of typhoid and 18 studies of paratyphoid in the systematic review and meta-analysis. The estimated prevalence of laboratory-confirmed typhoid and paratyphoid among individuals with fever across all hospital studies was 9.7% (95% CI: 5.7-16.0%) and 0.9% (0.5-1.7%) respectively. There was significant heterogeneity among studies (p-values<0.001). Typhoid was more likely to be detected among clinically suspected cases or during outbreaks and showed a significant decline in prevalence over time (odds ratio for each yearly increase in study date was 0.96 (0.92-0.99) in the multivariate meta-regression model). Paratyphoid did not show any trend over time and there was no clear association with risk-factors. Incidence of typhoid and paratyphoid was reported in 3 and 2 community cohort studies respectively (in Kolkata and Delhi, or Kolkata alone). Pooled estimates of incidence were 377 (178-801) and 105 (74-148) per 100,000 person years respectively, with significant heterogeneity between locations for typhoid (p<0.001). Children 2-4 years old had the highest incidence.

CONCLUSIONS

Typhoid remains a significant burden in India, particularly among young children, despite apparent declines in prevalence. Infant immunisation with newly-licensed conjugate vaccines could address this challenge.

Revisiting typhoid fever surveillance in low and middle income countries: lessons from systematic literature review of population-based longitudinal studies

Background

The control of typhoid fever being an important public health concern in low and middle income countries, improving typhoid surveillance will help in planning and implementing typhoid control activities such as deployment of new generation Vi conjugate typhoid vaccines.

Methods

We conducted a systematic literature review of longitudinal population-based blood culture-confirmed typhoid fever studies from low and middle income countries published from 1^st January 1990 to 31^st December 2013. We quantitatively summarized typhoid fever incidence rates and qualitatively reviewed study methodology that could have influenced rate estimates. We used meta-analysis approach based on random effects model in summarizing the hospitalization rates.

Results

Twenty-two papers presented longitudinal population-based and blood culture-confirmed typhoid fever incidence estimates from 20 distinct sites in low and middle income countries. The reported incidence and hospitalizations rates were heterogeneous as well as the study methodology across the sites. We elucidated how the incidence rates were underestimated in published studies. We summarized six categories of under-estimation biases observed in these studies and presented potential solutions.

Conclusions

Published longitudinal typhoid fever studies in low and middle income countries are geographically clustered and the methodology employed has a potential for underestimation. Future studies should account for these limitations.

Electronic supplementary material

The online version of this article (doi:10.1186/s12879-016-1351-3) contains supplementary material, which is available to authorized users.

Differential Killing of Salmonella enterica Serovar Typhi by Antibodies Targeting Vi and Lipopolysaccharide O:9 Antigen

Salmonella enterica serovar Typhi expresses a capsule of Vi polysaccharide, while most Salmonella serovars, including S. Enteritidis and S. Typhimurium, do not. Both S. Typhi and S. Enteritidis express the lipopolysaccharide O:9 antigen, yet there is little evidence of cross-protection from anti-O:9 antibodies. Vaccines based on Vi polysaccharide have efficacy against typhoid fever, indicating that antibodies against Vi confer protection. Here we investigate the role of Vi capsule and antibodies against Vi and O:9 in antibody-dependent complement- and phagocyte-mediated killing of Salmonella. Using isogenic Vi-expressing and non-Vi-expressing derivatives of S. Typhi and S. Typhimurium, we show that S. Typhi is inherently more sensitive to serum and blood than S. Typhimurium. Vi expression confers increased resistance to both complement- and phagocyte-mediated modalities of antibody-dependent killing in human blood. The Vi capsule is associated with reduced C3 and C5b-9 deposition, and decreased overall antibody binding to S. Typhi. However, purified human anti-Vi antibodies in the presence of complement are able to kill Vi-expressing Salmonella, while killing by anti-O:9 antibodies is inversely related to Vi expression. Human serum depleted of antibodies to antigens other than Vi retains the ability to kill Vi-expressing bacteria. Our findings support a protective role for Vi capsule in preventing complement and phagocyte killing of Salmonella that can be overcome by specific anti-Vi antibodies, but only to a limited extent by anti-O:9 antibodies.

Purification and characterization of a Shigella conjugate vaccine, produced by glycoengineering Escherichia coli

Shigellosis remains a major cause of diarrheal disease in developing countries and causes substantial morbidity and mortality in children. Glycoconjugate vaccines consisting of bacterial surface polysaccharides conjugated to carrier proteins are the most effective vaccines for controlling invasive bacterial infections. Nevertheless, the development of a multivalent conjugate vaccine to prevent Shigellosis has been hampered by the complex manufacturing process as the surface polysaccharide for each strain requires extraction, hydrolysis, chemical activation and conjugation to a carrier protein. The use of an innovative biosynthetic Escherichia coli glycosylation system substantially simplifies the production of glycoconjugates. Herein, the Shigella dysenteriae type 1 (Sd1) O-polysaccharide is expressed and its functional assembly on an E. coli glycosyl carrier lipid is demonstrated by HPLC analysis and mass spectrometry. The polysaccharide is enzymatically conjugated to specific asparagine residues of the carrier protein by co-expression of the PglB oligosaccharyltransferase and the carrier protein exotoxin A (EPA) from Pseudomonas aeruginosa. The extraction and purification of the Shigella glycoconjugate (Sd1-EPA) and its detailed characterization by the use of physicochemical methods including NMR and mass spectrometry is described. The report shows for the first time that bioconjugation provides a newly developed and improved approach to produce an Sd1 glycoconjugate that can be characterized using state-of-the-art techniques. In addition, this generic process together with the analytical methods is ideally suited for the production of additional Shigella serotypes, allowing the development of a multivalent Shigella vaccine.

Typhoid fever vaccination strategies

Typhoid vaccination is an important component of typhoid fever prevention and control, and is recommended for public health programmatic use in both endemic and outbreak settings. We reviewed experiences with various vaccination strategies using the currently available typhoid vaccines (injectable Vi polysaccharide vaccine [ViPS], oral Ty21a vaccine, and injectable typhoid conjugate vaccine [TCV]). We assessed the rationale, acceptability, effectiveness, impact and implementation lessons of these strategies to inform effective typhoid vaccination strategies for the future. Vaccination strategies were categorized by vaccine disease control strategy (preemptive use for endemic disease or to prevent an outbreak, and reactive use for outbreak control) and vaccine delivery strategy (community-based routine, community-based campaign and school-based). Almost all public health typhoid vaccination programs used ViPS vaccine and have been in countries of Asia, with one example in the Pacific and one experience using the Ty21a vaccine in South America. All vaccination strategies were found to be acceptable, feasible and effective in the settings evaluated; evidence of impact, where available, was strongest in endemic settings and in the short- to medium-term. Vaccination was cost-effective in high-incidence but not low-incidence settings. Experience in disaster and outbreak settings remains limited. TCVs have recently become available and none are WHO-prequalified yet; no program experience with TCVs was found in published literature. Despite the demonstrated success of several typhoid vaccination strategies, typhoid vaccines remain underused. Implementation lessons should be applied to design optimal vaccination strategies using TCVs which have several anticipated advantages, such as potential for use in infant immunization programs and longer duration of protection, over the ViPS and Ty21a vaccines for typhoid prevention and control.

Diagnostics for invasive Salmonella infections: Current challenges and future directions

Invasive Salmonellosis caused by Salmonella enterica serotype Typhi or Paratyphi A, B, C, or invasive non-typhoidal Salmonella serotypes, is an immensely important disease cluster for which reliable, rapid diagnostic tests are not available. Blood culture remains the gold standard but is insensitive, slow, and resource-intensive. Existing molecular diagnostics have poor sensitivity due to the low organism burden in bodily fluids. Commercially available serologic tests for typhoidal Salmonella have had limited sensitivity and specificity. In high burden, resource-limited settings, reliance on clinical diagnosis or inaccurate tests often results in frequent, unnecessary treatment, which contributes selective pressure for the emergence of antimicrobial resistance. This practice also results in inadequate therapy for other etiologies of acute febrile illnesses, including leptospirosis and rickettsial infections. A number of novel serologic, molecular, transcriptomic and metabolomic approaches to diagnostics are under development. Target product profiles that outline specific needs may focus development and investment, and establish benchmarks for accuracy, cost, speed, and portability of new diagnostics. Of note, a critical barrier to diagnostic assay rollout will be the low cost and low perceived harm of empiric therapy on behalf of providers and patients, which leaves few perceived incentives to utilize diagnostics. Approaches that align incentives with societal goals of limiting inappropriate antimicrobial use, such as subsidizing diagnostics, may be essential for stimulating development and uptake of such assays in resource-limited settings. New diagnostics for invasive Salmonellosis should be developed and deployed alongside diagnostics for alternative etiologies of acute febrile illnesses to improve targeted use of antibiotics.

One more shot for the road: a review and update of vaccinations for pediatric international travelers

Increasing numbers of children are traveling to developing countries where they are often at a higher risk than adults of acquiring vaccine-preventable diseases. Yet, they are less likely to receive pretravel medical advice and preventive care. This article reviews the current recommendations for pediatric travel immunizations, including specific travel vaccines such as typhoid, yellow fever, Japanese encephalitis virus, and rabies as well as prospective vaccines for significant global diseases like malaria, dengue, chikungunya, and Ebola.

Structural analysis of the O-acetylated O-polysaccharide isolated from Salmonella paratyphi A and used for vaccine preparation

Salmonella paratyphi A is increasingly recognized as a common cause of enteric fever cases and there are no licensed vaccines against this infection. Antibodies directed against the O-polysaccharide of the lipopolysaccharide of Salmonella are protective and conjugation of the O-polysaccharide to a carrier protein represents a promising strategy for vaccine development. O-Acetylation of S. paratyphi A O-polysaccharide is considered important for the immunogenicity of S. paratyphi A conjugate vaccines. Here, as part of a programme to produce a bivalent conjugate vaccine against both S. typhi and S. paratyphi A diseases, we have fully elucidated the O-polysaccharide structure of S. paratyphi A by use of HPLC-SEC, HPAEC-PAD/CD, GLC, GLC-MS, 1D and 2D-NMR spectroscopy. In particular, chemical and NMR studies identified the presence of O-acetyl groups on C-2 and C-3 of rhamnose in the lipopolysaccharide repeating unit, at variance with previous reports of O-acetylation at a single position. Moreover HR-MAS NMR analysis performed directly on bacterial pellets from several strains of S. paratyphi A also showed O-acetylation on C-2 and C-3 of rhamnose, thus this pattern is common and not an artefact from O-polysaccharide purification. Conjugation of the O-polysaccharide to the carrier protein had little impact on O-acetylation and therefore should not adversely affect the immunogenicity of the vaccine.

Nasal immunization of mice with AFCo1 or AFPL1 plus capsular polysaccharide Vi from Salmonella typhi induces cellular response and memory B and T cell responses

The response to infection against Salmonella involves both B and T cell mediated immunity. An effective immunization can activate an adequate immune response capable to control the primary infection and protect against a secondary infection. Mucosal vaccination, by inducing local pathogen-specific immune responses, has the potential to counter mucosally transmitted pathogens at the portal of entry, thereby increasing the efficacy of vaccines. The aim of this work was to explore the efficacy of AFCo1 or AFPL1, as mucosal adjuvants to stimulate cell immunity and memory responses against Vi polysaccharide antigen of Salmonella typhi (PsVi). Mice immunized with 3 intranasal doses exhibited high levels of PsVi-specific IgG (p<0.05), IgG2a and IgG2c subclasses. Also, an amplified recall response after a booster immunization with a plain polysaccharide vaccine was induced. Avidities index were higher in mice immunized with adjuvanted formulations at different chaotropic concentrations. Furthermore, IL-12 and IFN-γ levels in nasally vaccinated mice with both adjuvants were induced. Moreover, priming with 3 doses followed by booster immunization with VaxTyVi(®) resulted in high levels of anti-Vi specific IgG, IgG subclasses and antibody avidity. Long lived plasma cells in bone marrow, memory B cells and long-term memory T cells after booster dose were induced. The combined formulation of Vi polysaccharide with mucosal adjuvants provides an improved immunogenicity, in particular with regard to cellular responses and long lasting cells responses.

The effects of vaccination and immunity on bacterial infection dynamics in vivo

Salmonella enterica infections are a significant global health issue, and development of vaccines against these bacteria requires an improved understanding of how vaccination affects the growth and spread of the bacteria within the host. We have combined in vivo tracking of molecularly tagged bacterial subpopulations with mathematical modelling to gain a novel insight into how different classes of vaccines and branches of the immune response protect against secondary Salmonella enterica infections of the mouse. We have found that a live Salmonella vaccine significantly reduced bacteraemia during a secondary challenge and restrained inter-organ spread of the bacteria in the systemic organs. Further, fitting mechanistic models to the data indicated that live vaccine immunisation enhanced both the bacterial killing in the very early stages of the infection and bacteriostatic control over the first day post-challenge. T-cell immunity induced by this vaccine is not necessary for the enhanced bacteriostasis but is required for subsequent bactericidal clearance of Salmonella in the blood and tissues. Conversely, a non-living vaccine while able to enhance initial blood clearance and killing of virulent secondary challenge bacteria, was unable to alter the subsequent bacterial growth rate in the systemic organs, did not prevent the resurgence of extensive bacteraemia and failed to control the spread of the bacteria in the body.

The bacterium Salmonella enterica causes gastroenteritis and the severe systemic diseases typhoid, paratyphoid fever and non-typhoidal septicaemia (NTS). Treatment of systemic disease with antibiotics is becoming increasingly difficult due to the acquisition of resistance. Licensed vaccines are available for the prevention of typhoid, but not paratyphoid fever or NTS. Vaccines can be either living (attenuated strains) or non-living (e.g. inactivated whole cells or surface polysaccharides) and these different classes potentially activate different components of the host immune system. Improvements in vaccine design require a better understanding of how different vaccine types differ in their ability to control a subsequent infection. We have improved a previously developed experimental system and mathematical model to investigate how these different vaccine types act. We show that the inactivated vaccine can only control bacterial numbers by a transient increase in bactericidal activity whereas the living vaccine is superior as it can induce an immune response that rapidly kills, then restrains the growth and spread of infecting bacteria.

Vaccines against invasive Salmonella disease: current status and future directions

Though primarily enteric pathogens, Salmonellae are responsible for a considerable yet under-appreciated global burden of invasive disease. In South and South-East Asia, this manifests as enteric fever caused by serovars Typhi and Paratyphi A. In sub-Saharan Africa, a similar disease burden results from invasive nontyphoidal Salmonellae, principally serovars Typhimurium and Enteritidis. The existing Ty21a live-attenuated and Vi capsular polysaccharide vaccines target S. Typhi and are not effective in young children where the burden of invasive Salmonella disease is highest. After years of lack of investment in new Salmonella vaccines, recent times have seen increased interest in the area led by emerging-market manufacturers, global health vaccine institutes and academic partners. New glycoconjugate vaccines against S. Typhi are becoming available with similar vaccines against other invasive serovars in development. With other new vaccines under investigation, including live-attenuated, protein-based and GMMA vaccines, now is an exciting time for the Salmonella vaccine field.

Phase I clinical trial of O-acetylated pectin conjugate, a plant polysaccharide based typhoid vaccine

BACKGROUND

Typhoid fever remains an important cause of morbidity and mortality in the developing countries. Vi capsular polysaccharide conjugate vaccine demonstrated safety and efficacy in young children in high endemic regions. A novel typhoid conjugate vaccine based on plant polysaccharide pectin was studied in a phase I trial.

METHODS

Fruit pectin, having the same carbohydrate backbone structure as Vi, was purified from citrus peel and used as the polysaccharide source to prepare a semi-synthetic typhoid conjugate vaccine. Pectin was chemically O-acetylated (OAcPec) to antigenically resemble Vi and conjugated to carrier protein rEPA, a recombinant exoprotein A from Pseudomonas aeruginosa. 25 healthy volunteers, 18-45 years old, were injected once with OAcPec-rEPA. Safety and IgG antibodies reactive with Vi and pectin were analyzed.

RESULTS

No vaccine associated serious adverse reaction was reported. Six weeks after the injection of OAcPec-rEPA, 64% of the volunteers elicited >4-fold rise of anti-Vi IgG. At 26 weeks the level declined, but the difference between the levels at 6 and 26 weeks are not statistically significant. There is a direct correlation between the level of anti-Vi IgG before and after the injection (R(2)=0.96). The anti-Vi IgG can be absorbed by Vi, but not by pectin. There was no corresponding increase of anti-pectin after the injection, indicating the antibody response to OAcPec-rEPA was specific to Vi. There is no Vi-rEPA data in US adults for comparison of immune responses. The OAcPec-rEPA elicited significantly less IgG anti-Vi in US adults than those by Vi-rEPA in Vietnamese adults.

CONCLUSION

The O-acetylated pectin conjugate, a plant based typhoid vaccine, is safe and immunogenic in adult volunteers. ClinicalTrial.gov identifier: NCT00277147, NIH Protocol ID number: OH06-CH-0070, FDA BB Investigation New Drug (IND) number 6989.

Re-examination of immune response and estimation of anti-Vi IgG protective threshold against typhoid fever-based on the efficacy trial of Vi conjugate in young children

BACKGROUND

The capsular polysaccharide of Salmonella enterica serovar Typhi, Vi antigen, is an essential virulence factor and a protective antigen. Similar to other polysaccharide vaccines, the protective action of Vi, both to the polysaccharide alone or when presented as a conjugate, is mediated by serum IgG Vi antibodies. The evaluation of Vi capsular polysaccharide based vaccines to prevent typhoid fever would be significantly facilitated by the identification of a "protective level" of serum antibodies to Vi antigen.

METHODS

The protective level of anti-Vi IgG against typhoid fever was derived from the protective efficacy and immune response of a Vi-rEPA conjugate vaccine efficacy trial. The estimation was derived by two methods: correlation of the percent efficacy and the antibody distribution profile in the vaccine group at a given period of observation, and use of the relative ratio of anti-Vi IgG levels between the vaccine and placebo groups greater or equal to the Relative Risk of typhoid fever used in the efficacy determination.

RESULTS

Both methods predicted a similar range of a minimum protective level of anti-Vi IgG between 1.4 and 2.0μg/ml (short term threshold). When applying a protective threshold of 10μg/ml at 6 months post immunization, an IgG level in excess of 1.4μg/ml was achieved by 90% of children at 46 months post immunization, consistent with an 89% level of protection over the duration of the study. We thus suggest that the proportion of children with Vi IgG>10μg/ml (long term threshold) 6 months after immunization may reflect the proportion protected over at least a 4 year period.

CONCLUSION

The current assignment of an anti-Vi IgG protective level may be of value when evaluating vaccine performance of future Vi conjugate vaccines.

[Vaccinations for international travelers]

Vaccinations are a prominent part of health preparations before international travel. They can avoid or significantly reduce the risk of numerous infectious diseases. Until recently, vaccination against yellow fever was the only obligatory vaccination. However, according to updated international health regulations, other vaccinations and prophylactic measures may be required at entry from certain countries. For all routine vaccinations as recommended in Germany, necessary revaccination and catch-up of missed vaccinations should be administered before travel. At most destinations the risk of infection is higher than in Germany. Hepatitis A vaccine is generally recommended for travelers to areas of increased risk, polio vaccine for all destinations where eradication is not yet confirmed (Asia and Africa). The indications for other travel vaccines must take into consideration travel destination and itinerary, type and duration of travel, individual risk of exposure as well as the epidemiology of the disease to be prevented. Several vaccines of potential interest for travel medicine, e.g., new vaccines against malaria and dengue fever, are under development.