Onset and duration of protective immunity in challenged volunteers after vaccination with live oral cholera vaccine CVD 103-HgR

Controlled human infectious models, a path forward in uncovering immunological correlates of protection: Lessons from enteric fevers studies

Enteric infectious diseases account for more than a billion disease episodes yearly worldwide resulting in approximately 2 million deaths, with children under 5 years old and the elderly being disproportionally affected. Enteric pathogens comprise viruses, parasites, and bacteria; the latter including pathogens such as Salmonella [typhoidal (TS) and non-typhoidal (nTS)], cholera, Shigella and multiple pathotypes of Escherichia coli (E. coli). In addition, multi-drug resistant and extensively drug-resistant (XDR) strains (e.g., S. Typhi H58 strain) of enteric bacteria are emerging; thus, renewed efforts to tackle enteric diseases are required. Many of these entero-pathogens could be controlled by oral or parenteral vaccines; however, development of new, effective vaccines has been hampered by lack of known immunological correlates of protection (CoP) and limited knowledge of the factors contributing to protective responses. To fully comprehend the human response to enteric infections, an invaluable tool that has recently re-emerged is the use of controlled human infection models (CHIMs) in which participants are challenged with virulent wild-type (wt) organisms. CHIMs have the potential to uncover immune mechanisms and identify CoP to enteric pathogens, as well as to evaluate the efficacy of therapeutics and vaccines in humans. CHIMs have been used to provide invaluable insights in the pathogenesis, host-pathogen interaction and evaluation of vaccines. Recently, several Oxford typhoid CHIM studies have been performed to assess the role of multiple cell types (B cells, CD8+ T, Tregs, MAIT, Monocytes and DC) during S. Typhi infection. One of the key messages that emerged from these studies is that baseline antigen-specific responses are important in that they can correlate with clinical outcomes. Additionally, volunteers who develop typhoid disease (TD) exhibit higher levels and more activated cell types (e.g., DC and monocytes) which are nevertheless defective in discrete signaling pathways. Future critical aspects of this research will involve the study of immune responses to enteric infections at the site of entry, i.e., the intestinal mucosa. This review will describe our current knowledge of immunity to enteric fevers caused byS. Typhi and S. Paratyphi A, with emphasis on the contributions of CHIMs to uncover the complex immunological responses to these organisms and provide insights into the determinants of protective immunity.

Human Challenge Studies for Cholera

The human challenge model permits an estimate of the vaccine protection against moderate and severe cholera. It eliminates the difficulty in setting up a vaccine study in endemic area including uncertainties about the incidence of cholera and the logistic arrangements for capturing those who do/do not become ill. Valuable information from small groups of subjects can be obtained in a short period. Under proper precautions and study design, the challenge model is safe and efficient. Although the model has evolved since it was introduced over 50 years ago, it has been used extensively to test vaccine efficacy. Vaccine licensure has resulted from data obtained using the human challenge model. In addition, the model has been used to: (1) Establish and validate a standardized inoculum, (2) Identify immune markers and immune responses, (3) Determine natural immunity (in re-challenge studies), (4) Identify the role of the gastric acid barrier in preventing cholera infection, (5) Show homologous and heterologous infection-derived immunity, and (6) Test the efficacy of anti-diarrheal/anti-secretory small molecules. The aim of this chapter is to present an overview on the state of the art for human challenge models used to study cholera and new medical interventions against it.

Peculiarities of Zika Immunity and Vaccine Development: Lessons from Dengue and the Contribution from Controlled Human Infection Model

The Zika virus (ZIKV) was first isolated from a rhesus macaque in the Zika forest of Uganda in 1947. Isolated cases were reported until 2007, when the first major outbreaks of Zika infection were reported from the Island of Yap in Micronesia and from French Polynesia in 2013. In 2015, ZIKV started to circulate in Latin America, and in 2016, ZIKV was considered by WHO to be a Public Health Emergency of International Concern due to cases of Congenital Zika Syndrome (CZS), a ZIKV-associated complication never observed before. After a peak of cases in 2016, the infection incidence dropped dramatically but still causes concern because of the associated microcephaly cases, especially in regions where the dengue virus (DENV) is endemic and co-circulates with ZIKV. A vaccine could be an important tool to mitigate CZS in endemic countries. However, the immunological relationship between ZIKV and other flaviviruses, especially DENV, and the low numbers of ZIKV infections are potential challenges for developing and testing a vaccine against ZIKV. Here, we discuss ZIKV vaccine development with the perspective of the immunological concerns implicated by DENV-ZIKV cross-reactivity and the use of a controlled human infection model (CHIM) as a tool to accelerate vaccine development.

Update on CVD 103-HgR single-dose, live oral cholera vaccine

Cholera remains endemic in >50 countries, putting millions at risk, especially young children for whom killed vaccines offer limited protection. An oral, live attenuated vaccine - CVD 103-HgR (Vaxchora vaccine) - was licensed by the US FDA in 2016 for adults aged 18-64 years traveling to endemic regions, based on clinical trials in human volunteers showing the vaccine was well tolerated and conferred 90% efficacy within 10 days. The evidence base for Vaxchora vaccine has expanded with additional clinical trial data, in older adults (aged 46-64 years) and children (aged 2-17 years), demonstrating that the vaccine produces a strong vibriocidal antibody response. Over 68,000 doses have been administered in the United States, with no new safety signals. The dose volume has been reduced in children to improve acceptability, and cold chain requirements are less st ringent, at +2°C─+8°C. The vaccine has recently been licensed in the Untied States for children aged 2-17 years, in Europe for individuals aged ≥2 years, and for home administration in Europe. Next steps include a Phase 4 study in infants (6-23 months). Additional information is needed regarding duration of immunity, the need for and timing of revaccination, and efficacy data from lower-middle-income countries.

Systemic, Mucosal, and Memory Immune Responses following Cholera

Vibrio cholerae O1, the major causative agent of cholera, remains a significant public health threat. Although there are available vaccines for cholera, the protection provided by killed whole-cell cholera vaccines in young children is poor. An obstacle to the development of improved cholera vaccines is the need for a better understanding of the primary mechanisms of cholera immunity and identification of improved correlates of protection. Considerable progress has been made over the last decade in understanding the adaptive and innate immune responses to cholera disease as well as V. cholerae infection. This review will assess what is currently known about the systemic, mucosal, memory, and innate immune responses to clinical cholera, as well as recent advances in our understanding of the mechanisms and correlates of protection against V. cholerae O1 infection.

Longitudinal analysis of human humoral responses after vaccination with a live attenuated V. cholerae vaccine

Vibrio cholerae is a bacterial pathogen which causes the severe acute diarrheal disease cholera. Given that a symptomatic incident of cholera can lead to long term protection, a thorough understanding of the immune response to this pathogen is needed to identify parameters critical to the generation and durability of immunity. To approach this, we utilized a live attenuated cholera vaccine to model the response to V. cholerae infection in 12 naïve subjects. We found that this live attenuated vaccine induced durable vibriocidal antibody titers that were maintained at least one year after vaccination. Similar to what we previously reported in infected patients from Bangladesh, we found that vaccination induced plasmablast responses were primarily specific to the two immunodominant antigens lipopolysaccharide (LPS) and cholera toxin (CT). Interestingly, the magnitude of the early plasmablast response at day 7 predicted the serological outcome of vaccination at day 30. However, this correlation was no longer present at later timepoints. The acute responses displayed preferential immunoglobulin isotype usage, with LPS specific cells being largely IgM or IgA producing, while cholera toxin responses were predominantly IgG. Finally, CCR9 was highly expressed on vaccine induced plasmablasts, especially on IgM and IgA producing cells, suggesting a role in migration to the gastrointestinal tract. Collectively, these findings demonstrate that the use of a live attenuated cholera vaccine is an effective tool to examine the primary and long-term immune response following V. cholerae exposure. Additionally, it provides insight into the phenotype and specificity of the cells which likely return to and mediate immunity at the intestinal mucosa. A thorough understanding of these properties both in peripheral blood and in the intestinal mucosae will inform future vaccine development against both cholera and other mucosal pathogens. Trial Registration: NCT03251495.

Long-Term Immunogenicity of Live Oral Cholera Vaccine CVD 103-HgR in Adolescents Aged 12-17 Years in the United States

As part of a phase 4, randomized, double-blind, placebo-controlled trial to assess the immunogenicity and safety of PXVX0200 in children and adolescents aged 2-17 years, a subset of 73 adolescent subjects aged 12-17 years was followed for 2 years after vaccination and had blood collected for antibody assays on days 1, 11, 29, 91, 181, 365, 547, and 730. Endpoints included serum vibriocidal antibody (SVA) seroconversion, defined as a 4-fold or greater rise in antibody titer over baseline; geometric mean titers (GMTs); and geometric mean fold increase (GMFI) over baseline. Serum vibriocidal antibody seroconversion persisted in most subjects, with a rate of 64.5% noted at day 730. Geometric mean titers and GMFI both peaked at day 11 and remained greater than baseline at all time points, including day 730. Vaccination with PXVX0200 produces an immune response which persists for at least 2 years in adolescents aged 12-17 years.

Vaccines against gastroenteritis, current progress and challenges

Enteric viral and bacterial infections continue to be a leading cause of mortality and morbidity in young children in low-income and middle-income countries, the elderly, and immunocompromised individuals. Vaccines are considered an effective and practical preventive approach against the predominantly fecal-to-oral transmitted gastroenteritis particularly in the resource-limited countries or regions where implementation of sanitation systems and supply of safe drinking water are not quickly achievable. While vaccines are available for a few enteric pathogens including rotavirus and cholera, there are no vaccines licensed for many other enteric viral and bacterial pathogens. Challenges in enteric vaccine development include immunological heterogeneity among pathogen strains or isolates, a lack of animal challenge models to evaluate vaccine candidacy, undefined host immune correlates to protection, and a low protective efficacy among young children in endemic regions. In this article, we briefly updated the progress and challenges in vaccines and vaccine development for the leading enteric viral and bacterial pathogens including rotavirus, human calicivirus, Shigella, enterotoxigenic Escherichia coli (ETEC), cholera, nontyphoidal Salmonella, and Campylobacter, and introduced a novel epitope- and structure-based vaccinology platform known as MEFA (multiepitope fusion antigen) and the application of MEFA for developing broadly protective multivalent vaccines against heterogenous pathogens.

Safety and Immunogenicity of Live Oral Cholera Vaccine CVD 103-HgR in Children and Adolescents Aged 6-17 Years

The attenuated recombinant Vibrio cholerae O1 vaccine strain CVD 103-HgR, redeveloped as PXVX0200, elicits a rapid serum vibriocidal antibody (SVA) response and protects against cholera-induced diarrhea in adult volunteer challenge trials but has not been studied in children in developed countries. We performed a phase 4, placebo-controlled, double-blind, multicenter study to assess the safety, immunogenicity, and tolerability of a single, oral dose of PXVX0200 in children and adolescents aged 6–17 years in the United States and bridged immunogenicity to adults aged 18–45 years from a separate lot consistency study. Volunteers were randomized to receive a single dose of 1 × 10⁹ colony forming units (CFU) of PXVX0200 or placebo. Immunogenicity endpoints included SVA levels on days 1, 11, and 29 in volunteers aged 6–17 years and also on days 91 and 181 in volunteers aged 12–17 years. Safety was assessed by comparing solicited signs and symptoms on days 1–8, unsolicited adverse events (AEs) through day 29, and serious AEs through day 181. A total of 374 participants were enrolled, comprising 321 vaccine and 53 placebo recipients. The SVA seroconversion rates 10 days after immunization were 98.6% and 2.1% in vaccine and placebo recipients, respectively, and the vaccine seroconversion rate was non-inferior to the 93.5% rate seen in adults aged 18–45 years. Most reactogenicity was mild to moderate, and there were no vaccine-related serious AEs. The complete dose was consumed in 95.3% and 98.1% of vaccine and placebo recipients, respectively. PXVX0200 appears safe, immunogenic, and well tolerated in children and adolescents aged 6–17 years.

Distal Consequences of Oral Inflammation

Periodontitis is an incredibly prevalent chronic inflammatory disease, which results in the destruction of tooth supporting structures. However, in addition to causing tooth and alveolar bone loss, this oral inflammatory disease has been shown to contribute to disease states and inflammatory pathology at sites distant from the oral cavity. Epidemiological and experimental studies have linked periodontitis to the development and/or exacerbation of a plethora of other chronic diseases ranging from rheumatoid arthritis to Alzheimer's disease. Such studies highlight how the inflammatory status of the oral cavity can have a profound impact on systemic health. In this review we discuss the disease states impacted by periodontitis and explore potential mechanisms whereby oral inflammation could promote loss of homeostasis at distant sites.

Oral immunization with a probiotic cholera vaccine induces broad protective immunity against Vibrio cholerae colonization and disease in mice

Oral cholera vaccines (OCVs) are being increasingly employed, but current killed formulations generally require multiple doses and lack efficacy in young children. We recently developed a new live-attenuated OCV candidate (HaitiV) derived from a Vibrio cholerae strain isolated during the 2010 Haiti cholera epidemic. HaitiV exhibited an unexpected probiotic-like activity in infant rabbits, preventing intestinal colonization and disease by wild-type V. cholerae before the onset of adaptive immunity. However, it remained unknown whether HaitiV would behave similarly to other OCVs to stimulate adaptive immunity against V. cholerae. Here, we orally immunized adult germ-free female mice to test HaitiV’s immunogenicity. HaitiV safely and stably colonized vaccinated mice and induced known adaptive immune correlates of cholera protection within 14 days of administration. Pups born to immunized mice were protected against lethal challenges of both homologous and heterologous V. cholerae strains. Cross-fostering experiments revealed that protection was not dependent on vaccine colonization in or transmission to the pups. These findings demonstrate the protective immunogenicity of HaitiV and support its development as a new tool for limiting cholera.

Oral cholera vaccines are increasingly used as public health tools for prevention of cholera and curtailing the spread of outbreaks. However, current killed vaccines provide minimal protection in young children, who are especially susceptible to this diarrheal disease, and require ~7–14 days between vaccination and development of protective immunity. We recently created HaitiV, a live-attenuated oral cholera vaccine candidate derived from a clinical isolate from the Haiti cholera outbreak. Unexpectedly, HaitiV protected against cholera-like illness in infant rabbits within 24 hours of administration, before the onset of adaptive immunity. However, HaitiV’s capacity to stimulate adaptive immune responses against the cholera pathogen were not investigated. Here, we report that HaitiV induces immunological correlates of protection against cholera in adult germ-free mice and leads to protection against disease in their offspring. Protection against disease was transferable through the milk of the immunized mice and was not due to transmission or colonization of HaitiV in this model. Coupling the immunogenicity data presented here with our previous observation that HaitiV can protect from cholera prior to the induction of adaptive immunity, we propose that HaitiV may provide both rapid-onset short-term protection from disease while eliciting stable and long-lasting immunity against cholera.

Lipopolysaccharide-specific memory B cell responses to an attenuated live cholera vaccine are associated with protection against Vibrio cholerae infection

BACKGROUND

The single-dose live attenuated vaccine CVD 103-HgR protects against experimental Vibrio cholerae infection in cholera-naïve adults for at least 6 months after vaccination. While vaccine-induced vibriocidal seroconversion is associated with protection, vibriocidal titers decline rapidly from their peak 1-2 weeks after vaccination. Although vaccine-induced memory B cells (MBCs) might mediate sustained protection in individuals without detectable circulating antibodies, it is unknown whether oral cholera vaccination induces a MBC response.

METHODS

In a study that enrolled North American adults, we measured lipopolysaccharide (LPS)- and cholera toxin (CtxB)-specific MBC responses to PXVX0200 (derived from the CVD 103-HgR strain) and assessed stool volumes following experimental Vibrio cholerae infection. We then evaluated the association between vaccine-induced MBC responses and protection against cholera.

RESULTS

There was a significant increase in % CT-specific IgG, % LPS-specific IgG, and % LPS-specific IgA MBCs which persisted 180 days after vaccination as well as a significant association between vaccine-induced increase in % LPS-specific IgA MBCs and lower post-challenge stool volume (r = -0.56, p < 0.001).

DISCUSSION

Oral cholera vaccination induces antigen-specific MBC responses, and the anamnestic LPS-specific responses may contribute to long-term protection and provide correlates of the duration of vaccine-induced protection.

CLINICAL TRIALS REGISTRATION

NCT01895855.

Safety and immunogenicity of single-dose live oral cholera vaccine strain CVD 103-HgR in healthy adults age 18-45

The attenuated recombinant Vibrio cholerae O1 vaccine strain CVD 103-HgR, re-developed as PXVX0200, elicits a rapid serum vibriocidal antibody (SVA) response and protects against cholera diarrhea in volunteer challenge studies. We performed a phase 3, placebo controlled, double blind, multi-center study to further assess the safety, immunogenicity, and lot-to-lot consistency of PXVX0200. Adult volunteers 18-45 years of age were randomized 8:1 to receive a single dose of 1 × 10⁹ CFU of PXVX0200 from three production lots or saline placebo. Immunogenicity endpoints included SVA and anti-cholera toxin (CT) antibody levels on days 1, 11, 29, 91 and 181. Safety was assessed by comparing solicited signs and symptoms on days 1-8, unsolicited adverse events through day 29 and serious adverse events through day 181. A total of 3146 participants were enrolled, including 2795 vaccine and 351 placebo recipients. The SVA seroconversion rates at day 11 were 94% and 4% in the PXVX0200 and placebo recipients, respectively (P < .0001). Cumulative SVA seroconversion occurred among 96% of vaccine recipients. PXVX0200 SVA GMTs peaked on day 11 and remained significantly higher than placebo through day 181 while the fold-rise over baseline in PXVX0200 anti-CT antibody was significantly greater than placebo at every post-vaccination time point. Most reactogenicity was mild and resolved within 1-3 days with headache and diarrhea more frequently reported in PXVX0200 recipients. There were no differences in unsolicited adverse events and no study-related serious adverse events. Immunogenicity and safety endpoints were equivalent between the three production lots. PXVX0200 is immunogenic and well tolerated across multiple production lots.

CLINICAL TRIALS REGISTRATION

Clinicaltrials.gov NCT02094586.

Antibody-Based Correlates of Protection Against Cholera Analysis of a Challenge Study in a Cholera-Naïve Population

Immunologic correlates of protection can be used to infer vaccine efficacy for populations in which challenge trials or field studies are infeasible. In a recent cholera challenge trial (WH Cohen et al, Clinical Infectious Disease 62: 1329-1335, 2016), 134 North American cholera-naïve volunteers were randomized to receive either the live, attenuated single-dose cholera vaccine CVD 103-HgR or placebo, and titers of vibriocidal antibodies against classical Inaba were assessed 10 days after treatment. Subsequent to the immunologic evaluation, each subject ingested a fixed quantity of virulent V. cholerae O1 El Tor Inaba. Data from this trial suggest that vaccine-induced increase in vibriocidal antibody titer prior to challenge is tightly linked with protection: 51/51 vaccinees with post-vaccination vibriocidal titers >= 2560 were protected against moderate/severe cholera, and 60/62 vaccinees who seroconverted, or experienced a 4-fold or greater increase in vibriocidal titer relative to pre-vaccination levels, were similarly protected. Atypically high vibriocidal titers were observed in some placebo subjects; protection was limited in these individuals and differed substantially from the level of protection experienced by vaccinees with the same post-vaccination titers. Since only 1 of 66 placebo recipients experienced seroconversion, seroconversion was found to be uniquely associated with vaccination and insensitive to the effects of factors that can cause titers to be elevated but are weakly associated with protection. Thus, vibriocidal seroconversion was found to be better than vibriocidal titer for inferring vaccine efficacy in cholera-naïve populations for which studies based upon exposure to V. cholerae are impractical.

Improving immunization approaches to cholera

INTRODUCTION

Cholera's impact is greatest in resource-limited countries. In the last decade several large epidemics have led to a global push to improve and implement the tools for cholera prevention and control. Areas covered: PubMed, Google Scholar and the WHO website were searched to review the literature and summarize the current status of cholera vaccines to make recommendations on improving immunization approaches to cholera. Oral cholera vaccines (OCVs) have demonstrated their effectiveness in endemic, outbreak response and emergency settings, highlighting their potential for wider adoption. While two doses of the currently available OCVs are recommended by manufacturers, a single dose would be easier to implement. Encouragingly, recent studies have shown that cold chain requirements may no longer be essential. The establishment of the global OCV stockpile in 2013 has been a major advance in cholera preparedness. New killed and live-attenuated vaccines are being actively explored as candidate vaccines for endemic settings and/or as a traveller's vaccine. The recent advances in cholera vaccination approaches should be considered in the global cholera control strategy. Expert commentary: The development of affordable cholera vaccines is a major success to improve cholera control. New vaccines and country specific interventions will further reduce the burden of this disease globally.

Single-dose Live Oral Cholera Vaccine CVD 103-HgR Protects Against Human Experimental Infection With Vibrio cholerae O1 El Tor

BACKGROUND

No licensed cholera vaccine is presently available in the United States. Cholera vaccines available in other countries require 2 spaced doses. A single-dose cholera vaccine that can rapidly protect short-notice travelers to high-risk areas and help control explosive outbreaks where logistics render 2-dose immunization regimens impractical would be a major advance.PXVX0200, based on live attenuated Vibrio cholerae O1 classical Inaba vaccine strain CVD 103-HgR, elicits seroconversion of vibriocidal antibodies (a correlate of protection) within 10 days of a single oral dose. We investigated the protection conferred by this vaccine in a human cholera challenge model.

METHODS

Consenting healthy adult volunteers, 18-45 years old, were randomly allocated 1:1 to receive 1 oral dose of vaccine (approximately 5 × 10(8) colony-forming units [CFU]) or placebo in double-blind fashion. Volunteers ingested approximately 1 × 10(5) CFU of wild-type V. cholerae O1 El Tor Inaba strain N16961 10 days or 3 months after vaccination and were observed on an inpatient research ward for stool output measurement and management of hydration.

RESULTS

The vaccine was well tolerated, with no difference in adverse event frequency among 95 vaccinees vs 102 placebo recipients. The primary endpoint, moderate (≥3.0 L) to severe (≥5.0 L) diarrheal purge, occurred in 39 of 66 (59.1%) placebo controls but only 2 of 35 (5.7%) vaccinees at 10 days (vaccine efficacy, 90.3%; P < .0001) and 4 of 33 (12.1%) vaccinees at 3 months (vaccine efficacy, 79.5%; P < .0001).

CONCLUSIONS

The significant vaccine efficacy documented 10 days and 3 months after 1 oral dose of PXVX0200 supports further development as a single-dose cholera vaccine.

CLINICAL TRIALS REGISTRATION

NCT01895855.

High-resolution spatial analysis of cholera patients reported in Artibonite department, Haiti in 2010-2011

BACKGROUND

Cholera is caused by Vibrio cholerae, and is transmitted through fecal-oral contact. Infection occurs after the ingestion of the bacteria and is usually asymptomatic. In a minority of cases, it causes acute diarrhea and vomiting, which can lead to potentially fatal severe dehydration, especially in the absence of appropriate medical care. Immunity occurs after infection and typically lasts 6-36 months. Cholera is responsible for outbreaks in many African and Asian developing countries, and caused localised and episodic epidemics in South America until the early 1990s. Haiti, despite its low socioeconomic status and poor sanitation, had never reported cholera before the recent outbreak that started in October 2010, with over 720,000 cases and over 8700 deaths (Case fatality rate: 1.2%) through 8 december 2014. So far, this outbreak has seen 3 epidemic peaks, and it is expected that cholera will remain in Haiti for some time.

METHODOLOGY/FINDINGS

To trace the path of the early epidemic and to identify hot spots and potential transmission hubs during peaks, we examined the spatial distribution of cholera patients during the first two peaks in Artibonite, the second-most populous department of Haiti. We extracted the geographic origin of 84,000 patients treated in local health facilities between October 2010 and December 2011 and mapped these addresses to 63 rural communal sections and 9 urban cities. Spatial and cluster analysis showed that during the first peak, cholera spread along the Artibonite River and the main roads, and sub-communal attack rates ranged from 0.1% to 10.7%. During the second peak, remote mountain areas were most affected, although sometimes to very different degrees even in closely neighboring locations. Sub-communal attack rates during the second peak ranged from 0.2% to 13.7%. The relative risks at the sub-communal level during the second phase showed an inverse pattern compared to the first phase.

CONCLUSION/SIGNIFICANCE

These findings demonstrate the value of high-resolution mapping for pinpointing locations most affected by cholera, and in the future could help prioritize the places in need of interventions such as improvement of sanitation and vaccination. The findings also describe spatio-temporal transmission patterns of the epidemic in a cholera-naïve country such as Haiti. By identifying transmission hubs, it is possible to target prevention strategies that, over time, could reduce transmission of the disease and eventually eliminate cholera in Haiti.

Evidence for CVD 103-HgR as an effective single-dose oral cholera vaccine

We propose the ideal oral cholera vaccine (OCV) should be an inexpensive, single, oral dose that rapidly confers immunity for a long duration, and is well tolerated by individuals vulnerable to cholera. Vaccine trials in industrialized countries of a single oral dose of 5 × 108 colony forming units (CFU) of the live, attenuated cholera strain CVD 103-HgR have shown 88–97% serum vibriocidal antibody seroconversion rates, a correlate of protection and documented vaccine efficacy of ≥80% using volunteer challenge studies with wild-type cholera. For individuals of developing countries, a 5 × 109 CFU dose of CVD 103-HgR is necessary to elicit similar antibody responses. Presently, a reformulation of CVD 103-HgR is in late-stage clinical development for prospective US FDA licensure; making a cholera vaccine for US travelers potentially accessible in 2016. The availability of CVD 103-HgR should be a welcome addition to the currently available OCVs.

Design, recruitment, and microbiological considerations in human challenge studies

Since the 18th century a wealth of knowledge regarding infectious disease pathogenesis, prevention, and treatment has been accumulated from findings of infection challenges in human beings. Partly because of improvements to ethical and regulatory guidance, human challenge studies-involving the deliberate exposure of participants to infectious substances-have had a resurgence in popularity in the past few years, in particular for the assessment of vaccines. To provide an overview of the potential use of challenge models, we present historical reports and contemporary views from experts in this type of research. A range of challenge models and practical approaches to generate important data exist and are used to expedite vaccine and therapeutic development and to support public health modelling and interventions. Although human challenge studies provide a unique opportunity to address complex research questions, participant and investigator safety is paramount. To increase the collaborative effort and future success of this area of research, we recommend the development of consensus frameworks and sharing of best practices between investigators. Furthermore, standardisation of challenge procedures and regulatory guidance will help with the feasibility for using challenge models in clinical testing of new disease intervention strategies.

When, how, and where can oral cholera vaccines be used to interrupt cholera outbreaks?

Cholera continues to be a major global health problem, at times causing major and prolonged outbreaks in both endemic and nonendemic settings in developing countries. While improved water quality, sanitation, and hygiene (WASH) will provide the ultimate solution to prevention of this disease burden, this is a far-off goal for most developing countries. Oral cholera vaccines cholera vaccines (OCVs) have been demonstrated to be effective in the control of cholera outbreaks, and constitute useful tools to be used in conjunction with efforts to improve WASH. Two killed OCVs are prequalified by WHO for purchase by UN agencies for international use. Recently, WHO has launched a global stockpile stockpile of killed OCVs for use to control outbreaks. Rational deployment of OCV from this stockpile will require consideration of costs, feasibility, disease epidemiology epidemiology , and the protective characteristics of the vaccine deployed, as well as effective and rapid coordination of processes and logistics logistics used to make decisions on deployment and delivery of the vaccine to the population in need. Despite not having data on all the questions of relevance as to how to use OCVs to control cholera outbreaks in different settings, there is clearly more than enough evidence to initiate their use, as answers to remaining questions and refinement of policies will mainly come with experience.

Maximizing protection from use of oral cholera vaccines in developing country settings: an immunological review of oral cholera vaccines

When oral vaccines are administered to children in lower- and middle-income countries, they do not induce the same immune responses as they do in developed countries. Although not completely understood, reasons for this finding include maternal antibody interference, mucosal pathology secondary to infection, malnutrition, enteropathy, and previous exposure to the organism (or related organisms). Young children experience a high burden of cholera infection, which can lead to severe acute dehydrating diarrhea and substantial mortality and morbidity. Oral cholera vaccines show variations in their duration of protection and efficacy between children and adults. Evaluating innate and memory immune response is necessary to understand V. cholerae immunity and to improve current cholera vaccine candidates, especially in young children. Further research on the benefits of supplementary interventions and delivery schedules may also improve immunization strategies.

Live-attenuatedSalmonellaas a prototype vaccine vector for passenger immunogens in humans: are we there yet?

It has been nearly 20 years since the first Phase I clinical trial of a live-attenuated bacterial vaccine was created by recombinant DNA methods, opening the door to the use of these organisms as mucosal delivery vehicles for passenger antigens. Over this time, a number of animal studies have indicated the feasibility of this approach. These include studies showing that bacteria can deliver antigens expressed by the bacterium itself and that bacteria can deliver DNA vaccines to be expressed in target eukaryotic cells. Concomitant studies have identified a number of attenuating mutations that render the bacterial vectors both safe and immunogenic in humans. Both avenues of research indicate the significant promise of this approach to mucosal vaccine development; however, this promise remains largely unrealized at the level of human clinical trials. This review sketches the history of this problem and points toward possible solutions using Salmonella vaccine vectors as the prototypes.

The ABCs (Antibody, B Cells, and Carbohydrate Epitopes) of Cholera Immunity: Considerations for an Improved Vaccine

Cholera, a diarrheal disease, is known for explosive epidemics that can quickly kill thousands. Endemic cholera is a seasonal torment that also has a significant mortality. Not all nations with extensive rural communities can achieve the required infrastructure or behavioral changes to prevent epidemic or endemic cholera. For some communities, a single-dose cholera vaccine that protects those at risk is the most efficacious means to reduce morbidity and mortality. It is clear that our understanding of what a protective cholera immune response is has not progressed at the rate our understanding of the pathogenesis and molecular biology of cholera infection has. This review addresses V. cholerae lipopolysaccharide (LPS)-based immunogens because LPS is the only immunogen proven to induce protective antibody in humans. We discuss the role of anti-LPS antibodies in protection from cholera, the importance and the potential role of B cell subsets in protection that is based on their anatomical location and the intrinsic antigen-receptor specificity of various subsets is introduced.

Safety and immunogenicity of single-dose live oral cholera vaccine strain CVD 103-HgR, prepared from new master and working cell banks

Currently, no cholera vaccine is available for persons traveling from the United States to areas of high cholera transmission and who for reasons of occupation or host factors are at increased risk for development of the disease. A single-dose oral cholera vaccine with a rapid onset of protection would be particularly useful for such travelers and might also be an adjunct control measure for cholera outbreaks. The attenuated Vibrio cholerae O1 vaccine strain CVD 103-HgR harbors a 94% deletion of the cholera toxin A subunit gene (ctxA) and has a mercury resistance gene inserted in the gene encoding hemolysin A. We undertook a phase I randomized placebo-controlled two-site trial to assess the safety and immunogenicity of a preliminary formulation of CVD 103-HgR prepared from new master and working cell banks. Healthy young adults were randomized (5:1 vaccinees to placebo recipients) to receive a single oral dose of ∼4.4 × 10(8) CFU of vaccine or a placebo. Blood serum vibriocidal and cholera toxin-specific IgG antibodies were measured before and 10, 14, and 28 days following vaccination or placebo. Excretion of the vaccine strain in the stool was assessed during the first week postvaccination. A total of 66 subjects were enrolled, comprising 55 vaccinees and 11 placebo recipients. The vaccine was well tolerated. The overall vibriocidal and anti-cholera toxin seroconversion rates were 89% and 57%, respectively. CVD 103-HgR is undergoing renewed manufacture for licensure in the United States under the auspices of PaxVax. Our data mimic those from previous commercial formulations that elicited vibriocidal antibody seroconversion (a correlate of protection) in ∼90% of vaccinees. (This study has been registered at ClinicalTrials.gov under registration no. NCT01585181.).

Human challenge studies: a review of adequacy of reporting methods and results

Since the 1940s, researchers have purposefully infected healthy adult humans with pathogenic organisms to study how these pathogens cause disease and can be treated and prevented. 'Challenge studies' can be safe, ethical, extremely informative and an efficient use of resources during the clinical development of vaccines, but knowledge of this form of clinical research trial is not widespread. A review of the human challenge literature was performed to determine whether common elements of challenge studies can be identified in the articles published to date. The review demonstrated incomplete reporting of study characteristics deemed necessary for the correct interpretation and application of human challenge study results and for the accurate replication of study methodology. An unofficial extension of the Consolidated Standards of Reporting Trials (CONSORT) statement is proposed.

An overview of cholera vaccines and their public health implications

PURPOSE OF REVIEW

The current cholera pandemic now involves almost the entire developing world and represents an important global challenge. Though improved water and sanitation remain the mainstays of cholera prevention efforts, major improvements to infrastructure continue to be a goal far out of reach for many of those affected and near-term interventions, including vaccines, need consideration.

RECENT FINDINGS

Prolonged and frequent epidemics, increased antimicrobial resistance, and heightening awareness of the role of climate change in disease burden have returned cholera to the forefront of the international public health forum. The availability of new oral cholera vaccines and supporting data have prompted the WHO to recommend vaccination be used with other prevention and control strategies where disease is endemic, and their use should be considered in outbreak situations.

SUMMARY

Vibrio cholerae, a highly transmissible organism, is found in aquatic reservoirs and is not an eradicable disease. New variant strains appear to cause more severe clinical disease, and may be displacing earlier seventh pandemic organisms as the major cause of cholera. Licensed newer-generation oral vaccines have proven to be well tolerated, protective (including against new variant strains), and affordable and offer a new tool to control cholera.

Vaccines for enteric infections

PURPOSE OF REVIEW

To provide a review of currently licensed enteric vaccines and their efficacy based on completed field trials.

RECENT FINDINGS

In this review, we provide a brief description of the epidemiology of the most common enteric infections, in both developing and industrialized countries. We also describe the types, dosage, age-eligibility, availability, and efficacies of currently licensed vaccines, and review the results of recently completed clinical trials around the world.

SUMMARY

Several enteric vaccines are currently available. Although some vaccines have proven highly effective in industrialized countries where the disease burden is low (so-called travelers vaccines), they have demonstrated a lower protective effect in endemic countries where the disease is more prevalent. However, due to the magnitude of disease in endemic countries, even with lower efficacy, the potential for a vaccine to reduce the absolute number of cases remains considerable. Despite the continued reduction in overall disease burden with increased public health measures, such as improved sanitation, antimicrobials, and greater public awareness, enteric infections continue to cause significant morbidity and mortality in vulnerable populations. We contend that adoption and dissemination of available vaccines at affordable prices should be accelerated, particularly in areas where the disease burden is highest.

New-generation vaccines against cholera

Cholera is a major global health problem, causing approximately 100,000 deaths annually, about half of which occur in sub-Saharan Africa. Although early-generation parenteral cholera vaccines were abandoned as public health tools owing to their limited efficacy, newer-generation oral cholera vaccines have attractive safety and protection profiles. Both killed and live oral vaccines have been licensed, although only killed oral vaccines are currently manufactured and available. These killed oral vaccines not only provide direct protection to vaccinated individuals, but also confer herd immunity. The combination of direct vaccine protection and vaccine herd immunity effects makes these vaccines highly cost-effective and, therefore, attractive for use in developing countries. Administration of these oral vaccines does not require qualified medical personnel, which makes their use practical--even in developing countries. Although new-generation oral cholera vaccines should not be considered in isolation from other preventive approaches, especially improved water quality and sanitation, they represent important tools in the public health armamentarium to control both endemic and epidemic cholera.

The utility of human challenge studies in vaccine development: lessons learned from cholera

Experiments in which virulent infectious organisms are administered to healthy adult volunteers with the intent to deliberately induce infection have been practiced for centuries. Many useful applications have developed from these experiments such as the provision of evidence of microbial pathogenicity and the identification of key virulence factors. Challenge studies have also played an important role in the evaluation of preliminary efficacy of potential vaccine candidates. Over the past 40 years, these experimental human challenge studies have found particular utility with regards to the development of both living and nonliving attenuated cholera vaccines. This review highlights some of the important contributions made by these challenge studies to cholera vaccine research.

Oral vaccines against cholera

The current seventh pandemic of cholera, caused by serogroup O1, El Tor biotype, has now involved almost the entire developing world. The ongoing dynamic epidemiology of cholera, involving evolution of new strains, prolonged and more frequent epidemics, increased antimicrobial resistance, and awareness of the role of climate change upon the global burden has returned cholera to the forefront of global public health discussions. Improved water and sanitation should continue to be the mainstays of cholera-prevention efforts, but major improvements are a far-off goal for much of the cholera-affected developing world. The advent of safe and effective, new-generation oral vaccines against cholera has created renewed interest in the use of vaccines as a tool to control cholera.

Vibrio cholerae: lessons for mucosal vaccine design

The ability of Vibrio cholerae to persist in bodies of water will continue to confound our ability to eradicate cholera through improvements to infrastructure, and thus cholera vaccines are needed. We aim for an inexpensive vaccine that can provide long-lasting protection from all epidemic cholera infections, currently caused by O1 or O139 serogroups. Recent insights into correlates of protection, epidemiology and pathogenesis may help us design improved vaccines. This notwithstanding, we have come to appreciate that even marginally protective vaccines, such as oral whole-cell killed vaccines, if widely distributed, can provide significant protection, owing to herd immunity. Further efforts are still required to provide more effective protection of young children.

Oral vaccines for preventing cholera

EDITORIAL NOTE

This review is superseded by the published Cochrane Review, Saif‐Ur‐Rahman 2024 [https://doi.org/10.1002/14651858.CD014573], which considers only the oral killed vaccines because the live oral vaccines do not have World Health Organization (WHO) prequalification. Saif‐Ur‐Rahman 2024 also considered only currently available WHO pre‐qualified oral killed cholera vaccines (Dukoral, Shanchol, and Euvichol/Euvichol‐Plus).

BACKGROUND

Cholera is a cause of acute watery diarrhoea which can cause dehydration and death if not adequately treated. It usually occurs in epidemics, and is associated with poverty and poor sanitation. Effective, cheap, and easy to administer vaccines could help prevent epidemics.

OBJECTIVES

To assess the effectiveness and safety of oral cholera vaccines in preventing cases of cholera and deaths from cholera.

SEARCH STRATEGY

In October 2010, we searched the Cochrane Infectious Disease Group Specialized Register; Cochrane Central Register of Controlled Trials (CENTRAL); MEDLINE; EMBASE; LILACS; the metaRegister of Controlled Trials (mRCT), and the WHO International Clinical Trials Registry Platform (ICTRP) for relevant published and ongoing trials.

SELECTION CRITERIA

Randomized or quasi-randomized controlled trials of oral cholera vaccines in healthy adults and children.

DATA COLLECTION AND ANALYSIS

Each trial was assessed for eligibility and risk of bias by two authors working independently. Data was extracted by two independent reviewers and analysed using the Review Manager 5 software. Outcomes are reported as vaccine protective efficacy (VE) with 95% confidence intervals (CIs).

MAIN RESULTS

Seven large efficacy trials, four small artificial challenge studies, and twenty-nine safety trials contributed data to this review.Five variations of a killed whole cell vaccine have been evaluated in large scale efficacy trials (four trials, 249935 participants). The overall vaccine efficacy during the first year was 52% (95% CI 35% to 65%), and during the second year was 62% (95% CI 51% to 62%). Protective efficacy was lower in children aged less than 5 years; 38% (95% CI 20% to 53%) compared to older children and adults; 66% (95% CI 57% to 73%).One trial of a killed whole cell vaccine amongst military recruits demonstrated 86% protective efficacy (95% CI 37% to 97%) in a small epidemic occurring within 4 weeks of the 2-dose schedule (one trial, 1426 participants). Efficacy data is not available beyond two years for the currently available vaccine formulations, but based on data from older trials is unlikely to last beyond three years.The safety data available on killed whole cell vaccines have not demonstrated any clinically significant increase in adverse events compared to placebo.Only one live attenuated vaccine has reached Phase III clinical evaluation and was not effective (one trial, 67508 participants). Two new candidate live attenuated vaccines have demonstrated clinical effectiveness in small artificial challenge studies, but are still in development.

AUTHORS' CONCLUSIONS

The currently available oral killed whole cell vaccines can prevent 50 to 60% of cholera episodes during the first two years after the primary vaccination schedule. The impact and cost-effectiveness of adopting oral cholera vaccines into the routine vaccination schedule of endemic countries will depend on the prevalence of cholera, the frequency of epidemics, and access to basic services providing rapid rehydration therapy.

Immunogenicity and efficacy of oral vaccines in developing countries: lessons from a live cholera vaccine

Oral vaccines, whether living or non-living, viral or bacterial, elicit diminished immune responses or have lower efficacy in developing countries than in developed countries. Here I describe studies with a live oral cholera vaccine that include older children no longer deriving immune support from breast milk or maternal antibodies and that identify some of the factors accounting for the lower immunogenicity, as well as suggesting counter-measures that may enhance the effectiveness of oral immunization in developing countries. The fundamental breakthrough is likely to require reversing effects of the 'environmental enteropathy' that is often present in children living in fecally contaminated, impoverished environments.

Use of stabilized luciferase-expressing plasmids to examine in vivo-induced promoters in the Vibrio cholerae vaccine strain CVD 103-HgR

Live, attenuated Vibrio cholerae vaccines can induce potent immune responses after only a single oral dose. The strategy of harnessing these strains to present antigens from heterologous pathogens to the mucosal immune system shows great promise. To fully realize this possibility, V. cholerae strains must be created that stably express antigens in vivo in sufficient quantity to generate an immune response. In vivo-induced promoters have been shown to increase the stability and immunogenicity of foreign antigens expressed from multicopy plasmids. We report the construction of a series of genetically stabilized plasmids expressing luciferase as a heterologous protein from the following in vivo-induced promoters: V. cholerae P(argC), P(fhuC) and P(vca1008), and Salmonella enterica serovar Typhi P(ompC). We demonstrate that several of these expression plasmids meet two critical criteria for V. cholerae live vector vaccine studies. First, the plasmids are highly stable in the V. cholerae vaccine strain CVD 103-HgR at low copy number, in the absence of selective pressure. Second, real-time bioluminescent imaging (BLI) demonstrates inducible in vivo expression of the promoters in the suckling mouse model of V. cholerae colonization. Moreover, the use of BLI allows for direct quantitative comparison of in vivo expression from four different promoters at various time points.

Sialyloligosaccharides inhibit cholera toxin binding to the GM1 receptor

It is recognised that cholera toxin (Ctx) is a significant cause of gastrointestinal disease globally, particularly in developing countries where access to uncontaminated drinking water is at a premium. Ctx vaccines are prohibitively expensive and only give short-term protection. Consequently, there is scope for the development of alternative control strategies or prophylactics. This may include the use of oligosaccharides as functional mimics for the cell-surface toxin receptor (GM1). Furthermore, the sialic acid component of epithelial receptors has already been shown to contribute significantly to the adhesion and pathogenesis of Ctx. Here, we demonstrate the total inhibition of Ctx using GM1-competitive ELISA with 25mgmL(-1) of a commercial preparation of sialyloligosaccharides (SOS). The IC(50) value was calculated as 5.21mgmL(-1). One-hundred percent inhibition was also observed at all concentrations of Ctx-HRP tested with 500ngmL(-1) GM1-OS. Whilst SOS has much lower affinity for Ctx than GM1-OS, the commercial preparation is impure containing only 33.6% carbohydrate; however, the biantennary nature of SOS appears to give a significant increase in potency over constituent monosaccahride residues. It is proposed that SOS could be used as a conventional food additive, such as in emulsifiers, stabilisers or sweeteners, and are classified as nondigestible oligosaccharides that pass into the small intestine, which is the site of Ctx pathogenesis.

Acute infectious pediatric gastroenteritis: beyond oral rehydration therapy

Worldwide diarrheal diseases are a leading cause of childhood morbidity and mortality. Improvements in gastroenteritis management have reduced the annual number of pediatric deaths attributable to gastroenteritis from 5 million in 1982 to 2 million over 20 years. Recent advances are likely to contribute further to a reduction in morbidity and mortality secondary to acute infectious gastroenteritis. A new generation of vaccines against rotavirus is entering into routine use. Research into antisecretory agents has demonstrated that this class of medications may play a significant role in the future management of acute infectious gastroenteritis. A significant body of literature has recently emerged supporting the use of the antiemetic agent ondansetron. In developing countries, the routine use of zinc is now recommended by many experts, while, in developed countries, the use of probiotic agents has been associated with significant benefits in acute infectious gastroenteritis. Finally, more aggressive intravenous rehydration strategies are being employed; however, at present, limited data from randomized clinical trials are available to support its routine use.

Live attenuated oral cholera vaccines

Live, orally administered, attenuated vaccine strains of Vibrio cholerae have many theoretical advantages over killed vaccines. A single oral inoculation could result in intestinal colonization and rapid immune responses, obviating the need for repetitive dosing. Live V. cholerae organisms can also respond to the intestinal environment and immunological exposure to in vivo expressed bacterial products, which could result in improved immunological protection against wild-type V. cholerae infection. The concern remains that live oral cholera vaccines may be less effective among partially immune individuals in cholera endemic areas as pre-existing antibodies can inhibit live organisms and decrease colonization of the gut. A number of live oral cholera vaccines have been developed to protect against cholera caused by the classical and El Tor serotypes of V. cholerae O1, including CVD 103-HgR, Peru-15 and V. cholerae 638. A number of live oral cholera vaccines have also been similarly developed to protect against cholera caused by V. cholerae O139, including CVD 112 and Bengal-15. Live, orally administered, attenuated cholera vaccines are in various stages of development and evaluation.

Oral cholera vaccines: use in clinical practice

Cholera continues to occur globally, particularly in sub-Saharan Africa and Asia. Oral cholera vaccines have been developed and have now been used for several years, primarily in traveller populations. The licensure in the European Union of a killed whole cell cholera vaccine combined with the recombinant B subunit of cholera toxin (rCTB-WC) has stimulated interest in protection against cholera. Because of the similarity between cholera toxin and the heat-labile toxin of Escherichia coli, a cause of travellers' diarrhoea, it has been proposed that the rCTB-WC vaccine may be used against travellers' diarrhoea. An analysis of trials of this vaccine against cholera (serotype O1) shows that for 4-6 months it will protect 61-86% of people living in cholera-endemic regions; lower levels of protection continue for 3 years. Protection wanes rapidly in young children. Because the risk of cholera for most travellers is extremely low, vaccination should be considered only for those working in relief or refugee settings or for those who will be travelling in cholera-epidemic areas and who will be unable to obtain prompt medical care. The vaccine can be expected to prevent 7% or less of cases of travellers' diarrhoea and should not be used for this purpose.

Analysis of efficacy of CVD 103-HgR live oral cholera vaccine against all-cause travellers’ diarrhoea in a randomised, double-blind, placebo-controlled study

Enterotoxigenic Escherichia coli (ETEC), which produces heat labile toxin (LT) and/or heat stable toxin (ST), is considered to be the most common known cause of travellers' diarrhoea (TD). Owing to the antigenic similarity between cholera toxin and LT, immunization with inactivated oral B-subunit/whole-cell cholera vaccine (BS-WC) offers short term (3 months) but significant (>67%) protection against TD caused by LT-related ETEC. Since it expresses the cholera toxin B (CTB) subunit, the live attenuated oral cholera vaccine strain CVD 103-HgR, may induce similar protection. A trial was performed to determine if CVD 103-HgR live oral cholera vaccine would provide a protective efficacy of at least 50% against TD. In addition, the protective efficacy of the vaccine against TD specifically due to LT-ETEC and LT/ST-ETEC was determined. Volunteers (n=134) travelling to Indonesia, India, Thailand or West-Africa were randomised to receive either a placebo (n=65) or the vaccine (n=69). In the placebo group, 46% reported an episode of diarrhoea, compared to 52% in the vaccine group. No significant group differences were found with regard to incidence, duration or severity of all caused TD or ETEC-associated TD. However, ETEC-associated TD occurred earlier in the placebo group (median 5 days), compared to the vaccine group (median 15 days). In conclusion, CVD 103-HgR live oral cholera vaccine failed to provide a 50% protection against TD. This study does not exclude that the vaccine may offer a short-lived protection against ETEC-associated TD. However, the power of the study was limited by the unexpected low incidence of LT-ETEC-associated diarrhoea (9% of all TD) compared to ST-associated TD (24% of all TD).

Vaccines against diarrheal diseases

At least 2 million persons succumb annually to enteric infection, and in countless other patients, diarrheal disease aggravates malnutrition and susceptibility to other infections. Prevention of enteric illness by virtue of improved hygiene and provision of sanitation and water treatment is impractical in most developing countries, where morbidity and mortality rates are highest. For this reason, development of vaccines against the most important gastrointestinal infections remains a high priority.