Randomized, controlled human challenge study of the safety, immunogenicity, and protective efficacy of a single dose of Peru-15, a live attenuated oral cholera vaccine

Vibrio cholerae, classification, pathogenesis, immune response, and trends in vaccine development

Vibrio cholerae is the causative agent of cholera, a highly contagious diarrheal disease affecting millions worldwide each year. Cholera is a major public health problem, primarily in countries with poor sanitary conditions and regions affected by natural disasters, where access to safe drinking water is limited. In this narrative review, we aim to summarize the current understanding of the evolution of virulence and pathogenesis of V. cholerae as well as provide an overview of the immune response against this pathogen. We highlight that V. cholerae has a remarkable ability to adapt and evolve, which is a global concern because it increases the risk of cholera outbreaks and the spread of the disease to new regions, making its control even more challenging. Furthermore, we show that this pathogen expresses several virulence factors enabling it to efficiently colonize the human intestine and cause cholera. A cumulative body of work also shows that V. cholerae infection triggers an inflammatory response that influences the development of immune memory against cholera. Lastly, we reviewed the status of licensed cholera vaccines, those undergoing clinical evaluation, and recent progress in developing next-generation vaccines. This review offers a comprehensive view of V. cholerae and identifies knowledge gaps that must be addressed to develop more effective cholera vaccines.

Controlled Human Infection Models To Accelerate Vaccine Development

The timelines for developing vaccines against infectious diseases are lengthy, and often vaccines that reach the stage of large phase 3 field trials fail to provide the desired level of protective efficacy. The application of controlled human challenge models of infection and disease at the appropriate stages of development could accelerate development of candidate vaccines and, in fact, has done so successfully in some limited cases. Human challenge models could potentially be used to gather critical information on pathogenesis, inform strain selection for vaccines, explore cross-protective immunity, identify immune correlates of protection and mechanisms of protection induced by infection or evoked by candidate vaccines, guide decisions on appropriate trial endpoints, and evaluate vaccine efficacy. We prepared this report to motivate fellow scientists to exploit the potential capacity of controlled human challenge experiments to advance vaccine development. In this review, we considered available challenge models for 17 infectious diseases in the context of the public health importance of each disease, the diversity and pathogenesis of the causative organisms, the vaccine candidates under development, and each model's capacity to evaluate them and identify correlates of protective immunity. Our broad assessment indicated that human challenge models have not yet reached their full potential to support the development of vaccines against infectious diseases. On the basis of our review, however, we believe that describing an ideal challenge model is possible, as is further developing existing and future challenge models.

Emerging Concepts in Cholera Vaccine Design

Cholera is a severe diarrheal disease caused by the bacterium Vibrio cholerae and constitutes a significant public health threat in many areas of the world. V. cholerae infection elicits potent and long-lasting immunity, and efforts to develop cholera vaccines have been ongoing for more than a century. Currently available inactivated two-dose oral cholera vaccines are increasingly deployed to both prevent and actively curb cholera outbreaks, and they are key components of the global effort to eradicate cholera. However, these killed whole-cell vaccines have several limitations, and a variety of new oral and nonoral cholera vaccine platforms have recently been developed. Here, we review emerging concepts in cholera vaccine design and implementation that have been driven by insights from human and animal studies. As a prototypical vaccine-preventable disease, cholera continues to be an excellent target for the development and application of cutting-edge technologies and platforms that may transform vaccinology. Expected final online publication date for the Annual Review of Microbiology, Volume 76 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Nontoxigenic Vibrio cholerae Challenge Strains for Evaluating Vaccine Efficacy and Inferring Mechanisms of Protection

Human challenge studies are instrumental for testing cholera vaccines, but these studies use outdated strains and require inpatient facilities. Here, we created next-generation isogenic Ogawa and Inaba O1 V. cholerae challenge strains (ZChol strains) derived from a contemporary Zambian clinical isolate representative of current dominant pandemic V. cholerae. Since the primary mechanism of immune protection against cholera is thought to be antibody responses that limit V. cholerae colonization and not the diarrheagenic actions of cholera toxin, these strains were rendered nontoxigenic. In infant mice, the ZChol strains did not cause diarrhea and proved to accurately gauge reduction in intestinal colonization mediated by effective vaccination. ZChol strains were also valuable as targets for measuring vibriocidal antibody responses. Using barcoded ZChol strains, we discovered that vaccination and passive immunity in the infant mouse model tightens the infection bottleneck without restricting pathogen expansion during intestinal infection. Collectively, our findings suggest that ZChol strains have the potential to enhance the safety, relevance, and scope of future cholera vaccine challenge studies and be valuable reagents for studies of immunity to cholera. IMPORTANCE Human challenge studies are a valuable method for testing the efficacy of cholera vaccines. However, challenge studies cannot be performed in countries of cholera endemicity due to safety concerns; also, contemporary pandemic Vibrio cholerae strains are not used in current challenge studies. To facilitate cholera research, we derived nontoxigenic challenge strains of both V. cholerae serotypes from a 2016 clinical isolate from Zambia and demonstrated how they can be used to gauge cholera immunity accurately and safely. These strains were also genetically barcoded, adding the potential for analyses of V. cholerae population dynamics to challenge studies. Preclinical analyses presented here suggest that these strains have the potential to enhance the safety, relevance, and scope of future cholera vaccine challenge studies and be valuable reagents for studies of immunity to cholera.

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.

The Interface of Vibrio cholerae and the Gut Microbiome

The bacterium Vibrio cholerae is the etiologic agent of the severe human diarrheal disease cholera. The gut microbiome, or the native community of microorganisms found in the human gastrointestinal tract, is increasingly being recognized as a factor in driving susceptibility to infection, in vivo fitness, and host interactions of this pathogen. Here, we review a subset of the emerging studies in how gut microbiome structure and microbial function are able to drive V. cholerae virulence gene regulation, metabolism, and modulate host immune responses to cholera infection and vaccination. Improved mechanistic understanding of commensal-pathogen interactions offers new perspectives in the design of prophylactic and therapeutic approaches for cholera control.

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.

The Emergency Response Capacity of Plant-Based Biopharmaceutical Manufacturing-What It Is and What It Could Be

Several epidemic and pandemic diseases have emerged over the last 20 years with increasing reach and severity. The current COVID-19 pandemic has affected most of the world's population, causing millions of infections, hundreds of thousands of deaths, and economic disruption on a vast scale. The increasing number of casualties underlines an urgent need for the rapid delivery of therapeutics, prophylactics such as vaccines, and diagnostic reagents. Here, we review the potential of molecular farming in plants from a manufacturing perspective, focusing on the speed, capacity, safety, and potential costs of transient expression systems. We highlight current limitations in terms of the regulatory framework, as well as future opportunities to establish plant molecular farming as a global, de-centralized emergency response platform for the rapid production of biopharmaceuticals. The implications of public health emergencies on process design and costs, regulatory approval, and production speed and scale compared to conventional manufacturing platforms based on mammalian cell culture are discussed as a forward-looking strategy for future pandemic responses.

A live vaccine rapidly protects against cholera in an infant rabbit model

Outbreaks of cholera, a rapidly fatal diarrheal disease, often spread explosively. The efficacy of reactive vaccination campaigns-deploying Vibrio cholerae vaccines during epidemics-is partially limited by the time required for vaccine recipients to develop adaptive immunity. We created HaitiV, a live attenuated cholera vaccine candidate, by deleting diarrheagenic factors from a recent clinical isolate of V. cholerae and incorporating safeguards against vaccine reversion. We demonstrate that administration of HaitiV 24 hours before lethal challenge with wild-type V. cholerae reduced intestinal colonization by the wild-type strain, slowed disease progression, and reduced mortality in an infant rabbit model of cholera. HaitiV-mediated protection required viable vaccine, and rapid protection kinetics are not consistent with development of adaptive immunity. These features suggest that HaitiV mediates probiotic-like protection from cholera, a mechanism that is not known to be elicited by traditional vaccines. Mathematical modeling indicates that an intervention that works at the speed of HaitiV-mediated protection could improve the public health impact of reactive vaccination.

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.

Vaccine development for enteric bacterial pathogens: Where do we stand?

Gut infections triggered by pathogenic bacteria lead to most frequently occurring diarrhea in humans accounting for million deaths annually. Currently, only a few licensed vaccines are available against these pathogens for mostly travelers moving to diarrheal endemic areas. Besides commercialized vaccines, there are many formulations that are either under clinical or pre-clinical stages of development and despite several efforts to improve safety, immunogenicity and efficacy, none of them can confer long-term protective immunity, for which repeated booster doses are always recommended. Further in many countries, financial, social and political constraints have jeopardized vaccine development program against these pathogens that enforce us to gather knowledge on safety, tolerability, immunogenicity and protective efficacy regarding the same. In this review, we analyze safety and efficacy issues of vaccines against five major gut bacteria causing enteric infections. The article also simultaneously describes several barriers for vaccine development and further discusses possible strategies to enhance immunogenicity and efficacy.

Impact of lower challenge doses of enterotoxigenic Escherichia coli on clinical outcome, intestinal colonization and immune responses in adult volunteers

A reliable and effective human challenge model is needed to help down-select the most promising ETEC vaccines currently under development. Such a model would need to reliably induce diarrhea in a high proportion of volunteers using the lowest possible inoculum to maximize safety and sensitivity. Previously we validated a challenge model that utilized a dose of 2x107 CFU of ETEC strain H10407 (LT+, ST+, CFA/I+ and O78+) to induce attack rates for moderate to severe diarrhea (MSD) of ~60-70%. Here we detail efforts to further refine the model in an attempt to determine if a lower challenge dose of H10407 can be used. Thirty subjects were randomized 1:1 to receive an oral administration of H10407 at doses of 106 or 105 CFU in bicarbonate buffer. After challenge, subjects were monitored for signs and symptoms of enteric illness and stool samples were collected to detect shedding of the challenge strain. Systemic and mucosal immune responses were measured using serum, antibody in lymphocyte supernatant and fecal samples. The attack rate was 13.3% (2/15) and 26.7% (4/15) for MSD in the 105 and 106 groups, respectively. Four MSD cases met criteria for early antibiotic treatment. All subjects but one shed the challenge strain in fecal samples. The frequency and magnitude of anti-LT toxin, CFA/I and LPS O78 immune responses were antigen, dose, severity of diarrhea and shedding levels dependent. Notably, although of lower magnitude, there were considerable immune responses in the subjects with no diarrhea. This may indicate that immune responses to asymptomatic infections of ETEC in children in the endemic countries may contribute to protection. Based on this and our prior studies, we conclude that a dose of 2x107 H10407 remains the lowest practical dose for use in future volunteer studies evaluating candidate vaccines and other preventive or therapeutic ETEC interventions.

TRIAL REGISTRATION

ClinicalTrials.gov NCT00844493.

Cholera

Cholera is an acute, watery diarrhoeal disease caused by Vibrio cholerae of the O1 or O139 serogroups. In the past two centuries, cholera has emerged and spread from the Ganges Delta six times and from Indonesia once to cause global pandemics. Rational approaches to the case management of cholera with oral and intravenous rehydration therapy have reduced the case fatality of cholera from more than 50% to much less than 1%. Despite improvements in water quality, sanitation, and hygiene, as well as in the clinical treatment of cholera, the disease is still estimated to cause about 100 000 deaths every year. Most deaths occur in cholera-endemic settings, and virtually all deaths occur in developing countries. Contemporary understanding of immune protection against cholera, which results from local intestinal immunity, has yielded safe and protective orally administered cholera vaccines that are now globally stockpiled for use in the control of both epidemic and endemic cholera.

Impact of CD4+ T Cell Responses on Clinical Outcome following Oral Administration of Wild-Type Enterotoxigenic Escherichia coli in Humans

Enterotoxigenic Escherichia coli (ETEC) is a non-invasive enteric pathogen of considerable public health importance, being one of the most common attributable causes of diarrheal illness in infants and young children in developing countries and the most common cause of traveler’s diarrhea. To enhance study-to-study consistency of our experimental challenge model of ETEC in volunteers, and to allow concomitant multi-site trials to evaluate anti-ETEC immunoprophylactic products, hundreds of vials, each containing a standardized inoculum of virulent wild-type (wt) ETEC strain H10407 (serotype O78:H11 expressing colonization factor antigen I and heat-labile and heat-stable enterotoxins), were prepared under current Good Manufacturing Practices (cGMP) and frozen. Following thawing, the contents of each vial can be used (diluted as necessary) to prepare consistent challenge inoculum, even at different study sites. A preliminary human experimental challenge study using this cGMP inoculum was conducted on a research isolation ward and the clinical and cell-mediated immune responses evaluated. Of the 6 healthy adult volunteers challenged 83% (5/6) developed diarrhea and 50% developed moderate-to-severe diarrhea (MSD). Moderate and severe diarrhea were defined as passage of ≥ 1 liter or ≥ 3 liters of diarrheal stool respectively. We compared the CD4+ T cell responses of volunteers who developed MSD against those who did not and identified significant differences in ETEC-specific cytokine production and gut homing potential. We furthermore demonstrated that increased expression of the gut-homing molecule integrin α4β7 by peripheral T follicular helper cells (pT_fh) correlated with decreased stool volume and increased ETEC-specific IgA B memory cell (B_M) development. Collectively, despite small numbers of volunteers, our results indicate a potential role for CD4+ T cells, in particular pT_fh, in modulating disease outcome following exposure to wt ETEC in a volunteer experimental challenge model.

Enterotoxigenic Escherichia coli (ETEC) is an important cause of diarrheal illness in infants and young children in the developing world, as well as in individuals traveling to endemic areas. Due to the lack of suitable animal models for human ETEC infection, we performed a human challenge study in which volunteers ingested wild-type ETEC in a controlled clinical setting. In addition to closely monitoring their clinical status, we studied their ETEC-specific T cell responses prior to and after challenge and studied the presence of associations between CD4+ T cell responses and clinical outcome. We observed differences in the immunological responses of individuals who developed moderate to severe diarrhea following challenge compared to those who did not. These results indicate that T cells may be an important component of the immune response against ETEC.

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.

Vaccines for viral and bacterial pathogens causing acute gastroenteritis: Part I: Overview, vaccines for enteric viruses and Vibrio cholerae

Efforts to develop vaccines for prevention of acute diarrhea have been going on for more than 40 y with partial success. The myriad of pathogens, more than 20, that have been identified as a cause of acute diarrhea throughout the years pose a significant challenge for selecting and further developing the most relevant vaccine candidates. Based on pathogen distribution as identified in epidemiological studies performed mostly in low-resource countries, rotavirus, Cryptosporidium, Shigella, diarrheogenic E. coli and V. cholerae are predominant, and thus the main targets for vaccine development and implementation. Vaccination against norovirus is most relevant in middle/high-income countries and possibly in resource-deprived countries, pending a more precise characterization of disease impact. Only a few licensed vaccines are currently available, of which rotavirus vaccines have been the most outstanding in demonstrating a significant impact in a short time period. This is a comprehensive review, divided into 2 articles, of nearly 50 vaccine candidates against the most relevant viral and bacterial pathogens that cause acute gastroenteritis. In order to facilitate reading, sections for each pathogen are organized as follows: i) a discussion of the main epidemiological and pathogenic features; and ii) a discussion of vaccines based on their stage of development, moving from current licensed vaccines to vaccines in advanced stage of development (in phase IIb or III trials) to vaccines in early stages of clinical development (in phase I/II) or preclinical development in animal models. In this first article we discuss rotavirus, norovirus and Vibrio cholerae. In the following article we will discuss Shigella, Salmonella (non-typhoidal), diarrheogenic E. coli (enterotoxigenic and enterohemorragic), and Campylobacter jejuni.

Peru-15 (Choleragarde(®)), a live attenuated oral cholera vaccine, is safe and immunogenic in human immunodeficiency virus (HIV)-seropositive adults in Thailand

BACKGROUND

Many areas with endemic and epidemic cholera report significant levels of HIV transmission. According to the World Health Organization (WHO), over 95% of reported cholera cases occur in Africa, which also accounts for nearly 70% of people living with HIV/AIDS globally. Peru-15, a promising single dose live attenuated oral cholera vaccine (LA-OCV), was previously found to be safe and immunogenic in cholera endemic areas. However, no data on the vaccine's safety among HIV-seropositive adults had been collected.

METHODS

This study was a double-blinded, individually randomized, placebo-controlled trial enrolling HIV-seropositive adults, 18-45 years of age, conducted in Bangkok, Thailand, to assess the safety of Peru-15 in a HIV-seropositive cohort.

RESULTS

32 HIV infected subjects were randomized to receive either a single oral dose of the Peru-15 vaccine with a buffer or a placebo (buffer only). No serious adverse events were reported during the follow-up period in either group. The geometric mean fold (GMF) rise in V. cholerae O1 El Tor specific antibody titers between baseline and 7 days after dosing was 32.0 (p<0.001) in the vaccine group compared to 1.6 (p<0.14) in the placebo group. Among the 16 vaccinees,14 vaccinees (87.5%) had seroconversion compared to 1 of 16 placebo recipients (6.3%). V. cholerae was isolated from the stool of one vaccinee, and found to be genetically identical to the Peru-15 vaccine strain. There were no significant changes in HIV viral load or CD4 T-cell counts between vaccine and placebo groups.

CONCLUSION

Peru-15 was shown to be safe and immunogenic in HIV-seropositive Thai adults.

Cholera toxin-B (ctxB) antigen expressing Salmonella Typhimurium polyvalent vaccine exerts protective immune response against Vibrio cholerae infection

Live attenuated vaccines are cost effective approach for preventing a broad range of infectious diseases, and thus are of great interest. However, immune-defects can predispose the patient to infections by the vaccine candidate itself. So far, few live vaccine candidates have been designed specifically for immune compromised individuals. Recently, we reported a new Salmonella Typhimurium Z234-vaccine strain (Periaswamy et al., PLoS ONE 2012;7:e45433), which was specifically attenuated in the NADPH-oxidase deficient host. In the present study, the Z234-vaccine strain was further engineered to express heterologous antigen (Vibrio cholerae toxin antigen subunit-B, i.e. CtxB) with the intention of creating a vector for simultaneous protection against Cholera and Salmonellosis. The primary aim of this study was to ensure the expression of CtxB antigen by the recombinant vaccine strain Z234-pMS101. The antigen CtxB was expressed through Z234 as a fusion protein with N-terminal signal sequence of Salmonella outer protein (SopE), an effector protein from Salmonella under the control of SopE promoter. The CtxB-expressing plasmid construct pMS101 (pM968-pSopE-ctxB) was found to be stable both in vitro and in vivo. In an oral mouse infection model, the vaccine strain Z234-pMS101 efficiently colonized the host gut. The extent of protection was confirmed after challenging the immunized hosts with live V. cholerae. Vaccinated mice showed reduced gut colonization by V. cholerae. Further assessment of immunological parameters supported the possibility of conferring effective immune response by Z234-pMS101 vaccine strain. Overall, the Z234-pMS101 vaccine strain showed potential as a promising polyvalent vaccine candidate to protect against S. Typhimurium and V. cholerae infection simultaneously.

Safety and immunogenicity of escalating dosages of a single oral administration of peru-15 pCTB, a candidate live, attenuated vaccine against enterotoxigenic Escherichia coli and Vibrio cholerae

Enterotoxigenic Escherichia coli (ETEC) organisms are a leading cause of infectious diarrhea in developing countries. A live, attenuated cholera strain that expresses high levels of the nontoxic B subunit of cholera toxin, which might also serve as an ETEC protective antigen, was evaluated for safety, excretion, and immunogenicity in healthy volunteers. We enrolled four inpatient dose-escalation cohorts of 15 to 16 eligible subjects to randomly (3:1) receive a single oral dose of vaccine or placebo (buffer alone), evaluating 1 ×10(7), 1 ×10(8), 1 ×10(9), and 1 ×10(10) CFU of the vaccine. The vaccine was well tolerated, although some subjects experienced moderate diarrhea. The serum Inaba vibriocidal antibody response appeared to display a dose-response relationship with increasing dosages of vaccine, plateauing at the 10(9)-CFU dosage. The serum antitoxin (cholera toxin and heat-labile enterotoxin) antibody seroconversion rate (4-fold increase over baseline) also appeared to display a dose-response relationship. The vaccine strain was excreted in stool cultures, displaying a dose-response relationship. A single oral dose of Peru-15 pCTB at dosages up to 1 ×10(10) CFU was safe and immunogenic in this first-in-human trial. These encouraging data support the ongoing clinical development of this candidate combined cholera and ETEC vaccine. (This study has been registered at ClinicalTrials.gov under registration no. NCT00654108.).

Estimating the per-exposure effect of infectious disease interventions

The average effect of an infectious disease intervention (eg, a vaccine) varies across populations with different degrees of exposure to the pathogen. As a result, many investigators favor a per-exposure effect measure that is considered independent of the population level of exposure and that can be used in simulations to estimate the total disease burden averted by an intervention across different populations. However, while per-exposure effects are frequently estimated, the quantity of interest is often poorly defined, and assumptions in its calculation are typically left implicit. In this article, we build upon work by Halloran and Struchiner (Epidemiology. 1995;6:142-151) to develop a formal definition of the per-exposure effect and discuss conditions necessary for its unbiased estimation. With greater care paid to the parameterization of transmission models, their results can be better understood and can thereby be of greater value to decision-makers.

Evaluation of dengue virus strains for human challenge studies

Discordance between the measured levels of dengue virus neutralizing antibody and clinical outcomes in the first-ever efficacy study of a dengue tetravalent vaccine (Lancet, Nov 2012) suggests a need to re-evaluate the process of pre-screening dengue vaccine candidates to better predict clinical benefit prior to large-scale vaccine trials. In the absence of a reliable animal model and established correlates of protection for dengue, a human dengue virus challenge model may provide an approach to down-select vaccine candidates based on their ability to reduce risk of illness following dengue virus challenge. We report here the challenge of flavivirus-naïve adults with cell culture-passaged dengue viruses (DENV) in a controlled setting that resulted in uncomplicated dengue fever (DF). This sets the stage for proof-of-concept efficacy studies that allow the evaluation of dengue vaccine candidates in healthy adult volunteers using qualified DENV challenge strains well before they reach field efficacy trials involving children. Fifteen flavivirus-naïve adult volunteers received 1 of 7 DENV challenge strains (n=12) or placebo (n=3). Of the twelve volunteers who received challenge strains, five (two DENV-1 45AZ5 and three DENV-3 CH53489 cl24/28 recipients) developed DF, prospectively defined as ≥2 typical symptoms, ≥48h of sustained fever (>100.4°F) and concurrent viremia. Based on our study and historical data, we conclude that the DENV-1 and DENV-3 strains can be advanced as human challenge strains. Both of the DENV-2 strains and one DENV-4 strain failed to meet the protocol case definition of DF. The other two DENV-4 strains require additional testing as the illness approximated but did not satisfy the case definition of DF. Three volunteers exhibited effusions (1 pleural/ascites, 2 pericardial) and 1 volunteer exhibited features of dengue (rash, lymphadenopathy, neutropenia and thrombocytopenia), though in the absence of fever and symptoms. The occurrence of effusions in milder DENV infections counters the long-held belief that plasma leakage syndromes are restricted to dengue hemorrhagic fever/dengue shock syndromes (DHF/DSS). Hence, the human dengue challenge model may be useful not only for predicting the efficacy of vaccine and therapeutic candidates in small adult cohorts, but also for contributing to our further understanding of the mechanisms behind protection and virulence.

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.

Cholera

Cholera is an acute, secretory diarrhoea caused by infection with Vibrio cholerae of the O1 or O139 serogroup. It is endemic in more than 50 countries and also causes large epidemics. Since 1817, seven cholera pandemics have spread from Asia to much of the world. The seventh pandemic began in 1961 and affects 3-5 million people each year, killing 120,000. Although mild cholera can be indistinguishable from other diarrhoeal illnesses, the presentation of severe cholera is distinct, with pronounced diarrhoeal purging. Management of patients with cholera involves aggressive fluid replacement; effective therapy can decrease mortality from more than 50% to less than 0·2%. Antibiotic treatment decreases volume and duration of diarrhoea by 50% and is recommended for patients with moderate to severe dehydration. Prevention of cholera depends on access to safe water and sanitation. Two oral cholera vaccines are available and the most effective use of these in integrated prevention programmes is being actively assessed.

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.

Refinement of a human challenge model for evaluation of enterotoxigenic Escherichia coli vaccines

Enterotoxigenic Escherichia coli (ETEC) strain H10407 (serotype O78:H11 producing heat-labile toxin [LT], heat-stable toxin [ST], and colonization factor I [CFA/I]) induces reliably high diarrheal attack rates (ARs) in a human challenge model at doses of ≥10(9) CFU. A descending-dose challenge study was conducted with changes to the standard fasting time and buffer formulation, seeking conditions that permit lower inocula while maintaining reproducibly high ARs. In cohort 1, 20 subjects were fasted overnight and randomized 1:1:1:1 to receive H10407 at doses of 10(8) CFU with bicarbonate, 10(8) CFU with CeraVacx, 10(7) CFU with bicarbonate, or 10(7) CFU with CeraVacx. Subsequent cohorts received H10407 (10(7) CFU with bicarbonate) with similar fasting conditions. Cohort 2 included 15 ETEC-naïve volunteers. Cohort 3 included 10 ETEC-naïve volunteers and 10 rechallenged volunteers. In all, 25/35 (71%) ETEC-naïve recipients of 10(7) CFU of H10407 developed moderate or severe diarrhea (average maximum stool output/24 h = 1,042 g), and most (97%) shed H10407 (maximum geometric mean titer = 7.5 × 10(7) CFU/gram of stool). Only one of 10 rechallenged volunteers developed diarrhea. These rechallenged subjects had reduced intestinal colonization, reflected by quantitative microbiology of fecal samples. Among the 35 ETEC-naïve subjects, anti-lipopolysaccharide (LPS) O78 serum antibody responses were striking, with positive IgA and IgG antibody responses in 33/35 (94%) and 25/35 (71%), respectively. Anti-heat-labile enterotoxin (LTB) serum IgA and IgG responses developed in 19/35 (54%) and 14/35 (40%) subjects, respectively. Anti-CFA/I serum IgA and IgG responses were detected in 15/35 (43%) and 8/35 (23%) subjects. After the second challenge, participants exhibited blunted anti-LPS and -LTB responses but a booster response to CFA/I. This ETEC model should prove useful in the future evaluation of ETEC vaccine candidates.

Vaccination strategies to combat an infectious globe: oral cholera vaccines

Cholera is a substantial health burden in many countries in Africa and Asia, where it is endemic. It is as well responsible for ongoing epidemics in sub-Saharan Africa which are becoming greater in terms of frequency, extension, and duration. Given the availability of two oral cholera vaccines and the new data on their efficacy, field effectiveness, feasibility, and acceptance in cholera-affected populations and in travelers, these vaccines should be used in endemic areas, in travelers for these areas and should be considered in areas at risk for outbreaks. The two vaccines currently available in worldwide are: (1) The killed oral vaccine (Dukoral, licensed by SBL–Sweden to Crucell–Holland) is recommended since 1999 by WHO and consists of a mixture of four preparations of heat or formalin killed whole cell Vibrio cholera O1 (Inaba and Ogaba serotypes, and classical and El Tor biotypes) that are then added with purified recombinant cholera toxin (CT) B subunit. Because CT cross-reacts with Escherichia coli LT the vaccine also provides short-term protection against ETEC (enterotoxigenic E. coli) which is of added benefit for travelers. It is available in more than 60 countries. (2) A bivalent O1 and O139 whole cell oral vaccine without CT B subunit (Shanchol) has been lately developed in Vietnam (licensed by VaBiotech–Viet Nam to Shantha Biotechnics–India. It is available in India and Indonesia. A structured search of papers in PubMed and reports on cholera vaccines by WHO and CDC, as well as critical reading and synthesis of the information was accomplished. Inclusion criteria were defined according to reports quality and relevance.

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.

Exploiting cholera vaccines as a versatile antigen delivery platform

The development of safe, immunogenic and protective cholera vaccine candidates makes possible their use as a versatile antigen delivery platform. Foreign antigens can be delivered to the immune system with cholera vaccines by expressing heterologous antigens in live attenuated vectors, as fusion proteins with cholera toxin subunits combined with inactivated Vibrio cholerae whole cells or by exposing them on the surface of V. cholerae ghosts. Progress in our understanding of the genes expressed by V. cholerae during infection creates unprecedented opportunities to develop an improved generation of vaccine vectors to induce immune protection against a broad range of pathogenic organisms.

The true burden and risk of cholera: implications for prevention and control

Cholera is a substantial health burden on the developing world and is endemic in Africa, Asia, South America, and Central America. The exact scale of the problem is uncertain because of limitations in existing surveillance systems, differences in reporting procedures, and failure to report cholera to WHO; official figures are likely to greatly underestimate the true prevalence of the disease. We have identified, through extensive literature searches, additional outbreaks of cholera to those reported to WHO, many of which originated from the Indian subcontinent and southeast Asia. Such underestimation of cholera can have important implications for decisions on provision of health interventions for indigenous populations, and on risk assessments for travellers. Furthermore, until recently, it has not been possible to implement public-health interventions in low-income countries to eliminate disease, and the prevention of cholera in travellers has been limited to restrictive guidelines. However, a vaccine against cholera is now available that has proven efficacy and tolerability in mass vaccination campaigns in low-income countries, and among travellers.

A Vibrio cholerae serogroup O1 vaccine candidate against CTX ET Phi infection

Cholera is a severe diarrheal disease that may spread rapidly. Vaccination is considered a valid measure against it. We developed a new vaccine candidate, IEM109, against Vibrio cholerae. To generate this candidate, a chromosomal fragment containing the TLC element, attB of the CTX Phi integration site, and RTX cluster responsible for the cytotoxic activity for mammalian cells was deleted through homologous recombination from the previously described El Tor biotype, IEM101. The protective genes ctxB and rstR, which establish resistance to CTX Phi infections, were inserted into that same location on the chromosome of IEM109 to enhance the safety and genetic stability of the vaccine candidate and to prevent horizontal gene transfer. In in vivo tests, cell cultures showed that the cytotoxic effect of IEM109 on Hep-2 was negative. Furthermore, the infection rate of El Tor biotype CTX Phi to that of IEM109 in the rabbit intestine is 3000-fold lower than that of IEM101. Intraintestinal vaccination of rabbits with a single dose of IEM109 elicits high titers of anti-CTB IgG and vibriocidal antibodies. When challenged with 0.5-2 microg CT and 10(5) to 10(8)CFU of four wild toxigenic strains of different biotypes and serogroups, IEM109 conferred full protection. Thus, IEM109 is a stable vaccine candidate that evokes not only antitoxic and vibriocidal immunities, but also resistance to the El Tor biotype CTX Phi infection.

Peru-15, a live attenuated oral cholera vaccine, is safe and immunogenic in Bangladeshi toddlers and infants

A live oral Vibrio cholerae O1 El Tor vaccine, Peru-15 was tested in a double-blind, randomized placebo controlled study for safety and immunogenicity in Phase I and Phase II studies in 240 Bangladeshi children aged 9 months-5 years of age. Two different doses (2x10(7) and 2x10(8)cfu) were tested. Vaccination did not elicit adverse events and the strain was genetically stable. Vibriocidal antibody responses developed in 42/50 (84%) toddlers (2-5 years) and 35/50 (70%) of younger children (9-23 months) and overall 77/100 (77%) who received the high dose. LPS-IgA-antibody responses were seen in 60% of toddlers and 34% of infants; 40% responded with IgA antibodies to cholera toxin. The responses to the reduced dose was lower. These studies demonstrate that Peru-15 at a dose of 2x10(8)cfu is safe and immunogenic in children in Bangladesh.

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.

Biosafety evaluation of recombinant live oral bacterial vaccines in the context of European regulation

Live bacterial vaccines represent a highly valid preventive strategy in the fight against infectious disease. However, the road from research to market is peppered with hurdles, one of which is the requirement for high biosafety characteristics, which the candidate vaccine has to display. In Europe, the European Agency for the evaluation of medicinal products (EMEA) is the relevant authority regulating the licensure of genetically engineered vaccines. For this purpose, the agency may rely on several directives and guidelines defined in the past 15 years. As for live vaccines containing genetically modified organisms (GMOs) susceptible to be released into the environment, Directive 2001/18/EC determines the framework and principles of an environmental risk assessment (ERA) process, the results of which constitute an important section of the vaccine registration package submitted to registration authorities. In this article, we address the implications of current European regulations for the approval of live oral bacterial vaccines with emphasis on the assessment of potential risks associated with environmental release. Biosafety aspects of already registered and some promising live bacterial vaccine strains will be briefly discussed.

Enteric infections and the vaccines to counter them: future directions

While it is well-recognized that diarrheal diseases remain the second most frequent cause of mortality among children <60 months of age in the developing world, there is nevertheless a need to obtain more precise mortality and hospitalization burden data in populations living in the world's least developed areas. There is also a glaring need to obtain robust etiology data in relation to the different diarrheal disease clinical syndromes, including serotypes of Shigella and antigenic types of ETEC. Because of the poor uptake of the new typhoid and cholera vaccines licensed since 1985, it will be important to create reliable, long-term demand for the next generation of enteric vaccines, including new rotavirus, Shigella and ETEC vaccines. The first priority is to get individual vaccines licensed. Post-licensure, it will then be simpler to investigate the clinical acceptability, immunogenicity and effectiveness of various combinations of the individual licensed enteric vaccines. The extensive gut mucosal surface with its many sites for induction of immune responses make it likely that co-administrations will be successful. Partnerships of public and private agencies in the developing and the industrialized world will have to be forged to create a reliable demand for new enteric vaccines and to assure adequate, sustained supplies of affordable products. Systematic implementation programs will have to be created in the least developed, high burden, high mortality countries to deliver enteric vaccines and to document their impact after introduction.

Virulence factors, pathogenesis and vaccine protection in cholera and ETEC diarrhea

Recent work has provided new insights into the pathogenesis of the potentially life-threatening diarrheas caused by Vibrio cholerae and enterotoxigenic Escherichia coli (ETEC): a new mechanism (post-translational degradation), which is involved in the control of cholera toxin expression, has been discovered. Recent evidence also suggests that vibrios upregulate cholera toxin expression in response to intestinal fluid components, and enterotoxin-carrying bacterial outer membrane vesicles might have a function in ETEC pathogenesis. An important role of the environment is supported by the correlation between cholera incidence and elevated sea surface temperature, which supports the notion that the zooplankton is a V. cholerae reservoir. Additionally, environmental lytic cholera phages could influence cholera seasonality by 'terminating' the seasonal epidemic. Finally, the strong herd immunity elicited by an oral cholera vaccine indicates that cholera vaccination could have a significant public health impact.

TcpF is a soluble colonization factor and protective antigen secreted by El Tor and classical O1 and O139 Vibrio cholerae serogroups

Vibrio cholerae causes diarrhea by colonizing the human small bowel and intoxicating epithelial cells. Colonization is a required step in pathogenesis, and strains defective for colonization are significantly attenuated. The best-characterized V. cholerae colonization factor is the toxin-coregulated pilus (TCP). It has been demonstrated that TCP is required for V. cholerae colonization in both humans and mice. TCP enhances bacterial interactions that allow microcolony formation and thereby promotes survival in the intestine. We have recently discovered that the TCP biogenesis apparatus also serves as a secretion system, mediating the terminal step in the extracellular secretion pathway of TcpF. TcpF was identified in classical isolates of V. cholerae O1 as a soluble factor essential for colonization in the infant mouse cholera model. In the present study, we expanded our analysis of TcpF to include the O1 El Tor and O139 serogroups and investigated how TCP and TcpF act together to mediate colonization. Additionally, we demonstrated that antibodies generated against TcpF are protective against experimental V. cholerae infection in the infant mouse cholera model. This observation, coupled with the fact that TcpF is a potent mediator of colonization, suggests that TcpF should be considered as a component of a polyvalent cholera vaccine formulation.