Background: Despite efforts to control cholera, outbreaks continue to occur in Kenya. Oral cholera vaccines (OCVs) are a critical tool in cholera prevention strategies. This study evaluates the immunogenicity of extended dosing intervals for Euvichol-S, a WHO-prequalified OCV, in a Phase IV non-inferiority trial.

Overview Design: This trial is a parallel-group, open-label, randomized, non-inferiority study. It aims to compare the immune response of three dosing schedules of the Euvichol-S OCV: a standard 2-week interval, a 2-month interval, and a 12-month interval. The study will enroll 1071 participants, stratified by age into three groups (1-4 years, 5-14 years, 15+ years. The primary endpoint is the plasma vibriocidal geometric mean titer (GMT) measured two weeks after the second dose.

Primary Objective: To assess and compare the immune response to Euvichol-S measured by vibriocidal GMT two weeks after the second vaccine dose across different dosing intervals.

Study Sites: The study will be conducted in the Mukuru informal settlement in Nairobi, Kenya, a high-priority cholera hotspot area. The Kenya Medical Research Institute (KEMRI) will manage the study, leveraging its established relationship with the community.

Study Population: Inclusion criteria include residents of Mukuru ≥1 year, who are healthy as determined by medical history and physical examination. Exclusion criteria include pregnant women, severe malnutrition, and non-HIV immunosuppressive conditions or severe chronic diseases.

Study Interventions: Participants will be randomized to one of three dosing arms stratified by age: a standard 2-week interval, a 3-month interval, or an annual booster interval. Participants will receive the Euvichol-S OCV according to the assigned schedule. The follow-up period for participants will be 18 months, during which they will undergo regular scheduled visits and additional unscheduled visits as needed (i.e., standard dosing arm: six scheduled visits; 3-month interval arm: 7 visits; annual booster arm: 6 visits). Blood samples will be collected at each vaccination visit and 14 days later and at 6 months, 1 year and 18 months after enrollment to measure plasma vibriocidal GMT and other immunological markers. Older children in the PBMC cohort will have two additional samples collected five days after each vaccine dose and a larger blood volume (10mls) collected at 14 days after the second dose.

Outcome Measures: Primary outcome measures include the plasma vibriocidal GMT two weeks post-second dose. Secondary outcomes include antibody seroconversion rates, longitudinal GMT changes, incidence of cholera disease, and the safety profile of the vaccine. The PBMC sub-cohort will provide detailed insights into memory B cell and plasmablast responses.

Sample Size: The study will enroll 1071 participants with equal distribution across the three dosing arms and age strata. The PBMC sub-cohort will include 240 participants (40 per arm among 5-14 years, 40 per arm 1-4 years), with detailed immunological assessments.

Data Analysis: The immunogenicity of the vaccine across different dosing schedules will be compared to determine non-inferiority. Data will be analyzed descriptively to summarize the by-grade incidence of treatment-emergent adverse events (AEs), serious adverse events (SAEs), and other safety indicators.

Impact: This trial aims to generate evidence on the optimal dosing schedule for Euvichol-S OCV, potentially informing future vaccination strategies in cholera-endemic regions and improving cholera prevention in resource-limited settings.

Study Rationale Between October 2022 and February 2024, Kenya has experienced a cholera outbreak with over 10,000 reported cases and 166 deaths across 23 counties. Alarmingly, children under 10 years of age account for one-third of the documented cases, indicating their heightened vulnerability to the disease. In February 2023, Kenya vaccinated more than 2.2 million people with a single dose of oral cholera vaccine (OCV). However, this reactive campaign alone has proven insufficient to control the outbreak.

To address endemic cholera and prevent future outbreaks, the Kenya MoH has applied to Gavi, the Vaccine Alliance, to receive OCV for a preventive vaccination campaign in 2025 to 2028. Based on recent hotspot mapping led by partners in our consortium, the government plans to conduct mass vaccination campaigns in 94 hotspot sub-counties, reaching 19 million individuals. This campaign will mark the first national preventive campaign of OCV in East Africa. Insights gained will benefit not only Kenya but also neighbouring countries experiencing cholera outbreaks.

Studies evaluating the relative impact of campaigns utilizing two doses with extended intervals are limited. Additionally, prior studies assessing extended dosing intervals have not compared durability and immunogenicity within specific age groups, nor have they consistently included extended follow-up periods. Given the specific vulnerability of children, the Global Task Force for Cholera Control ranks research on optimal cholera vaccine schedules for children one to five years as the highest priority in the Cholera Roadmap Research Agenda.

Vaccination with OCV is a proven, effective strategy to prevent cholera outbreaks. However, the current manufacturer-recommended dosing regimen is challenging to implement programmatically and offers only short-term protection, particularly in young children, the most vulnerable population. This study aims to address this gap by generating evidence on the immunogenicity and durability of immunity across different dosing schedules. By evaluating the immune response, particularly in children, this trial will provide critical data to inform optimal dosing strategies. The findings will support the Kenyan Ministry of Health in enhancing the impact of preventive OCV campaigns and improving long-term cholera control efforts in endemic regions.

Risks and Benefits

Known Potential Risks

Associated with oral vaccine administration:

Mild to moderate side effects, such as gastrointestinal discomfort, nausea, or diarrhea, may occur.

Serious allergic reactions, though rare, are a possibility and will be monitored.

Behaviour Change:

Although the literature on preventative behaviors suggest that it is highly unlikely, it is theoretically possible that participants might alter their risk behaviors assuming they are protected post-vaccination. They will be advised to continue following cholera prevention measures.

Specific Risks from Euvichol-S Vaccine:

Euvichol-S is generally well-tolerated. In the phase 3 clinical trial, of 1,595 participants who received the product, 151 (<10%) reported any adverse event. The most common adverse events after vaccination were pyrexia (3.6%) and nasopharyngitis (2.2%).

The specific risks associated with different dosing intervals of Euvichol-S are under study. Participants will be informed of any known risks related to the vaccine.

Community Perceptions:

Participation in the vaccine trial might affect community perceptions towards the participant. Efforts will be made to maintain confidentiality and manage community relations carefully.

Associated with Venous Puncture:

While generally safe, venous puncture can result in discomfort or minor reactions such as pain at the puncture site, bruising, or occasional bleeding. Participants will receive instructions to minimize and manage these potential effects.

Vaccine efficacy risks:

One dose of oral cholera vaccine offers short-term protection against cholera, necessitating a second dose for longer-term immunity. Our hypothesis is that longer intervals between doses (3-months and 1 year) will result in similar long-term immunity at 1 year after the delayed booster dose as with the standard 2-week dosing interval at 1 year after the booster dose; however, it is possible that protection against cholera may diminish prior to the 2nd dose. These risks will be clearly communicated to participants, and any new information regarding vaccine efficacy will be communicated as it becomes available.

Known Potential Benefits Euvichol-S oral cholera vaccine induces a protective immune response against cholera. Thus, participants in this study who receive the vaccine are expected to have a lower risk of cholera compared to those who are not enrolled in the trial and do not receive immunization. Additionally, participants in this trial will receive the vaccine before it becomes widely available through government vaccination campaigns. This early access to the vaccine provides an immediate benefit in reducing their risk of cholera infection. Furthermore, this study aims to enhance the understanding of cholera vaccination strategies, potentially leading to more effective public health interventions against cholera globally.

Assessment of Potential Risks and Benefits

The trial balances the mild to moderate side effects and the rare but serious risks of allergic reactions from the Euvichol-S oral cholera vaccine against the expected benefit of reduced cholera risk among participants. While there are uncertainties regarding the longevity of protection, especially between dosing intervals, the study is critical for optimizing vaccination schedules and enhancing global cholera prevention strategies.

Objectives and Endpoints The primary objective is to assess and compare the immune response, specifically plasma vibriocidal GMT, two weeks post-second dose of Euvichol-S, across three dosing intervals: 2-weeks, 3-months and 1-year. Our secondary objectives expand on this by evaluating seroconversion, immune response within different age groups, monitoring long-term GMT differences, assessing cholera incidence, and documenting the vaccine's safety profile over extended dosing intervals. Additionally, our exploratory objectives include, analyzing V. cholerae O-specific polysaccharide (OSP)-specific plasma antibody, circulating gut-homing plasmablasts (a surrogate marker for the mucosal immune response), and memory B cell response magnitude and dynamics, to gain comprehensive insights into the vaccine's immunogenicity over 18 months.

Primary Objective To assess and compare the immune response to Euvichol-S, measured by plasma vibriocidal geometric mean titer (GMT) two weeks after the second vaccine dose, across different dosing intervals in participants aged 1 year and older.

Hypotheses:

The immune response to a three-month or twelve-month dosing interval of Euvichol-S is non-inferior to the recommended two-week interval in participants aged 1 year and older two weeks after the second dose.

This non-inferiority persists within specific age groups: young children (1-4 years), older children (5-14 years), and adults (15+ years), as measured at 6 months, 12 months, and 18 months after enrollment.

Secondary Objectives To compare plasma vibriocidal GMT across different dosing intervals within specific age groups.

To evaluate vibriocidal antibody seroconversion in relation to dosing intervals among all participants and within each age group.

To monitor and document the changes in vibriocidal GMT over an 18-month follow-up period, comparing between dosing interval groups and within age groups.

To assess and compare the incidence of cholera disease over an 18-month follow-up, focusing on the association with age groups and dosing intervals.

To evaluate and describe the safety and tolerability profile of extended dosing intervals of Euvichol-S.

Exploratory Objectives Assess and compare the following measures a) among all study participants and b) within each age group: Anti-O-specific polysaccharide (OSP) IgA and IgG responses in plasma over an 18-month follow-up period

Circulating gut-homing anti-OSP plasmablasts at baseline and 5 days after each dose of vaccine in a subcohort of children under 15 years Circulating anti-OSP memory B cells at baseline and 14 days after booster doses after each dose of vaccine in a subcohort of children under 15 years Assess and compare the persistence of plasma anti-OSP and vibriocidal immune response over an 18-month follow-up period by age group and dosing interval.

Cholera Disease Incidence Comparison: This involves comparing the incidence of cholera over 18 months across different age groups and dosing intervals. It is a direct measure of the vaccine's effectiveness in preventing the disease in the real-world setting.

Safety Profile Assessment: Investigators will record and analyze adverse events, spanning 18 months from the initial vaccination to the final follow-up, to assess the vaccine's safety over different dosing schedules.

Inclusion Criteria:

• Residents of Mukuru, Nairobi, Kenya
• Individuals aged 1 year and above
• Voluntary written informed consent for study participation provided by an individual or his/her legally acceptable representative. Children aged 13 years and above will also provide assent, with parental permission required for all children.
• Ability to comply with study requirements and attend follow-up visits during the study period.

Participants must be in good health, as determined by medical history, physical examination, and the clinical judgment of the investigators. Clinical judgment will consider factors such as the absence of acute illness or, uncontrolled or severe chronic conditions that may affect participation in the study.

• Lactating women may be enrolled following clinical assessment and informed consent. The vaccine contains killed, formalin-inactivated bacteria that are not systemically absorbed and act locally in the gastrointestinal tract.

Exclusion Criteria:

• Known history of hypersensitivity reactions to other vaccines.
• Pregnant women, due to differences in immune response. A pregnancy test will be administered to all female participants who have reached menarche and are under 50 years.
• Reported diarrhea or abdominal pain lasting 2 weeks or longer within 6 months prior to study initiation; to avoid confounding the vaccine's effects with pre-existing conditions.
• Received a cholera vaccine in the last 24 months: Ensures that the study assesses the vaccine in question without interference from prior vaccinations.
• History of cholera disease in the last 24 months: Recent history of cholera infection can interfere with the measurement of vaccine response.
• Severely malnourished individuals as determined by mid-upper arm circumference (MUAC) and age-specific body mass index (BMI) measurements: malnutrition can affect immune response and vaccine efficacy. In children below 510 years, severe malnutrition will be defined as MUAC < 11.5 cm, for older children (age 5 - 17y) severe malnutrition will be defined as BMI-for-age z score < -3 (WHO Child Growth Standards), for children <10y, presence of bilateral pitting oedema will be considered indicative of severe malnutrition. For adults (18y and above), severe malnutrition will be while in older children and adults it will be defined as BMI < 16.
• Non-HIV/AIDS immunosuppressive condition or on immunosuppressive therapy: Such conditions can significantly alter vaccine response.
• Presence of bleeding disorders or medical contraindication for blood draws: To ensure participant safety during blood collection.
• Participation in another clinical trial with investigational product dosing within 6 months prior to study initiation.
• An individual thought to have difficulty in participating in the study due to severe chronic diseases, based on the judgment of the investigator.
• An individual thought to have difficulty participating in the study due to reasons, such as significant logistical constraints, or communication barriers, or likely to be away for a period of at least 3 consecutive months in the first 6 months of enrollment based on the judgment of the investigator.