The goal of this clinical trial is to test a new Shigella vaccine (InvaplexAR-DETOX) in combination with a new adjuvant (dmLT) in healthy participants. The main questions it aims to answer are:

• Is the new Shigella vaccine (with and without the new adjuvant) safe and well tolerated?
• How wel does the new Shigella vaccine stimulate the immune system in combination with the new adjuvant, and without the new adjuvant?

Participants will receive three vaccinations at 28-day intervals. Researchers will compare the results of participants vaccinated with the vaccine in combination with the adjuvant to the results of participants vaccinated with the vaccine only and to the results of participants vaccinated with a placebo (fake vaccine).

Rationale: Shigella remains endemic in many places and occurs in epidemics that cause considerable morbidity and mortality. Vaccines are an attractive and potentially highly cost-effective tool for the prevention of shigellosis and can fill current gaps in effective prevention strategies. A challenge to effective Shigella vaccine development has been the reduced immunogenicity and protective efficacy of candidate Shigella vaccines in infants and children less than 3 years of age. The potential impact of including an adjuvant in candidate parenteral Shigella vaccine formulations needs to be evaluated. InvaplexAR-Detox is an injectable Shigella vaccine that uses a novel combination of conserved invasion plasmid antigen proteins with a serotype-specific bacterial lipopolysaccharide attenuated for safe intramuscular administration. The adjuvant dmLT has been shown to significantly enhance Shigella immune responses in mice and has safely been administered intramuscularly in healthy volunteers in combination with other antigens in phase I trials.

Objective: to evaluate the safety and immunogenicity of two strength formulations of a candidate Shigella vaccine (2.5 or 10 μg Sfl2a InvaplexAR-Detox) given with and without adjuvant (0.1 μg dmLT).

Study design: this is a phase Ia/b dose escalation, randomized, double-blind, placebo-controlled trial assessing the safety, tolerability and immunogenicity of three vaccinations given 4 weeks apart of Sfl2a InvaplexAR-Detox vaccine alone or in combination with the dmLT adjuvant in the Leiden University Medical Center in the Netherlands and at the Centre for Infectious Disease Research in Zambia (CIDRZ). The study will be initiated with the low vaccine dose in the Netherlands (Cohort A) and will not proceed to the high vaccine dose in the Netherlands (Cohort B) before the safety data of Cohort A has been reviewed by the Safety Monitoring Committe (SMC). The SMC will also review the safety data of Cohort B before the high vaccine dose will be administered in Zambian adults (Cohort C).

Study population: a total of 50 healthy Dutch and 35 healthy Zambian adults aged 18-50 years.

Intervention: a 2.5 μg or 10 μg intramuscular dose of the candidate Shigella vaccine Sfl2a InvaplexAR-Detox given with and without double mutant (LT R192G/L211A) enterotoxigenic Escherichia coli heat labile toxin adjuvant (0.1 μg dmLT) given at day 1, day 29 and day 57 compared to a placebo (saline).

Main study endpoints: Primary endpoint measures are the occurrence of solicited and unsolicited adverse events considered to be related to vaccination. Secondary outcome measures are humoral and cellular immune responses to vaccination with and without adjuvant compared to placebo.

Inclusion Criteria:

1. Healthy adult, male or female, aged 18 to 50 years (inclusive) at the time of inclusion (=vaccination).
2. Provide written informed consent before initiation of any study procedures.
3. Available to complete all study visits and procedures.
4. Negative stool PCR test for Shigella.
5. Women of childbearing potential: negative pregnancy test at screening and before each study vaccine administration. Women are considered not of childbearing potential if they are postmenopausal (no menses for 12 months without an alternative medical cause), or if they have no uterus or no ovaries. Women of childbearing potential must agree to use continuous adequate contraception to avoid pregnancy during the study, for at least 4 weeks before the first vaccination and for 3 months following the last vaccine dose.

Adequate methods of contraception for this study include:

1. hormonal contraception

1. combined (estrogen and progestogen containing) hormonal contraception associated with inhibition of ovulation (oral, intravaginal, or transdermal)

2. progestogen-only hormonal contraception associated with inhibition of ovulation (oral, injectable, or implantable) 2. intrauterine device (IUD) 3. intrauterine hormone-releasing system (IUS) 4. bilateral tubal occlusion/litigation procedure 5. vasectomized partner (the vasectomized partner should be the sole male sexual partner for that participant).

   6. sexual abstinence (defined as refraining from heterosexual intercourse from signing the informed consent until 3 months after the last vaccine dose).

   1. Any history or evidence of clinically relevant chronic medical conditions (such as: psychiatric conditions, diabetes mellitus, hypertension [treated by medication], autoimmune disorders, immunodeficiencies, cardiovascular, renal disease or inflammatory bowel disease). Trial physicians (in consultation with the principal investigator) will use clinical judgement on a case-by-case basis to assess safety risks under this criterion.
   2. Current use of immunosuppressive medications (except for antihistamines and topical or inhalation corticosteroids).
   3. Women who are a) currently nursing or b) who are pregnant or planning to become pregnant during the study period plus 3 months beyond the last vaccine dose.
   4. Participation in research involving another investigational product (defined as receipt of an investigational product or exposure to an invasive investigational device) 30 days before the first vaccination or anytime through the last in-clinic study safety visit.
   5. Positive blood test for hepatitis B surface antigen (HbsAg), hepatitis C virus (HCV), or human immunodeficiency virus (HIV).
   6. Clinically significant abnormalities on basic laboratory screening tests.
   7. Systemic antimicrobial treatment (i.e., topical treatments are not an exclusion criterion) within 1 week before the first vaccine dose (temporary exclusion).
   8. Known hypersensitivity to compounds in the vaccine or adjuvant or other known drug allergies that may increase the risk of adverse events.
   9. Regular use (weekly or more often) of antidiarrheal, anti-constipation, or antacid therapy.
   10. Abnormal stool pattern (fewer than 3 stools per week or more than 3 stools per day) on a regular basis; loose or liquid stools on other than an occasional basis.
   11. Personal or family history of inflammatory arthritis.
   12. Proven allergy to any substance in the InvaplexAR-Detox vaccine or dmLT or history of anaphylactic reaction to any other vaccine.
   13. Exclusionary skin disease history/findings that would confound assessment or prevent appropriate monitoring of local AEs, or possibly increase the risk of local AEs.
   14. Recent (<3 months) history of gastroenteritis.
   15. Received previous licensed or experimental Shigella vaccine, dmLT or live Shigella challenge.
   16. Any severe medical condition that might place the participant at increased risk of adverse events according to the clinical judgment of the study clinicians in consultation with the PI.
   17. Any planned vaccination within 14 days before the first vaccine dose until the last in-clinic study visit, with the exception of SARS-CoV-2 vaccines or influenza vaccines.

Self-identified male or female

Data of the trial will follow the FAIR (findable, accessible, interoperable and re-usable) principles for data collection and sharing outlined by force11 and metadata will be entered in a repository which will be open access after completion of the trial.

After completion of the trial. Specific timepoint to be decided.

Double blind