This is a Phase II b, double-blind, randomized, placebo-controlled study in healthy adults (age 18-65 years) to evaluate the safety, immunogenicity, different diagnostic tools and efficacy of ETVAX. Participants will travel to Grand Popo, Africa for 12 days. Prior travelling participants will be vaccinated with two doses of vaccine or placebo. Vaccine Preventable Outcome will be identified and then characterized as to incidence, duration, severity and frequency of Moderate or Severe Travellers diarrhea. Health related information and assessments will be recorded during the travel.

This is a Phase II b, double-blind, randomized, placebo-controlled study in healthy adults (age 18-65 years) to evaluate the safety, immunogenicity, different diagnostic tools and efficacy of two doses of ETVAX. Study participants will be recruited among students, and personnel at the University of Helsinki and Helsinki University Hospital and among those responding to recruitment advertisements. To be eligible, the participants have to commit themselves to comply with the study protocol which involves in addition to vaccination, study visits and sampling, and commitment to travel to Grand Popo, Benin and stay there for 12 days. After providing written informed consent, eligible participants will be randomized (ratio 1:1, and block randomization in groups of 6) and immunized with two doses of vaccine or placebo, given 14±7 days apart while in Finland. The last dose of vaccine will be administered at least one week before departure (and no more than 30 days prior to travel) to Benin. Eligible participants will pay four pre-travel visits (V0 + V 1-3) to Aava Travel Clinic / University of Helsinki prior to travel. They will be asked to fill in pre-travel questionnaire latest at visit 1 (Q1) and fill in an Adverse Event Form (AEF1 and AEF2) after each dose, and give blood and fecal samples and saliva before travel. The participants will go together to Benin in groups of 25-35 individuals at a time.

In Benin, the participants will be seen within 48 hours of arrival to the study site in Grand Popo. Once the participants arrive at the study site in Grand Popo, they will be provided with practical information and contact details of the study personnel. They will receive the health card (HC1) and a stool collection kit with instructions.

Diarrhea reporting, stool collection, and stool submission procedures will be reviewed with the study participants. One routine visit at day 4 is planned for review of the participant's health status and collection of a routine stool sample. Other visits will take place when and if the participant gets Travelers Diarrhea (TD) episodes for collection of study specific stool samples and health related information and assessments. If more than one diarrhea episode occurs, 48 diarrhea and symptom (TD defined symptoms) free hours must have passed between the episodes for the new episode to be counted as a separate one. One or two days prior to departure from the Grand Popo site, all participants will have a final review of their HC1s. Health Card ( HC2) will be given. After 1-6 days back in Finland the participant has to give blood and a routine stool sample and they fill in Questionnaire 2 (Q2), HC2 will be reviewed. Approx. 30 days after return to Finland the last stool and blood sample will be collected and the post-travel questionnaire (Q3) filled and HC2 reviewed. Participants who get urinary tract infection will fill in Urinary questionnaire 4 (QU4) form at the time of infection. This follow-up is valid until V5.

Inclusion criteria:

1. Male or female age ≥18 and ≤ 65 years
2. General good health at the time of first vaccination
3. Female participants of childbearing potential must not be pregnant
4. Females of childbearing potential must agree to use an efficacious hormonal or barrier method of birth control during the study
5. Willingness to participate in the study after all aspects of the protocol have been explained and written informed consent obtained
6. Availability for the study duration, including all planned follow-up visits
7. Intake of atovaquone + proguanil (Malarone) as anti-malaria prophylax according to prescription guidelines mandatory before, during and after travel to Benin

Exclusion Criteria:

1. Presence of a significant medical or psychiatric condition, which in the opinion of the investigator precludes participation in the study
2. Known or suspected history of drug, chemical or alcohol abuse, as deemed by the investigator/physician; AUDIT > 13 points
3. Known recent history of impaired immune function which, according to the judgement of the investigator could influence the immune response
4. Intends to receive any other investigational vaccine during the study period, or within two weeks prior to study vaccination
5. Intends to donate blood at any time during the study.
6. An acute or chronic medical condition that, in the opinion of the investigator/physician, would render ingestion of the investigational products unsafe or would interfere with the evaluation of responses. This includes, but is not limited to gastrointestinal diseases and autoimmune diseases requiring treatment
7. Any history of psychosis or bipolar disorder or on-going significant mental disorder
8. Regular (daily) use of laxatives or agents which lower stomach acidity (antacids, proton pump inhibitors) less than one week before visit V1
9. Use of any oral or parenteral medication known to affect the immune function (e.g., corticosteroids and others) within 30 days preceding the first vaccination or planned use during the active study period
10. Traveled to ETEC-endemic areas within the last year or visit for > two months in ETEC endemic areas during the last 10 years
11. Receipt of Dukoral or other ETEC or cholera vaccines within 3 years or planned receipt of such vaccine except ETVAX during the study
12. Antibiotic therapy within two weeks prior to the vaccination
13. History of diarrhea in the 7 days prior to vaccination (defined as ≥ 3 unformed loose stools in 24 hours)
14. Any other criteria which, in the investigator's opinion, would compromise the ability of the traveler to participate in the study, the safety of the study, or the results of the study

Other parts will be blinded except unblinded person who prepares the doses.