This randomized, controlled, double blind trial aims at assessing the safety and immunogenicity profiles of the co-administered Live Attenuated Dengue and Chikungunya vaccines comparatively to the isolated administration, in the adult population aged 18 to 59 years without prior exposure to either arbovirus.

A Phase 3b multicenter, randomized, controlled, double-blind clinical trial was designed to evaluate the Immunogenicity (non-inferiority), 28 days post-immunization, for each Dengue and Chikungunya serotypes, as well as the safety, 21 days post-immunization, of the co-administration of the live attenuated Dengue and Chikungunya vaccines compared to the separate administration in adults aged 18 to 59 years without prior exposure to either arbovirus.

Inclusion Criteria:

1. Male or female adults aged 18 to 59 years at the time of vaccination.
2. Signed informed consent by the participant or their legal representatives.
3. Ability to understand, based on the investigator's assessment, and agree to comply with all study procedures, including blood collection.

Exclusion Criteria:

1. Participation in another clinical trial within 28 days prior to screening or planned participation in another clinical study during the trial period.
2. Pre-existing unstable health condition. An unstable health condition is defined as a disease requiring a change in treatment or hospitalization due to disease worsening within 90 days prior to screening.
3. Vaccination within 14 days prior to screening with any inactivated vaccine or within 28 days prior to screening with any live attenuated vaccine, or planned vaccination with any vaccine up to 28 days after study vaccination.
4. Known hypersensitivity to any component of the vaccines.
5. Thrombocytopenia or bleeding disorders that contraindicate intramuscular vaccination or venipuncture for blood collection.
6. Receipt of immunoglobulins, blood, or blood products within 180 days prior to screening.
7. Altered immunocompetence (immunosuppression, immunodeficiency, or immunocompromise) primary or secondary due to: Clinical conditions (including but not limited to renal failure, liver failure with cirrhosis, heart failure class III or IV according to the New York Heart Association, HIV infection, and asplenia).
8. Use of systemic corticosteroids (oral, intravenous, or intramuscular) at a dose equivalent to ≥20 mg/day of prednisone for more than 14 days or a cumulative dose greater than 280 mg within the last 90 days prior to screening. Topical, inhaled, and intranasal corticosteroids are allowed. Intermittent use (a single dose within the last 30 days prior to screening) of intra-articular corticosteroids is also allowed.
9. Receipt of antineoplastic agents, immunosuppressants, immunomodulators, or radiotherapy within the last 180 days prior to screening.
10. Malignancy at the time of screening or a history of malignancy with <5 years of disease-free status at screening (except for basal cell carcinoma of the skin and localized prostate cancer under active surveillance).
11. Abuse of alcohol and illicit drugs within the past 12 months before screening that may compromise study compliance, at the investigator's discretion.
12. Being part of the study team, having a first-degree relative (parents, children, in-laws, stepchildren, sons-in-law, or daughters-in-law) or living in the same household as a study team member.
13. Any other clinical condition that, in the investigator's opinion, may interfere with the study results or pose an additional risk to the participant due to study inclusion.
14. Prior exposure to dengue and chikungunya viruses, i.e., non-reactive IgM and IgG as screened by specific ELISA for both viruses. In case of doubt or indeterminate ELISA results, at least two consecutive samples will be collected. If doubt persists after two test collections, the participant will be excluded.
15. For female participants of childbearing potential: Pregnancy (confirmed by a positive β-hCG test), breastfeeding, or intention to engage in sexual activity with reproductive potential without using a contraceptive method for 90 days following vaccination.
16. Previous receipt of any dengue or chikungunya vaccine.

To ensure blinding of both participants and the study team performing safety assessments, vaccine preparation will be conducted by unblinded team members in a separate room, following the four-eyes principle (two individuals supervising the process). The syringes for different vaccination regimens will be pre-masked before being delivered to the study sites, ensuring the blinding process is maintained.