The aim of the study was to assess and describe the booster effect of a tetravalent CYD dengue vaccine dose administered about 5 years or more after the completion of a 3-dose vaccination schedule in Singapore.

Primary Objective:

• To demonstrate the non-inferiority in terms of geometric mean of titer ratios (GMTRs) of a CYD dengue vaccine booster compared to the third CYD dengue vaccine injection in participants from CYD28 trial (participants from Group 1 only).

Secondary Objectives:

• If the primary objective of non-inferiority achieved: To demonstrate the superiority, in terms of GMTRs, of a CYD dengue vaccine booster compared to the third CYD dengue vaccine injection in participants from CYD28 trial (participants from Group 1 only).
• To describe the immune responses elicited by the CYD dengue vaccine booster or placebo injection in participants who received three doses of the CYD dengue vaccine in the CYD28 trial in all participants.
• To describe the neutralizing antibody levels of each dengue serotype Post Dose 3 (CYD28 participants) and immediately prior to booster or placebo injection in all participants.
• To describe the neutralizing antibody persistence 6 months, 1 year and 2 years post booster or placebo injection in all study participants.
• To evaluate the safety of booster vaccination with CYD dengue vaccine in all participants.

Healthy adolescents and adults received 3 doses of the tetravalent dengue vaccine 5-6 years earlier in a previous CYD dengue vaccine trial (CYD28 - NCT00880893) received either a booster injection of CYD dengue vaccine or a placebo on Day 0. They were evaluated for safety and antibody persistence of the booster injection up to 2 years post-vaccination.

Inclusion Criteria:

• Had been identified as a potential participant by the Sponsor, and was included in the list provided to the investigator (i.e., aged 9 to 45 years on the day of first injection of CYD dengue vaccine in CYD28, had received 3 doses of CYD dengue vaccine in the CYD28 trial, and had a post-dose 3 serum sample available).
• Participants with good health, based on medical history and physical examination.
• Informed consent form (ICF) had been signed and dated by participant (based on local regulations), and ICF had been signed and dated by the parent(s) or another legally acceptable representative (and by an independent witness if required by local regulations).
• Participant and parent(s)/legally acceptable representative(s) was able to attend all scheduled visits to comply with all trial procedures.

Exclusion Criteria:

• Participant who received any other dengue vaccination that was not part of CYD28.
• Participant was pregnant, or lactating, or of childbearing potential (considered of non childbearing potential, a female must be pre-menarche or post-menopausal for at least 1 year, surgically sterile, or using an effective method of contraception or abstinence from at least 4 weeks prior to vaccination until at least 4 weeks after vaccination).
• Participation at the time of study enrollment (or in the 4 weeks preceding the trial vaccination) or planned participation during the present trial period in another clinical trial investigating a vaccine, drug, medical device, or medical procedure.
• Receipt of any vaccine in the 4 weeks preceding the trial vaccination or planned receipt of any vaccine in the 4 weeks following the trial vaccination
• Receipt of immune globulins, blood or blood-derived products in the past 3 months.
• Known or suspected congenital or acquired immunodeficiency; or receipt of immunosuppressive therapy, such as anti-cancer chemotherapy or radiation therapy, within the preceding 6 months; or long-term systemic corticosteroid therapy (prednisone or equivalent for more than 2 consecutive weeks within the past 3 months).
• Known systemic hypersensitivity to any of the vaccine components, or history of a life-threatening reaction to the vaccines used in the trial or to a vaccine containing any of the same substances.
• Chronic illness that, in the opinion of the Investigator, is at a stage where it might interfere with trial conduct or completion.
• Receipt of blood or blood-derived products in the past 3 months, which might interfere with assessment of the immune response.
• Deprived of freedom by an administrative or court order, or in an emergency setting, or hospitalized involuntarily.
• Current alcohol abuse or drug addiction.
• Moderate or severe acute illness/infection (according to investigator judgment) on the day of vaccination or febrile illness (temperature >= 38.0°C). A prospective participant should not be included in the study until the condition had resolved or the febrile event had subsided.
• Identified as an Investigator or employee of the Investigator or study center with direct involvement in the proposed study, or identified as an immediate family member (i.e., parent, spouse, natural or adopted child) of the Investigator or employee with direct involvement in the proposed study.

Qualified researchers may request access to patient level data and related study documents including the clinical study report, study protocol with any amendments, blank case report form, statistical analysis plan, and dataset specifications. Patient level data will be anonymized and study documents will be redacted to protect the privacy of trial participants. Further details on Sanofi's data sharing criteria, eligible studies, and process for requesting access can be found at: https://vivli.org

An observer-blind procedure was followed for the injection of CYD dengue vaccine or placebo. Neither the blind-observer Investigator nor the participants (and/or participants' parent(s)/legally acceptable representative(s) for participants aged < 21 years) knew which product was administered.