Long-Term Safety and Antibody Persistence of TDV and the Impact of a Booster Dose

The purpose of this study is to describe antibody persistence for each of the 4 dengue serotypes for up to 63 months after the first vaccination in the primary vaccination series for participants from parent trial DEN-315 (NCT03341637) (Mexico) and for up to 36 months after the first vaccination in the primary vaccination series for participants from parent trial DEN-304 (NCT03423173) (United States \[US\]) and to describe the impact of a tetravalent dengue vaccine (TDV) booster dose vs placebo on antibody response for each of the 4 dengue serotypes at 1 month and 6 months post administration of the TDV booster or placebo.

The vaccine tested in this study is Takeda's Dengue Tetravalent Vaccine (Live, Attenuated) (TDV). This study will look at the long-term antibody persistence and safety of Takeda's TDV in healthy adolescents and adults and will assess the impact of a booster dose.

The study has enrolled 365 healthy participants. Participants who previously received TDV in two parent trials (DEN-304 \[NCT03423173\] and DEN-315 \[NCT03341637\]), will be invited to participate in this follow-up trial. Participants will be assessed for antibody persistence and safety from Baseline (Month 0) through Month 15 (for participants from parent trial DEN-304 \[US\]) or Month 42 (for participants from parent trial DEN-315 \[Mexico\]). At Month 15 (for participants from parent trial DEN-304 \[US\]) or at Month 42 (for participants from parent trial DEN-315 \[Mexico\]), eligible participants will be randomized in 1:1 ratio to one of two trial groups to receive TDV or placebo:

A. Group 1- TDV 0.5 mL subcutaneous (SC) injection at Month 15 for participants from parent trial DEN-304 (US) or at Month 42 for participants from parent trial DEN-315 (Mexico\]).

B. Group 2- Takeda's tetravalent dengue placebo (dummy SC injection - this is a liquid that looks like the study drug but has no active ingredient), 0.5 mL, subcutaneous injection at Month 15 for participants from parent trial DEN-304 (US) or at Month 42 for participants from parent trial DEN-315 (Mexico).

This multi-centre trial will be conducted in US and Mexico. The overall time to participate in this study is up to 21 months for parent trial DEN-304 (US) and up to 48 months for parent trial DEN-315 (Mexico). Participants from parent trial DEN-304 (US) and participants from parent trial DEN-315 (Mexico) will come for 5 visits to the clinic which includes a final visit (Visit 5) 6 months after the booster dose for a follow-up assessment.

Eligibility Criteria:
Inclusion Criteria:

1. Male or female participants (irrespective of serostatus at baseline in the parent trials (DEN-304 \[(NCT03423173)\] and DEN-315 \[NCT03341637\]) who received at least one dose of Takeda's tetravalent dengue vaccine candidate (TDV) in the parent trials and have data from at least one blood draw post-vaccination.

Exclusion Criteria:

1. Participants with a prolonged period of habitation (≥1 year) in a dengue endemic area within the 2 years prior to Visit 1 Day 1 (Month 0).
2. Previous and planned vaccination (during the trial conduct), against any flavivirus including dengue (other than Takeda's TDV), yellow fever (YF), Japanese encephalitis (JE) viruses or tick-borne encephalitis.

Booster Exclusion Criteria:

1. Participants for whom baseline serostatus is not defined in the parent trials (DEN-304 \[(NCT03423173)\] and DEN-315 \[NCT03341637\]).
2. Participants with any history of progressive or severe neurologic disorder, seizure disorder or neuro-inflammatory disease (eg, Guillain-Barré syndrome).
3. Known or suspected impairment/alteration of immune function, including:

   1. Chronic use of oral steroids (equivalent to 20 mg/day prednisone ≥12 weeks/≥2 mg/kg body weight/day prednisone ≥2 weeks) within 60 days prior to Month 42 for participants from parent trial DEN-315 (Mexico)/ Month 15 for participants from parent trial DEN-304 (US); use of inhaled, intranasal, or topical corticosteroids is allowed.
   2. Receipt of parenteral steroids (equivalent to 20 mg/day prednisone ≥12 weeks/≥2 mg/kg body weight/day prednisone ≥2 weeks) within 60 days prior to Month 42 for participants from parent trial DEN-315 (Mexico)/ Month 15 for participants from parent trial DEN-304 (US).
   3. Administration of immunoglobulins and/or any blood products within the 3 months prior to administration of the TDV booster or placebo at Month 42 for participants from parent trial DEN-315 (Mexico)/ Month 15 for participants from parent trial DEN-304 (US); consider whether applicable as an exclusion criterion or criterion for delay.
   4. Receipt of immunostimulants within 60 days prior to Month 42 for participants from parent trial DEN-315 (Mexico)/ Month 15 for participants from parent trial DEN-304 (US).
   5. Immunosuppressive therapy such as anti-cancer chemotherapy or radiation therapy within 6 months prior to Month 42 for participants from parent trial DEN-315 (Mexico) / Month 15 for participants from parent trial DEN-304 (US).
   6. Known human immunodeficiency virus (HIV) infection or HIV-related disease.
   7. Hepatitis C virus infection.
   8. Genetic immunodeficiency.
4. Abnormalities of splenic or thymic function.
5. Participants with a known bleeding diathesis, or any condition that may be associated with a prolonged bleeding time.
6. Participants with history of current or previous infection with a flavivirus such as dengue, Zika, YF, JE, West Nile fever, tick-borne encephalitis or Murray Valley encephalitis and participants with a prolonged period of habitation (≥1 year) in a dengue endemic area during trial conduct.