The study aims to retrospectively describe the demographic characteristics, comorbidity profiles, vaccination history and vaccination patterns of all individuals who have received at least one dose of TAK-003 (Qdenga) dengue vaccine at all VNVC centers between September 26, 2024 and September 30, 2025. The study will utilize existing data from the VNVC immunization database. No new data collection or direct participant contact will occur.

This is a retrospective, observational, multicenter study using existing vaccination records from the Vietnam Vaccine Joint Stock Company (VNVC) immunization database. The study will analyze data from all individuals who received at least one dose of the TAK-003 (Qdenga) dengue vaccine between September 26, 2024 and September 30, 2025. No new data collection or participant contact will be performed.

The study will be conducted using data collected from all VNVC vaccination centers nationwide. VNVC is the most extensive private vaccination provider in Vietnam, with nearly 250 centers distributed across more than 50 provinces and cities. VNVC customers can be considered broadly representative of the Vietnamese population, reflecting a wide range of demographic, ethnic, and socioeconomic groups and levels.

Data will be obtained from the VNVC centralized immunization database (Web-based), which systematically records all vaccination activities across VNVC centers nationwide. The database includes detailed demographic, clinical, and vaccination-related information collected during each client's visit.

The VNVC data management team will extract relevant data fields for individuals who received at least one dose of TAK-003 during the study period. All data will be deidentified by VNVC before being shared with the research team in Tam Anh Research Institute (TAMRI). Personally identifiable information (e.g., name, phone numbers, address, or ID numbers) will be removed before transfer.

The de-identified dataset will then be securely transferred to TAMRI. TAMRI's data management team will be responsible for data cleaning (identifying missing, inconsistent or duplicate entries), basic data validation and preparing dataset for statistical analysis. The finalized dataset will then be analyzed using standard statistical software.

All analyses will be descriptive in nature, designed to characterize the demographic and clinical profile of individuals who received the TAK-003 vaccine and to estimate vaccination coverage and uptake patterns within the VNVC network. No inferential hypothesis testing will be performed.

Inclusion Criteria:

• All individuals who were vaccinated with at least one dose of TAK-003 at all VNVC vaccination centers in Vietnam from 26/09/2024 to 30/09/2025

Exclusion Criteria:

• Cases in which TAK-003 vaccination occurred outside the defined study period (from September 26, 2024, to September 30, 2025).
• Records that are duplicates, missing or contain inconsistent information on key demographic or clinical characteristics (age, sex, TAK-003 dose [first or second]) as such data are essential for analysis

All individuals who were vaccinated with at least one dose of TAK-003 at all VNVC vaccination centers in Vietnam from 26/09/2024 to 30/09/2025