New real-world evidence from Brazil confirms Qdenga’s impact on dengue outcomes
The Lancet Infectious Diseases published a pivotal study that marks the first real‑world assessment of Qdenga’s effectiveness after its approval. Conducted amid Brazil’s intense 2024 dengue surge, the observational research focused on adolescents in São Paulo, the epicenter of the outbreak, to understand how vaccination translated into tangible health benefits.
Study design and setting
Researchers from the DataHealth Lab at the Instituto de Investigação Sant Pau, led by Otavio Ranzani, in collaboration with Júlio Croda of Fiocruz, analyzed data from more than 90,000 adolescents with fever who were tested for dengue. During the year, health authorities launched a vaccination campaign that administered approximately 690,000 doses of Qdenga, a tetravalent dengue vaccine. The campaign prioritized youths aged 10 to 14, a demographic that sustains a large share of symptomatic dengue and faces a notable risk of hospitalization in endemic regions.
Key findings: hospitalizations and symptomatic disease
Using a test-positive versus test-negative design to estimate real-world vaccine effectiveness, the study found meaningful protection after vaccination. After the first dose, the vaccine reduced symptomatic dengue by about 50%. Following the second dose, protection against symptomatic disease rose to roughly 62%. Most strikingly, hospitalizations declined by 68% after just the first dose, underscoring a substantial reduction in severe dengue cases even with partial immunization.
Timing, duration, and the importance of completing the schedule
Protection started 14 days after the initial dose and remained through the first three months. Beyond that window, the protective effect waned, highlighting why completing the recommended two-dose schedule is essential for longer-lasting protection. The findings reinforce the vaccine’s two-dose design and suggest that full protection is best achieved when the full course is completed as approved.
Public health implications in Brazil and beyond
Until now, evidence for Qdenga’s effectiveness rested largely on phase 3 clinical trials. This study provides the first real-world confirmation of benefit during a large-scale dengue surge. Brazil faced a record year with about 6.6 million dengue cases and thousands of deaths, a sobering backdrop that tested the capacity of its health system. The observed reduction in hospitalizations could ease hospital pressure during future outbreaks and support vaccination campaigns focused on adolescents in high-risk areas.
Context and future directions
Experts caution that, while encouraging, these results require ongoing surveillance to monitor long-term vaccine performance and any shifts in circulating dengue virus strains. The researchers advocate maintaining and expanding two-dose vaccination programs, particularly in high-transmission regions, alongside core vector-control measures and community education to reduce dengue burden. The study’s authors emphasize that real-world data like this complements randomized trials, guiding evidence-based policy and protecting vulnerable populations as dengue dynamics evolve.
