Real-World Effectiveness of Dengue Vaccine Demonstrated During 2024 Outbreak in Brazil

Recent findings from an international research team indicate that the tetravalent dengue vaccine, known as Qdenga, offers substantial protection against dengue fever during the significant outbreak in São Paulo, Brazil, in 2024. This study, published in The Lancet Infectious Diseases, marks the first real-world evidence supporting the vaccine's effectiveness following its approval.

Dengue fever is rapidly becoming one of the most prevalent mosquito-borne viral diseases globally, exacerbated by climate change and the proliferation of the Aedes mosquito. The disease leads to millions of infections annually in tropical and subtropical regions, with increasing outbreaks occurring in previously non-endemic areas, including parts of Europe. The urgent need for effective vaccines backed by robust real-world data is critical for public health.

The research was conducted in São Paulo, which was at the center of the 2024 dengue epidemic, recording tens of thousands of reported cases. Health officials prioritized vaccinating adolescents aged 10 to 14, as this demographic constitutes a significant portion of symptomatic cases and faces a high risk of hospitalization in endemic areas. This age group aligns with the World Health Organization's current vaccination recommendations, and due to their daily activities, they are particularly exposed to the mosquito vector.

An extensive vaccination campaign led to the administration of nearly 690,000 doses of Qdenga throughout the year. The research team analyzed data from more than 90,000 adolescents who exhibited acute fever and underwent laboratory testing for dengue, employing a test-negative case-control design. This approach allowed the team to compare the vaccination rates among those testing positive for dengue against those who tested negative, providing a clear estimation of the vaccine's effectiveness under real-world conditions.

The results revealed that a single dose of the vaccine reduced symptomatic dengue cases by approximately 50%. With a second dose, the reduction increased to 62%, while hospitalizations were decreased by 68%. Notably, protection commenced 14 days after the first dose, remained effective for three months, and underscored the necessity of adhering to the complete two-dose schedule for optimal and lasting protection.

The study's design facilitated a prompt and precise assessment of the vaccine's effectiveness amid an ongoing epidemic, utilizing real patient data from the healthcare system. The design is particularly effective for generating evidence during health emergencies.

Despite the study's strengths, which include its status as the first evaluation of the Qdenga vaccine under real-world conditions and its execution amidst a significant outbreak, some limitations were acknowledged. The relatively low vaccine coverage limited the ability to analyze effectiveness in detail following the second dose. Additionally, the study lacked information regarding prior dengue immunity among participants, and the outbreak's serotype circulation was confined to DENV-1 and DENV-2, limiting the evaluation of effectiveness against all four dengue virus serotypes.

These findings contribute significantly to the ongoing discourse regarding dengue vaccination strategies. They indicate that Qdenga effectively reduces both the incidence of dengue and the associated risk of hospitalization, which is essential for guiding public health decisions.

Globally, the evidence gathered from this study is particularly relevant as it emerges in the context of rising dengue cases in Europe. Although the continent is not traditionally considered endemic for dengue, the Aedes mosquito's spread due to climate change has led to local outbreaks in countries like France and Italy. For instance, Catalonia has reported over 500 imported dengue cases from travelers visiting endemic regions between 2022 and 2024, with occasional local transmissions detected.

In light of these developments, the implementation of effective vaccines such as Qdenga is crucial for managing future outbreaks and safeguarding travelers. The study underscores the importance of completing the vaccination regimen to ensure prolonged protection against both mild and severe forms of dengue fever. These insights will assist policymakers and health agencies in formulating effective vaccination strategies.