The phase 3 DEN-03-IB study, double-blind and placebo-controlled, investigated the efficacy and safety of the single-dose tetravalent dengue vaccine Butantan over 5 years in participants aged 2–59 years in Brazil.[1][2] The vaccine was effective against DENV-2 and safe regardless of baseline serostatus.[1] The overall vaccine effectiveness reached nearly 75%, with 91% effectiveness against severe dengue and 100% effectiveness against dengue hospitalizations.[1][2] In the 12–59 age group, overall efficacy was 74.7%, 91.6% against severe dengue and dengue with severe symptoms, and 100% against hospitalizations.[2] The study involved more than 16,000 volunteers in 14 Brazilian states and ran from 2016 to 2024.[1][2] The vaccine was safe in people with and without previous infection, with mostly mild to moderate reactions such as pain and redness at the injection site, headache, and fatigue.[2] Serious adverse events associated with the vaccine were rare and all patients recovered.[2]