In 2013, the World Health Organization (WHO) withdrew the recommendation of booster doses of the yellow fever (YF) vaccine, based on retrospective and cross-sectional studies that showed lifelong protective immunity from a single dose. Currently, yellow fever transmission in Brazil occurs only through the jungle (sylvatic) cycle. However, the high vector density of Aedes aegypti, which transmits other orthoflaviviruses, is a concern for the expansion of YF in other regions of the country. We conducted a cohort study to assess the duration of vaccine-induced immunity in adults and children residing in an area without wild-type YF virus circulation but with a high incidence of other orthoflaviviruses. This phase IV, uncontrolled cohort study was conducted in three municipalities in northeastern Brazil. The 17DD strain vaccine was administered to children aged 9 months to 4 years and adults aged 18 to 50 years. Blood samples for antibody titration were collected before vaccination, 30-45 days after, and one year after vaccination. The following assays were used: µFRNT for yellow fever and dengue; PRNT for Zika; and chemiluminescence for Zika (IgG and IgM) and dengue (IgG). YF seroconversion rates 30-45 days post-vaccination increased with age, reaching 99% in adults, while 10% of infants remained without detectable antibodies. Seropositivity for dengue neutralizing antibodies was inversely associated with YF antibody titers 30-45 days post-vaccination. Previous Zika infection showed no substantial association with YF antibody titers post-vaccination. One year after vaccination, a considerable reduction in YF antibody titers was observed across all age groups, regardless of prior dengue or Zika infections. Our data support the Brazilian National Immunization Program's recommendation for a booster dose of the vaccine at 4 years of age. Current recommendations assuming lifelong protection from a single dose of the YF vaccine do not appear to provide sufficient protection in high-risk areas, particularly where infants are the primary target for vaccination.