Yellow fever in children and adolescents previously immunized in Minas Gerais State, Brazil.

Background - Yellow fever (YF) is a viral hemorrhagic fever that is transmitted by arthropods. It can occur with little symptomatic manifestations to the most fulminant forms. The most effective way to avoid YF is through vaccination. There is a lack of information about the immune response of the vaccine in childhood. Methods - We described children and adolescents with YF who had been previously immunized in Minas Gerais State from July 2017 to June 2018. Results - 527 cases of YF were observed representing an incidence of 7.6/100,000 inhabitants. Only 26 patients (4.9%) were ≤ 20 years and 501 (95.1%) were > 20 years. Only 9 vaccinated patients were ≤ 20 years and 15 were > 20 years. 34.6% (9/26) of YF patients ≤ 20 years were previously vaccinated and 3% (15/501) of those > 20 years (p < 0.001). The median age at vaccination was 1 year between those ≤ 20 years and 31 years between those > 20 years (p = 0.002). Among 9 vaccinated children and adolescents ≤ 20 years, age ranged from 7 to 18 years, the most described symptoms were fever (88%), headache (77%), myalgia (77%), and abdominal pain (66%). All patients recovered from the disease and none died. Conclusion - Prior YF vaccination may be associated with mild forms of the disease in children and adolescents. YF vaccination in the first years of life may be associated with poor vaccine response and high infection rates in this group as it fail to seroconvert a significant proportion of infants.