From 2004 to 2006, 658 patients with suspected dengue virus infections were enrolled in a clinical dengue cohort established in Recife, Pernambuco, located at the northeastern region of Brazil. A total of 2,364 blood samples were collected, and serum, plasma, and cells were cryopreserved. Among the suspected cases, 354 (54%) were confirmed as acute DENV-3 infection based on reverse transcription-polymerase chain reaction, virus isolation, and ELISA-IgM. According to WHO criteria, 29.4% of the positive acute cases were classified as dengue fever (DF) and 8.2% of the cases as dengue hemorrhagic fever (DHF), grade 1 or 2. The DHF cases represent 100% of those confirmed in Recife during the period of the study. The dengue cases that did not fulfill the definition of either DHF or DF were classified as DF complicated and accounted for 44.0% of the cases. All the acute cases were classified as either primary or secondary acute dengue virus infections. Secondary infection was predominant in patients with DF; however, there was no predominance of either primary or secondary infections in patients with DHF.

From September 2005 to March 2007, 238 individuals being vaccinated for the first time with the yellow fever (YF) -17DD vaccine were enrolled in a cohort established in Recife, Brazil. A prospective study indicated that, after immunization, anti-YF immunoglobulin M (IgM) and anti-YF IgG were present in 70.6% (IgM) and 98.3% (IgG) of the vaccinated subjects. All vaccinees developed protective immunity, which was detected by the plaque reduction neutralization test (PRNT) with a geometric mean titer of 892. Of the 238 individuals, 86.6% had IgG antibodies to dengue virus; however, the presence of anti-dengue IgG did not interfere significantly with the development of anti-YF neutralizing antibodies. In a separate retrospective study of individuals immunized with the 17DD vaccine, the PRNT values at 5 and 10 years post-vaccination remained positive but showed a significant decrease in neutralization titer (25% with PRNT titers < 100 after 5 years and 35% after 10 years).