Yellow fever (YF) vaccination does not increase dengue severity: A retrospective study based on 11,448 dengue notifications in a YF and dengue endemic region.

BACKGROUND: We study the association between prior yellow fever immunization and clinical outcomes of dengue infections in individuals of varying sexes and ages. Serological interactions between dengue virus and other flaviviruses could drive antibody dependent enhancement, which is associated with disease severity in dengue infections. This effect may influence disease severity in individuals subsequently affected by related flaviviruses, such as dengue. We compare the severity of dengue episodes between patients vaccinated and non-vaccinated against yellow fever. METHODS: We evaluated the severity of 11,448 lab-confirmed dengue cases reported in São José do Rio Preto, Brazil, in 7370 YF vaccinated patients compared to 4043 unvaccinated patients. We regressed dengue severity against YF vaccine status and a number of demographic, clinical, and laboratory variables as controls. We also evaluated the association between YF vaccination status and the clinical and laboratory symptoms of dengue patients. RESULTS: We did not find any evidence of increased risk for severe dengue in patients vaccinated against YF (odds ratio = 1.00; 95% confidence interval = 0.87-1.14). Most of the variables analyzed did not have a statistically significant association with YF vaccination status. CONCLUSIONS: We found no evidence that YF vaccination in dengue-endemic areas increases the risk of severe dengue fever.

Comparison between the traditional (1997) and revised (2009) WHO classifications of dengue disease: a retrospective study of 30 670 patients.

OBJECTIVE: To compare WHO's traditional (1997) and revised (2009) guidelines for dengue classification, using a large sample of patients of all ages with varying clinical conditions from a dengue-endemic area in Brazil. METHODS: We compared 30 670 laboratory-confirmed dengue cases (1998-2012) using both WHO's dengue classification guidelines. Stereotype ordinal logistic regressions were used to analyse the association between patients' demographics and signs and symptoms related to dengue infection severity, as defined in the 1997 and 2009 guidelines. We then compared the degree of agreement in dengue classification of both guidelines. RESULTS: Dengue signs and symptoms in patients were poorly correlated to disease severity as defined by both guidelines (Cramer's V test <0.2). Hypotensive shock was the exception for both classifications, presenting dependence (Z = 56.42; P < 0.001, and Z = 55.24; P < 0.001) and high agreement (Cramers's V = 1; P < 0.001, and Cramers's V = 0.97; P < 0.001) for WHO 1997 and 2009, respectively. Last, we also found substantial agreement in disease classification between both guidelines (Kendall tau-b = 0.79; P < 0.001), although 2009 guidelines were more sensitive in the detection of severe cases. CONCLUSIONS: We hope our results will inform the debate about dengue classification guidelines, particularly concerning clinical value, study comparability, and ways in which future guidelines can support the clinical management of dengue. Our results suggest that caution should be taken when using WHO guidelines to assess dengue severity to improve clinical management of patients.