Dengue and the risk of urban yellow fever reintroduction in Sao Paulo State, Brazil

Objective To propose a mathematical method for the estimation of the Basic Reproduction Number, R-0, of urban yellow fever in a dengue-infested area. Methods The method is based on the assumption that, as the same vector (Aedes aegypti) causes both infections, all the quantities related to the mosquito, estimated from the initial phase of dengue epidemic, could be applied to yellow fever dynamics. It is demonstrated that R-0 for yellow fever is, on average, 43% lower than that for dengue. This difference is due to the longer dengue viremia and its shorter extrinsic incubation period. Results In this study the analysis was expanded to the epidemiological situation of dengue in Sao Paulo in the year 2001. The total number of dengue cases increased from 3,582 in 2000 to 51,348 in 2001. It was then calculated R-0 for yellow fever for every city which have shown R-0 of dengue greater than 1. It was also estimated the total number of unprotected people living in highly risky areas for urban yellow fever. Conclusions Currently there is a great number of non-vaccinated people living in Aedes aegypti infested area in the state of Sao Paulo.