What to expect from the 2017 yellow fever outbreak in Brazil?

Dear Editor In the early beginning of January 2017, the Brazilian Ministry of Health notified the World Health Organization (WHO) about an increasing number of confirmed cases of yellow fever in the State of Minas Gerais, Southeastern Brazil. Since then, an apparent expansion of the transmission area and an increasing number of suspected and confirmed cases have been reported. Regarding this epidemiological context, there are emerging questions that could explain this apparently unexpected scenario and what to expect about the recent yellow fever outbreak in Brazil. In Brazil, between 1980 and 2004, 662 cases of sylvatic yellow fever (YF), most of them sporadic, were confirmed with a case fatality rate of 51% 1. In the outbreaks that occurred in the country between 2000 and 2009, an increased viral circulation was observed towards the Eastern and Southern areas of the country, and the virus was identified in areas that were silent for several decades 2. The epidemiological series of confirmed human cases of sylvatic yellow fever in Brazil present an irregular epidemiological pattern of occurrence marked by endemic transmission, interspersed by epizootic and/ or epidemic periods when the virus mainly affects populations with low vaccination coverage usually in the Central-Western, Southeast and South regions of Brazil, as occurred in the last epidemic period between 2008 and 2009 3-5. This time, the epicenter is in the State of Minas Gerais where yellow fever was quite common until the 1930's. After that, vector control programs associated with vaccination mass campaigns, as observed in other Brazilian States, interrupted the urban transmission cycle. Since then no cases/ outbreaks of the sylvatic cycle were observed until 2002, when an outbreak occurred in a region close to the now affected area in 2017 3. Unlike the previous one, which was restricted to two or three municipalities, this time there are suspect cases in more than 90 municipalities, and an apparent expansion of the transmission areas 6. The recent 2000's outbreaks in Brazil have followed a similar pattern: simultaneous small outbreaks popping out in quite extensive areas. In 2008-2009 there were cases in the States of São Paulo and Rio Grande do Sul (around 1,000 km apart), and eventually reached Asunción, in Paraguay, where " urban " transmission was established (though the spread was contained, with 20 some cases) 7,8. Unfortunately, the epidemiological data on this outbreak are no longer available at the Ministry of Health official sources …