Since the beginning of 2017, six countries and territories of the Region of the Americas reported confirmed cases of yellow fever: Bolivia, Brazil, Colombia, Ecuador, French Guiana, and Peru. The number of human cases and epizootics reported in the Region of the Americas during this period is the highest in decades.
In Brazil, yellow fever has a seasonal pattern, based on historical analysis of human cases and epizootics due to yellow fever in the past 20 years, with two different periods: higher transmission occurring between December and May (seasonal period) and lower or interrupted transmission occurring between June and November.
In the last 3 years, there has been an expansion of the historical area of transmission of yellow fever in the country. In 2014-2015, transmission that initially occurred in the North Region spread from east to south, and in 2015-2016, it mainly affected states in the Central Western Region. From the end of 2016 until June 2017, there was a large outbreak that mainly affected the states in the Southeast Region, with 778 human cases including 262 deaths and 1,655 epizootics due to yellow fever. A second wave of transmission was reported the following year during the same period (end of 2017 until June 2018) which also affected the Southeast Region though with higher transmission in the state of São Paulo; there were 1,376 human cases reported, including 483 deaths, and 864 epizootics. The reported cases in both periods, 2016-2017 and 2017-2018, exceeded what was reported in the last 50 years.
Aedes albopictus naturally infected with yellow fever virus has been reported in Brazil, during the peak of the 2017-2018 outbreak, mainly as a result of overlap in the distribution of both primate and mosquito populations in areas with active transmission. However, its involvement in the transmission of the virus to humans has not been established.
Given the size of the outbreaks that Brazil has faced during the last two years, the country has had to modify its vaccination policies for yellow fever, increasing the number of areas with recommended vaccination from 3,526 municipalities in 2010 to 4,469 municipalities in 2018, and to the entire country starting in 2019. In addition, the vaccination scheme shifted from two doses in children under 5 years and a booster after 5 years of age, to a single dose scheme from 9 months of age. The use of fractional doses to respond to outbreaks, especially in large cities, was also adopted. This strategy was implemented in São Paulo, Rio de Janeiro, and Bahia.
As of EW 39 of 2018, preliminary results of the mass vaccination campaign against yellow fever indicate that 13.3 million people in São Paulo, 6.5 million in Rio de Janeiro, and 1.85 million in Bahia states were vaccinated, which represents vaccination coverage of 53.6%, 55.6% and 55.0%, respectively.
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