During the first four weeks of 2018, a rapid increase in the number of confirmed human cases of yellow fever was observed in the states of Rio de Janeiro, São Paulo, and Minas Gerais. From 1 July 2017 through 16 February 2018, 464 confirmed human cases of yellow fever have been reported in Brazil, including 154 deaths. Confirmed cases were reported in São Paulo (181 cases, including 53 deaths), Minas Gerais (225 cases, including 76 deaths), and Rio de Janeiro (57 cases, including 24 deaths) states and in the Federal District (1 fatal case). Unlike the previous seasonal period, the current seasonal period has been characterized by more cases reported in São Paulo and Rio de Janeiro states along with the occurrence of cases in areas near large cities.
In Sao Paulo State, 57% of the confirmed cases were likely to have been infected in Mairiporã Municipality (a rural area located 15km north of São Paulo Municipality). In Rio de Janeiro State, 45% of the confirmed cases were among residents of Valença and Teresópolis municipalities; the latter is located 96km from Rio de Janeiro City. Likely places where infections occurred for all of the confirmed cases correspond to areas with documented epizootics in non-human primates. In Minas Gerais, 47% of the confirmed cases reside in municipalities located south and southeast of the city of Belo Horizonte, where no human cases were detected during the outbreak in the 2016/2017 seasonal period.
Two laboratory-confirmed cases of yellow fever (in France and in the Netherlands) have been reported among unvaccinated travellers who stayed in Brazil, in municipalities considered at-risk for yellow fever as described in the international travel health recommendations on the basis of the virus circulation, the distribution of yellow fever vectors and animal reservoirs. In addition, two laboratory-confirmed cases were reported in Argentinian citizens, likely to have been infected in Ilha Grande, municipality of Angra do Reis, state of Rio de Janeiro, and Isla Bella, state of São Paulo Brazil (both municipalities are known to be at risk for yellow fever). Furthermore, as of 26 February, three confirmed cases of yellow fever (two fatal) have been reported in Chilean citizens likely to also have been infected in Ilha Grande, municipality of Angra do Reis, Rio de Janeiro state, Brazil.
Epizootics have been reported continuously along the yellow fever pre-season and seasonal period, indicating sustained virus circulation in favourable ecosystem, which is expanding to areas previously not considered at risk for yellow fever. From 1 July 2017 through 6 February 2018, 3812 epizootics among non-human primates have been reported of which 517 have been laboratory-confirmed, 1157 remain under investigation, 1397 were classified as indeterminate, and 741 were ruled out. Epizootics have been reported in 22 of the 27 federal areas in the country. Epizootics with confirmed yellow fever circulation among non-human primates were reported in six states (Espírito Santo, Mato Grosso, Minas Gerais, Rio de Janeiro, São Paulo, and Tocantins). São Paulo has accounted for 42% of the total epizootics.
Additionally, the Brazil Ministry of Health reported the detection of yellow fever virus in Aedes albopictus mosquitoes captured in rural areas of two municipalities (Ituêta and Alvarenga) in Minas Gerais State in 2017 as part of an investigation conducted by the Ministry of Health of Brazil and the Evandro Chagas Institute. The significance of this finding requires further investigation, particularly to confirm vector competence to transmit yellow fever.