When infectious humans returning to urbanised areas are bitten by
Aedes aegypti
mosquitoes, an urban
transmission cycle can establish and spread rapidly in human populations with low vaccination coverage.
Aedes
aegypti
is present in all Brazilian states [17]. The season for the highest mosquito vector activity lasts from
December to July in the southern part of Brazil.
Since the outbreak of urban yellow fever in Acre in 1942, Brazil has only reported sylvatic yellow fever cases [18].
Entomological investigations conducted during the 2016/2017 outbreak in some of the affected states isolated
Haemagogus
mosquitoes positive for yellow fever virus, but no evidence of transmission by
Aedes aegypti
was
found
[19].
Outbreak of yellow fever in Brazil 2016/2017
From July 2016 to June 2017, 779 human yellow fever cases and 262 deaths were reported. In addition, 1 659
epizootics in non-human primates were reported, leading to the death of at least 2 504 animals [20]. The first
laboratory-confirmed cases were reported on 19 January 2017. In March 2017, a decreasing trend in yellow fever
cases was observed in the states of Minas Gerais and Espírito Santo, while an increasing trend was observed in the
state of Rio de Janeiro [16]. During the 2016/2017 outbreak, confirmed cases were reported in the states of Minas
Gerais, Espírito Santo, São Paulo, Rio de Janeiro, Pará, Distrito Federal, Goiás, Mato Grosso and Tocantins [21].
The 2016/2017 yellow fever outbreak in Brazil was declared over in September 2017 [22].
Upsurge of yellow fever cases in Brazil – 2017/2018
Between 1 September 2017 and 14 January 2018, 34 laboratory-confirmed cases, including 20 deaths (CFR=57%),
were reported by the Brazilian national authorities [23]. Four cases occurred between September and November
2017 and 30 cases in December 2017/January 2018. The confirmed cases were reported in the states of São Paulo
(20 cases, including eleven deaths), Minas Gerais (eleven cases, including seven deaths), Rio de Janeiro (two
cases, including one death) and in the Federal District (one fatal case).
The probable sites of infection of all confirmed human cases are areas with documented cases in non-human
primates [23]. Between 1 July 2017 and 8 January 2018, 2 242 yellow fever suspicions of epizootics among non-
human primates were reported, 411 of which were confirmed [23]. Confirmed non-human primate epizootics were
reported from the states of São Paulo (360 cases), Minas Gerais (47), Rio de Janeiro (3) and Mato Grosso (1). The
upsurge of epizootics among non-human primates has been observed since mid-September in São Paulo state.
In December 2017, non-human primates infected with yellow fever were detected in urban parks in the Greater
São Paulo [24]. As a consequence, the authorities closed several parks in the area [25]. In addition, the news
media reported the deaths of four monkeys near one of the access points to the urban Tijuca Forest, in the
northern part of Rio de Janeiro city. These four animal cases are being investigated for suspicion of yellow
fever [26].
A recent phylogenetic analysis of the south-eastern Brazilian outbreak of yellow fever in 2016/2017 suggests that
the outbreak was a result of the reintroduction of a modern-lineage (genotype I) variant from Venezuela or from
some endemic region in Brazil [27]. The genomic analysis of two complete yellow fever virus sequences from two
naturally infected howler monkeys from Espírito Santo state in 2017 showed an identical virus sequence. Seven
amino acid changes were identified by the alignment of the precursor polyproteins from Brazilian and Venezuelan
yellow fever viruses detected since 1980. Further investigations would be needed to asses if these genetic
modifications are found in other locations of the 2017 and current 2018 yellow fever outbreaks in Brazil. The
potential implications of these findings on infectivity and viral fitness remain to be determined [27].
Travel data and travel-related cases
Based on 2015 data from the International Air Transport Association (IATA), 940 000 travellers from the EU/EEA
are estimated to travel to Brazil by air during the period January to May. In 2015, countries of origin included Italy
(17%), Portugal (16%), France (15%), the United Kingdom (12%), Spain (12%) and Germany (12%).
The Carnival, one of the largest international mass gatherings in Brazil, will take place from 9 to 14 February 2018.
This event brings together millions of people throughout the country and an increased number of international
travellers, including travellers from the EU/EEA, is expected.
Since January 2017, three travel-associated cases of yellow fever have been identified among unvaccinated
EU/EEA travellers: one case in January 2017 returning from Bolivia, one case in March 2017 returning from
Suriname, and one case in January 2018 returning from Brazil (i.e. the surroundings of São Paulo city) [28-31]. By
comparison, there were six travel-related cases of yellow fever among EU/EEA travellers between 1999 and 2016,
highlighting an increased circulation of yellow fever virus in the Americas in 2017 [32].