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Epidemiological Update - Yellow Fever - 7 December 2018

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Situation summary in the Americas

Between January 2017 and November 2018, 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.

Since the 20 March 2018 Epidemiological Update on Yellow Fever published by the Pan American Health Organization / World Health Organization (PAHO/WHO), Bolivia, Brazil, Colombia, French Guiana, and Peru reported new yellow fever cases; the following is a summary of the epidemiological situation in these countries and territories.

In Bolivia, between epidemiological week (EW) 1 and EW 47 of 2018, 34 suspected yellow fever cases were reported in the departments of Beni, Cochabamba, La Paz, and Santa Cruz. Of these, one case was laboratory-confirmed in a 15-year-old male with no history of vaccination and who is a resident of San Ramón Municipality in Beni Department, which is an area considered to be at-risk for yellow fever. The case had traveled to the city of Costa Marques, Brazil, prior to onset of symptoms in EW 12 of 2018. The patient was discharged from the hospital and was laboratory-confirmed by ELISA IgM and reverse transcription polymerase chain reaction (RT-PCR). In 2017, 5 confirmed cases were reported.

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 CentralWestern 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. (Figure 1 and Figure 2)

During 2018, the epidemic curve for epizootics (Figure 3) shows that viral circulation continued during the period of low transmission (June to November). Between 1 July and 8 November 2018, a total of 271 suspected yellow fever human cases were reported, of which one (fatal) case was confirmed, 120 remain under investigation, and 150 were discarded. There were 1,079 epizootics among non-human primates reported, of which 13 were confirmed for yellow fever in the states of São Paulo (8), Rio de Janeiro (3), Minas Gerais (1), and Mato Grosso (1), which are the same areas or surrounding areas as those affected during the 2016-2017 outbreak, indicating that the risk of transmission to unvaccinated populations persists.

The fatal confirmed case was reported in EW 42 of 2018 and had a probable site of infection in Caraguatatuba Municipality in São Paulo State; epizootics among non-human primates due to yellow fever were detected within this area in prior months.

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.

In Colombia, between EW 1 and EW 36 of 2018, a laboratory-confirmed case of yellow fever was reported. The case is a 21-year-old male of the Desano indigenous community from Mitú Municipality, Vaupés Department. Symptom onset was in EW 35 of 2018 and the case died in EW 36. The case was confirmed for yellow fever by ELISA IgM and immunohistochemical testing of liver samples.

The last reported case of yellow fever in this department was in 2016 in Carurú Municipality. Yellow fever vaccination coverage for children under 18 months of age is 81.2% in Vaupés Department and 89.9% in Mitú Municipality.

In French Guiana, in EW 32 of 2018, a confirmed case of yellow fever was reported who had symptom onset in EW 31. The case is a 47-year-old male with a history of stay in the forest in Roura, French Guiana. The case was hospitalized in Cayenne, French Guiana, and in EW 32 presented with fulminant hepatitis and was transferred to Paris, France, for a liver transplant.
The case was confirmed for yellow fever by PCR.

In Peru, between EW 1 and EW 45 of 2018, there were 15 cases of yellow fever reported, of which 9 were laboratory-confirmed and the remaining 6 are under investigation. This figure is higher than those reported during the same period of 2017, when 6 yellow fever cases were reported.
In 2018, the confirmed cases are from the departments with forests: Loreto, San Martin,
Ucayali, and Madre de Dios.