Brazil is in the midst of a yellow fever outbreak, with the mosquito-borne virus reaching popular tourist destinations that do not normally see the disease. Since January 2018, 10 cases of yellow fever have been confirmed in international travelers visiting Brazil, including four deaths.

As a result, travelers should plan ahead and get vaccinated against yellow fever at least 10 days before visiting areas affected by the outbreak, including Rio de Janeiro state, Espírito Santo state, São Paulo state, and certain cities in Bahia state, according to a new article co-authored by a Boston University School of Public Health (BUSPH) researcher.

The article is an early-release Morbidity and Mortality Weekly Report published by the Centers for Disease Control and Prevention (CDC). The authors noted that most people who contract yellow fever do not have symptoms, but among the 15 percent of patients who develop severe illness, the fatality rate is between 20 percent and 60 percent.

"Given the potential severity of yellow fever and a substantial risk of dying from the disease, travelers should make sure that they are vaccinated," says lead author Davidson Hamer, professor of global health at BUSPH.

However, the authors wrote that getting the vaccine requires planning ahead for travelers from the United States. The Food and Drug Administration-approved yellow fever vaccine, YF-VAX, is currently unavailable in the US because of manufacturing difficulties, and the alternative yellow fever vaccine, Stamaril, is only available through a limited number of US yellow fever vaccination clinics.

"It may take several weeks to make an appointment, and potentially substantial travel time to reach a clinic in some parts of the country," Hamer says.

Brazil's current yellow fever outbreak began in December 2016. By July 2017, cases in both humans and nonhuman primates were reported from the states of São Paulo, Minas Gerais, and Rio de Janeiro. On January 16, 2018, the World Health Organization (WHO) updated yellow fever vaccination recommendations for Brazil to include anyone traveling to or living in Espírito Santo, São Paulo, and Rio de Janeiro states, and certain cities in Bahia state, in addition to areas where vaccination had been recommended before the recent outbreak.

Eight of the ten international travelers with confirmed cases of yellow fever acquired the disease on Ilha Grande, a forested island off the Rio de Janeiro coast. ProMed reported five cases among travelers to Ilha Grande, in two travelers from Argentina and three from Chile. Two of the travelers from Chile died. GeoSentinel reported cases in a Dutch man who traveled to São Paulo state; a French woman who traveled to Minas Gerais state; and a Romanian man, a Swiss man, and a German man who all visited Ilha Grande. The men from Switzerland and Germany later died from the disease.

These were the first yellow fever cases reported by GeoSentinel, which was initiated in 1995 by the International Society of Travel Medicine with support from CDC and now consists of 70 specialized travel and tropical medicine clinical sites in 31 countries around the world. Hamer is the principal investigator for GeoSentinel.

In addition to underscoring the importance of vaccination, authors wrote that clinicians should be aware and vigilant of yellow fever signs and symptoms in patients returning from Brazil.