With the recent battle against Zika in the United States still fresh in memory, health officials are keeping a cautious eye on news of a Brazilian outbreak of a historically devastating disease: yellow fever.
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For the first time in decades, the yellow fever virus is making its way from tropical areas toward major coastal cities in the southeast part of Brazil, including Sao Paulo and Rio de Janeiro, where tens of millions of people live.
In February, 234 confirmed infections and 80 confirmed deaths had been reported in Brazil and hundreds more cases – far more than typical for the region — were under investigation.
A devastating history
Yellow fever was likely imported into the Americas from Africa in the 1600s. In the 18th and 19th centuries, the virus claimed hundreds of thousands of lives – and it landed on U.S. soil: In 1793, about 10 percent of the population of Philadelphia died in a yellow fever epidemic.
It wasn’t until 1881 that the disease was found to be caused by mosquitoes, and not until 1937 that a live attenuated yellow fever vaccine was developed by virologist Max Theiler. That vaccine is still used today, and provides lifetime immunity in nearly everyone vaccinated.
In late 2015, a large urban outbreak of yellow fever occurred in Angola and the Democratic Republic of Congo, causing 961 confirmed cases and 137 deaths. That outbreak diminished the world’s emergency vaccine stockpile, and some inhabitants were given 1/5 of the standard dose to help extend the supply.
In January 2017, the Ministry of Health of Brazil distributed its routine supply of 650,000 doses of yellow fever vaccine across the country. But in response to the increasing threat, it’s now also sent an additional 4.2 million doses to Bahia, Minas Gerais, and Rio de Janeiro.
Why the outbreak now?
Few urban-dwelling Brazilians are vaccinated against yellow fever, raising concerns of a major epidemic if the virus does move into the cities. As of mid-March 2017, there had been no reports of human-to-human urban transmission caused by this vector. But this could change. Such epidemics can race through populations with little preexisting immunity and spread more widely due to national and international travel.
Yellow fever is now often considered a traveler’s disease, and those planning to visit Central and South America as well as Africa are encouraged to get the vaccine. While the World Health Organization advises that booster inoculations are no longer required after 10 years as the vaccine is considered to give lifelong immunity, the Centers for Disease Control is recommending that boosters are still advised for travelers going to areas of ‘active’ risk.” Vaccinations need to be received a minimum of 10 days before travel.
Few urban-dwelling Brazilians are vaccinated against yellow fever, raising concerns of a major epidemic if the virus does move into the cities.
Is the U.S. at risk?
As with other arboviruses, including dengue, chikungunya and Zika, urban transmission is normally sustained by the Aedes aegypti mosquito. While a yellow fever outbreak is highly unlikely in the continental United States, with its relatively low mosquito density and limited risk of exposure, there could be local transmission in warmer regions such as Puerto Rico and along the Gulf Coast, where A. aegypti are prevalent.
Travelers returning from affected regions should consider yellow fever when experiencing high fever, jaundice and other symptoms – physicians may not recognize the illness because of its rarity outside of the tropics.
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