Yellow fever is a mosquito-borne disease that often causes no symptoms. Some people may experience fever and nausea, and in about 15 percent of cases the infection can turn severe and lead to jaundice and multiple organ failure.
Since early 2017, the virus has been spreading in several of Brazil's eastern states, "including areas where yellow fever was not traditionally considered to be a risk," said the report by the US Centers for Disease Control and Prevention.
Affected areas include the states of Rio de Janeiro, Minas Gerais and Sao Paulo, including areas close to the city of Sao Paulo.
"Be protected or don't go," said Marty Cetron, director of the CDC division of global migration and quarantine.
He warned that the intensity of transmission was "highly unusual" and the risk to travelers is "somewhat unprecedented."
He urged potential travelers to be vigilant, even if they run into difficulties obtaining a vaccine due to low availability.
"Because of the challenge of getting the vaccine we don't want them to hesitate and think, 'Oh I will just go without it. It can't hurt, my friends have gone before and nothing has happened,'" he told reporters on a conference call.
The vaccine should be given 10 days before travel.
A total of 10 international visitors from Europe and South America who were not vaccinated prior to traveling to Brazil have been infected with yellow fever already this year.
Officials warned of a specific hotspot on Ilha Grande, a forested island off the coast of Rio de Janeiro, where eight international visitors have contracted yellow fever. Four of them died -- a Swiss national, a German and two travelers from Chile.
"This suggests Ilha Grande is an exceptionally hot spot for yellow fever virus transmission," said Cetron.
Brazil's health officials have confirmed 920 cases of yellow fever, including more than 300 deaths since July 2017 far higher than the previous year which saw 196 deaths.
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