Although it's "highly unlikely" there will be yellow fever outbreaks in the continental United States, the increase in domestic cases in Brazil and frequency of worldwide travel could lead to travel-related cases occurring in warmer parts of the United States, in the Gulf Coast states, and outbreaks in Puerto Rico and other US territories, where Zika has spread explosively, primarily through infected Aedes aegypti mosquitoes.
And in the current Brazilian outbreak, there have so far been 234 cases and 80 confirmed deaths, according to health authorities.
"This proximity raises concern that, for the first time in decades, urban transmission of yellow fever will occur in Brazil", the authors wrote. Health experts in the New England Journal of Medicine say that the likelihood of a yellow fever outbreak in the U.S.is low. The PAHO also said that the fatality rate is around 33% for confirmed cases and 11% for suspected cases. Yellow fever in the country has now claimed the lives of 220 people and the number could still rise over time.
To prevent the Brazil outbreak from spreading, "early identification of cases and rapid implementation of public health management and prevention strategies, such as mosquito control and appropriate vaccination, are critical", Fauci and Paules concluded.
Although there is a highly effective vaccine for yellow fever, it is not routinely given in Brazil's major urban centers, they said.
"In an era of frequent worldwide travel, any marked increase in domestic cases in Brazil raises the possibility of travel related cases and local transmission in regions where yellow fever is not endemic", they write.
The yellow fever outbreak comes as the Zika virus continues to affect countries throughout the Americas.
So far confirmed cases have been reported in three of Brazil's states: Minas Gerais, Espirito Santo, and Sao Paulo.
The recent yellow fever outbreak in Angola and the Democratic Republic of the Congo underscored the risk of global spread and exhausted the world's emergency vaccine stockpile, the two noted, adding that early identification, public health preparedness, and prevention strategies are critical. These people are infected by jungle-dwelling mosquito species and may travel to the city and spread the virus.
The outbreak of the fever in the United States is considered unlikely, but with Zika, the infections acquired through travel could easily get into the Aedes aegypti population, especially in warmer regions. The initial symptoms develop three to six days after infection and can include: high temperature, vomiting and muscle pain.
Even if Brazil's yellow fever outbreaks go to ground in the jungle, Fauci said the scenario now unfolding "reminds us of the things we need" to see potentially deadly epidemics coming - and respond to head them off.
Researchers for the US Centers for Disease Control and Prevention compared birth outcomes prior to the Zika epidemic's outbreak in 2015 - using data from three US state registries - to those of mothers infected by the virus in 2016.