The current outbreak of yellow fever began at the end of 2017 after several cases were confirmed in areas close to São Paulo, the CDC said.

A total of 53 people have died in Brazil after contracting the disease since July 1, the Brazilian Ministry of Health said.

But more than half of those deaths were reported in the past few weeks, according to WHO.

Two weeks ago, a Dutch traveler who had spent time near the São Paulo metropolitan area was diagnosed with yellow fever.

“The patient reported symptoms of high fever, headache, myalgia, nausea, vomiting and diarrhea,” WHO said.

The traveler had not received a yellow fever vaccination, officials said.

Limited vaccines

Yellow fever spreads to humans through the bite of an infected mosquito, and outbreaks usually occur in South America and Africa.

Symptoms of yellow fever usually begin three to six days after a bite and last about three or four days. They are similar to the flu and could become severe to include fever, headache, muscle pain, nausea, vomiting, fatigue and jaundice — hence its name yellow fever.

The yellow fever vaccine “is only effective if received at least 10 days prior to arrival in an at-risk area,” the CDC said.

Health officials say travelers should plan to get the vaccination before arriving in Brazil as the country is facing a shortage of the vaccine. However, American supplies of the vaccine can be also be limited.

Last year, the CDC announced that yellow fever vaccine supplies were expected to run out in the US after Sanofi Pasteur, which produces the YF-Vax vaccine, implemented restricted ordering following manufacturing problems.

Yellow fever in Brazil

Despite the limited supplies of the vaccine, the Brazilian Ministry of Health is launching a mass campaign Thursday to vaccinate nearly 22 million people.

Health officials will administer standard doses as well as one-fifth of the standard dose to extend the supply.

The large number of unvaccinated people, officials say, represents the biggest challenge for slowing down the disease because it many live in rural areas as well as urban centers where people live in denser communities.

A zoo in São Paulo was closed Tuesday to avoid the virus from spreading after a monkey infected with the virus was found dead, state-run news agency Agencia Brasil reported.

Yellow fever is endemic in tropical areas of 47 countries in Africa and Central and South America, the WHO said.

Brazil is the only country where new cases of yellow fever have been reported this year, the Pan American Health Organization said.