The Centers for Disease Control and Prevention says that an outbreak of the virus means that travelers should get the vaccine before a trip.
The Centers for Disease Control and Prevention has advice for people thinking about traveling to most parts of Brazil: get vaccinated against yellow fever, or stay home.
Brazil is currently suffering a major outbreak of the hemorrhagic fever, and it’s striking places that don’t normally see the virus. Since the beginning of 2017, 237 people, including several tourists, have died after becoming infected with the virus.
Carried by mosquitoes, the virus has also spread to the very edge of Brazil’s largest cities, São Paulo and Rio de Janeiro, and has also infected mosquitoes on the Ilha Grande, a major tourist destination not far from Rio.
“This is the time not to go into an outbreak area unprotected,” said Dr. Martin Cetron, director of the Division of Global Migration and Quarantine at theNational Center for Emerging and Zoonotic Infectious Diseases at the C.D.C.“We recommend people defer travel to these areas of transmission if they can’t be protected by the vaccine.”
Dr. Cetron said it’s also important for doctors seeing patients who have recently returned from Brazil to think about the virus as a possible cause if they develop a fever.
The mosquitoes that carry yellow fever normally stay in the upper canopies of the forest, said Dr. Edward Ryan, the director of Global Infectious Diseases at Massachusetts General Hospital in Boston. But for unknown reasons, in early 2017, mosquitoes with the virus began turning up in unexpected spots, including coastal areas.
Officials are now worried that the Aedes aegypti mosquito, which can also carry Zika, dengue and chikungunya, will pick up the virus. Aedes aegypti live close to the ground in cities and like feasting on humans, so if they become infected, it puts the population of major cities at high risk, Dr. Ryan said.
There’s no specific treatment for yellow fever, just supportive care, Dr. Ryan said.
Early symptoms include fever, chills, headache, backache and muscle aches, according to the C.D.C.
In about 15 percent of victims, the virus reaches a fulminant form, which causes liver damage that turns the whites of the eyes yellow, hence its name, Dr. Ryan said. The liver is where clotting factors are made, so liver damage can causes blood-thinning which can trigger bleeding. Yellow fever can kill as many as 20 to 50 percent of its victims, he said.
The best protection is vaccination. “As far as I know, no one who’s gotten the yellow fever vaccine has ever gotten wild type yellow fever,” Dr. Ryan said. The World Health Organization also recommends that people protect themselves against mosquito bites by using repellents, covering exposed skin and installing screens on windows.
The vaccine carries some risks, but they are far rarer than the risks of the disease itself, Dr. Cetron said.
The C.D.C. does not recommend the vaccine for those who have an increased risk of developing a severe reaction, including infants under 6 months old, people who are immune compromised or are on immunosuppressive drugs, those who have a rare disorder of the thymus or those with an allergy to the vaccine’s ingredients.
The vaccine, which provides immunity for at least 10 years, must be given a minimum of 10 days before travel, according to the C.D.C.
Unfortunately, there’s a shortage of the vaccine right now in the United States. Sanofi Pasteur, which manufactures the vaccine, ran into production troubles in 2017 and is currently upgrading its plants in the United States.
Sanofi has been importing a limited supply of a similar vaccine called Stamaril from Europe, but because of the limits, the vaccine is only available at about 250 clinics around the country. (The C.D.C. offers a searchable list of clinics here.)
So, travelers not only have to plan their vaccination ahead of time, they may have to go through more effort to get the shot, Dr. Cetron said.
Summer will shortly turn to fall in the Southern Hemisphere, which, Dr. Cetron said, makes him somewhat optimistic that the yellow fever outbreak will die down.
“There’s clearly a seasonality to yellow fever,” he said. But the transmission risk didn’t disappear entirely last winter, so public health experts don’t really know what to expect, Dr. Cetron said. “We really need to stay vigilant.”