American supplies of yellow fever vaccine are expected to run out this summer, the Centers for Disease Control and Prevention said Friday, and it released a report outlining a plan to ensure continuous access.

Yellow fever, a potentially deadly viral hemorrhagic disease transmitted by infected mosquitoes, is endemic in tropical areas of 47 countries in Africa and Central and South America, according to the World Health Organization.

There will be a depletion of the only US-licensed yellow fever vaccine available for US travelers by mid-2017, according to the CDC. Sanofi Pasteur, which produces the YF-Vax vaccine, implemented restricted ordering after manufacturing problems affected supplies. The company expects the vaccine to be available again in mid-2018.

The shortage is affecting Americans planning trips to endemic areas, according to CDC spokesman Tom Skinner.

It is “really, really important” to get the vaccine if your doctor recommends it, Skinner said, explaining that 18 countries require vaccination before they’ll allow a traveler to enter.

“If you are recommended to get it and for some reason are unable to get (it), then you should strongly consider not going,” he said.

Symptoms of yellow fever usually begin three to six days after a bite from an infected mosquito and last about three or four days. They include fever, headache, jaundice, muscle pain, nausea, vomiting and fatigue. A small proportion of patients develop severe symptoms, and nearly half of these die within 10 days.

Sanofi Pasteur will work with the CDC and the Food and Drug Administration to allow importation of an alternative yellow fever vaccine manufactured by Sanofi Pasteur France.

The alternative vaccine, Stamaril, has the same safety and efficacy profile as the US-licensed vaccine and is distributed within more than 70 countries. Sanofi Pasteur anticipates that production of YF-Vax will resume at a new facility in 2018.

The company has been working closely with the US military, which has been curtailing its own use of vaccines for those traveling in the next 30 days to areas where inoculation is recommended or required, said Cristine Schroeder, a spokeswoman for Sanofi Pasteur.

“In the interest of national security, we have engaged them in planning for future requirements, which will support continued force readiness until such time as product from the new facility is available,” Schroeder said.

The alternative vaccine will be available at a limited number of sites, based on 2016 administration numbers or an area’s need. Sanofi Pasteur identified about 250 U.S. clinics — approximately 4,000 usually distribute YF-Vax — to distribute Stamaril.

The global stockpile usually contains 6 million doses of the yellow fever vaccine, but that supply has become depleted due to recent outbreaks, according to the WHO.

The worst outbreak of yellow fever in three decades occurred in Angola beginning in December 2015. Sickness began in the capital of Luanda and soon spread to provinces. In the Democratic Republic of Congo, an outbreak of yellow fever was declared in April 2016, with some cases imported from Angola.

In response to the outbreak, the health ministries in Angola and Congo and the WHO coordinated with dozens of global partners, including Médecins sans Frontières, the International Federation of the Red Cross and UNICEF, on an emergency vaccination campaign.

After the crisis in Congo, six additional countries — Brazil, Chad, Colombia, Ghana, Peru and Uganda — reported either yellow fever outbreaks or sporadic cases not linked to the Angolan outbreak. As of April 3, yellow fever transmission continues to expand toward the Atlantic coast of Brazil in areas previously not deemed to be at risk, according to the WHO.

The vaccine was developed in the 1930s by Max Theiler, who earned a Nobel Prize for his work. A standard dose of vaccine offers lifelong protection against this disease. It’s been in use since 1937, and more than 650 million doses have been distributed in the past 75 years.