Amid rising concern over a deadly outbreak of yellow fever spreading from Angola, the World Health Organization has urged travelers to the African country to heed its warnings and get vaccinated.

At least 258 people have been killed and there have been around 1,975 suspected cases of the mosquito-borne disease since an epidemic erupted in December 2015. It has already grown to become the worst outbreak in decades.

Yellow fever is transmitted by the same mosquitoes that spread the Zika and dengue viruses, although it is a far more serious disease with death rates as high as 75 percent in severe cases requiring admission to hospital.

Angola’s outbreak has already spread to other countries in Africa, including the Democratic Republic of Congo (DRC), and at least 11 cases of yellow fever have been imported into China in people traveling from Angola.

“Cases of yellow fever linked to this outbreak have been detected in other countries of Africa and Asia,” WHO director-general Margaret Chan said in a statement. “We are particularly concerned that large urban areas are at risk and we strongly urge all travelers to Angola to ensure they are vaccinated against yellow fever and carry a valid certificate.”

The WHO’s regional office for Africa said last week that yellow fever in people who travelled from Angola has been reported in China (11 cases), DRC (10 cases with 1 in Kinshasa) and Kenya (2 cases). It said three further cases have been reported in Uganda, but these patients had no history of travel to Angola.

The WHO “is working with neighboring countries such as the DRC, Namibia and Zambia to bolster cross-border surveillance with Angola and information sharing to prevent and reduce the spread of infection,” it said.

Jack Woodall, a yellow fever expert who formerly worked for the WHO and the U.S. Centers of Disease Control and Prevention, said he is worried the outbreak could spread rapidly along a major trucking route from DRC to Uganda’s capital Kampala. “Surveillance of this trade route should be intensified and vaccination of people living along it should be top priority,” he said.

A spokesman for the WHO in Geneva said a nationwide vaccination program that began in Angola in February has reached 7 million people. But experts are warning the world’s stocks of yellow fever vaccines are under sever pressure form the outbreak, with some calling for a radical switch in strategy to use a tenth of the normal dose and aim to cover more people.