The WHO held an emergency meeting in Geneva, Switzerland, on Thursday to review the latest information about the outbreak that began in December. The committee said the outbreak does not constitute a public health emergency.
"It does require intensified control measures but is not a public health emergency of international concern," said professor Oyewale Tomori, chairman of the WHO yellow fever emergency committee.
More than 2,400 people have been infected since December, and 300 people have died. "This is a devastating disease with a high mortality rate and rapid spread," said Dr. Bruce Aylward, executive director of outbreaks and health emergencies for the WHO.
It's the worst outbreak of yellow fever in the southern African country in three decades.
The outbreak began in the capital of Luanda but has now spread to at least 14 of the 18 provinces in the country.
The Democratic Republic of Congo has nearly 50 cases, most of them linked to Angola. Uganda is experiencing local transmission of the virus and has reported nearly 60 cases. Kenya and China also have imported cases. Health officials are concerned that the outbreak is spreading to areas where people are not vaccinated, including neighboring countries Namibia and Zambia.
Yellow fever is a mosquito-borne disease transmitted by two types of mosquitoes. One is Aedes aegypti, the same type of mosquito that transmits Zika virus and dengue fever.
Yellow fever, a viral hemorrhagic illness, is transmitted to people when a mosquito becomes infected from biting an infected monkey and then bites a person.
Symptoms usually begin three to six days after a bite from an infected mosquito and last about three or four days. They include fever, muscle pain, nausea, vomiting and loss of appetite.
Yellow fever can be confused as dengue fever, malaria or another illness.
Fifteen percent of infected individuals experience a second phase of illness within 24 hours of when they first become sick. This can include jaundice and bleeding from the eyes, nose, mouth, stomach and, eventually, bloody vomit and feces.
There is no treatment for yellow fever. Those who are sick are given supportive care, which can include fluids, pain management and monitoring. Fifty percent of patients who experience this second phase of illness die within 10 to 14 days.
A vaccine can protect against the virus. Health officials launched a mass vaccination program in February to try to contain this outbreak. "They've built a large vaccinated population in affected areas," Aylward said.
In March, the WHO said the emergency stockpile of the vaccine, which usually has 6 million doses, was exhausted. On Thursday, Aylward said there are 5 million doses of the vaccine globally, and the WHO expects to have 7 million by the end of the month. This should be sufficient to vaccinate the affected population. He said the concern is if there are additional urban outbreaks.
There is a long-term plan to expand routine vaccination to "build up a robust barrier to this disease," Aylward said. This is in addition to continued mass vaccination efforts including targeting travelers, especially migrant workers, to and from Angola and the Democratic Republic of Congo.
The emergency committee also recommended increased surveillance and mosquito-control measures.
Travelers to the country -- or to any country where yellow fever circulates, including 33 other African countries -- are advised to be vaccinated before traveling. Many of those countries require proof of vaccination upon arrival.
In February, the WHO declared
a public health emergency for the Zika virus and related health problems. In doing so, it set in motion international coordination of tracking, research and response to the outbreak.