The Angola yellow fever epidemic that started in December is spreading to neighboring countries and has so far claimed the lives of 21 people in the Democratic Republic of Congo amid a vaccine shortage, according to a new report from the World Health Organization (WHO).

The WHO on Tuesday said that there have been a total of 151 cases of yellow fever and 21 deaths in the Democratic Republic of Congo from early January to March 22. Tests have linked the cases in Congo to last year's Angola outbreak of the mosquito-borne disease that has so far infected 490 and killed 198 Angolans since December 2015.

"Of the 151 suspected cases, 9 were confirmed by serology (IgM) at the Institute National Biomedical Research (INRB) in Kinshasa. Given possible cross-reactions with other arboviruses, IgM positive samples were sent to the Pasteur Institute of Dakar for confirmation where 4 tested positive for the infection. Of the 4 cases confirmed in Dakar, 3 were imported from Angola and were detected in the areas of Nsona-Pangu, Kimpese and Kitona in Kongo Central province (formerly, Bas-Congo), DRC. This province shares borders with Angola," the WHO said Friday in a released statement.

While 5.7 million people in the Angola's capital have now been vaccinated against yellow fever, WHO emphasizes that vaccines should first be given to individuals at highest risk due to the current worldwide vaccine shortage

"Whilst concerted efforts are being made to stop the outbreak, there is a global vaccine shortage, with the emergency stockpile completely depleted. An additional 1.5 million doses are needed to vaccinate the population at risk in Luanda province alone," WHO said in a news release. "With the spread of the outbreak to other provinces in Angola, and travelers bringing sporadic cases to other parts of Africa and China, WHO requests prioritizing vaccination of people at highest risk. WHO is in discussion with manufacturers and partners to divert shipments of vaccines for national routine immunization programs until the emergency stockpile is replenished."

Symptoms of the yellow fever virus, which is transmitted by infected mosquitos (Aedes aegypti), include headache, fever, nausea, muscle pain, fatigue and vomiting. In some cases, the virus, which is spread by the same mosquito that spreads the Zika virus, patients experiencing the second phase of the illness can have symptoms like high fever, jaundice and internal bleeding. According to WHO, at least half of patients experiencing severe symptoms and who don't receive treatment die within two weeks.

"This is the most serious outbreak of yellow fever that Angola has faced in 30 years," said Dr. Margaret Chan, WHO's director-general. "WHO is taking urgent action to support the Government to control this outbreak with a widespread vaccination campaign."

"The vaccination campaign has so far been effective. We are seeing case numbers dropping considerably, especially in Luanda," said Dr. Sergio Yactayo, expert on epidemic diseases at WHO. "We need to vaccinate the maximum number of people in Luanda and the affected provinces as possible to stop the spread of this deadly disease."

In light of the latest report, WHO experts warn that there is a "serious risk of further spread of the disease" in the Democratic Republic of Congo.

The WHO said that government officials have taken extreme precautions to control the outbreak.

"The Ministry of Health of DRC has activated the National Committee for outbreak management to respond to this event. Undertaken activities include the establishment of coordination mechanisms, social mobilization and community engagement, strengthening surveillance through the training of health workers, dissemination of case definitions, screening and sanitary controls at Points of Entry and screening of refugees' vaccination status. Other activities are targeting the vaccination of all individual traveling to Angola, strengthening vector control and case management," WHO said in a statement.

Health experts recommend that all individuals traveling to Angola should be vaccinated against yellow fever.

"WHO urges Members States especially those where the establishment of a local cycle of transmission is possible (i.e. where the competent vector is present) to strengthen the control of immunization status of travelers to all potentially endemic areas," the organization concluded.