One of the largest yellow fever outbreaks in decades could soon spread globally, aid group Save the Children has warned as the World Health Organisation (WHO) launched one of the largest emergency vaccination campaigns ever attempted in Africa.
The massive vaccination campaign begins this week in Congo and Angola, with the aim of vaccinating more than 14 million people in over 8,000 locations to stem the disease's spread.
More than 400 people have already died in this outbreak, the WHO says.
But supplies of the vaccine are limited and the more than 18 million vaccines that have been sent to the continent are far short of the 40 million doses some experts think are needed to contain the outbreak, according to a recent Associated Press investigation.
The WHO says it must now use one-fifth of the standard vaccine dose, which lasts about a year.
"Protecting as many people as possible is at the heart of this strategy. With a limited supply, we need to use these vaccines very carefully," said William Perea, the WHO's co-ordinator for the Control of Epidemic Diseases Unit.
Save The Children, which is sending a rapid reaction unit to support vaccinations in Congo, warned the epidemic could soon spread to the Americas, Asia and Europe and other cities in Africa.
"There is no known cure for yellow fever and it could go global," said Heather Kerr, Save the Children's country director for Congo.
Yellow fever is not highly contagious and is easily prevented with vaccines.
The mostly mosquito-spread virus was largely wiped out from the West following the development of two vaccines in the 1930s, but still sparks epidemics in Africa and Latin America.
The virus is transmitted by the same species of mosquito that spreads Zika, dengue and chikungunya.
Once infected, people often fall ill with fever and muscle pain but many recover after several days.
Others reach the more toxic phases, with possible bleeding from the eyes, ears and nose, organ failure and the jaundice which originally gave the disease its name, according to Save the Children.
The WHO in February announced the outbreak of yellow fever in Angola's capital, Luanda.
From Angola, the virus spread to Congo, with a total of 5,000 suspected cases.
Only two million people in Congo's capital, Kinshasa, have been vaccinated so far, the WHO said. It plans to vaccinate at least eight million people there.