As a yellow fever outbreak in central Africa exploded, 1 million vaccines disappeared in Angola. Thousands more vaccinations were delayed when accompanying syringes got waylaid. Ice packs to keep the shots potent went missing. And while the epidemic of the hemorrhagic fever spilled across international borders, a senior outbreak expert at the World Health Organization acknowledged their response had “lagged” for months.

This lack of oversight and mismanagement has undermined control of the outbreak in Central Africa, the worst yellow fever epidemic in decades, an Associated Press investigation has found. There is now a shortage of vaccines so severe that WHO has recommended doses be diluted by 80 percent to stretch the supply, even though there is limited evidence they will be effective in African populations.

“WE HAVE A MAJOR PROBLEM ON OUR HANDS,” UNICEF’s Robert Kezaala wrote in June to his colleagues at WHO, Doctors Without Borders and other partners.

WHO vowed after its fumbled response to the 2014 Ebola epidemic in West Africa that it would adopt a “no regrets” policy to better manage future disease outbreaks with its partners in other health organizations and national governments. But according to hundreds of pages of internal e-mails and documents obtained by AP, the U.N. health agency is facing many of the same problems that compromised its handling of Ebola, an outbreak that killed 11,000 people.

Some health officials now estimate they will be short about 22 million vaccines amid the worst yellow fever outbreak in decades. From Angola, the virus has since spread to Congo, with a total of 5,000 suspect cases and more than 450 deaths. The potential for the outbreak to be exported elsewhere remains high, since the disease is spreading largely unchecked in the Congolese capital of Kinshasa, a city of more than 10 million with strong international links.

“We could have prevented this from happening,” said Amanda McClelland, a senior emergency official at the International Federation of Red Cross and Red Crescent Societies. Unlike Ebola, there is a vaccine to stop yellow fever from spiraling out of control, and global agencies should have reacted faster, she said.

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. On February 12, WHO announced an outbreak of yellow fever in the Angolan capital of Luanda. In a bid to snuff out the disease, WHO and partners dispatched more than 6 million doses — the equivalent of the entire global emergency supply.

To ensure the vaccines reached those who needed them, WHO and partners requested a detailed explanation of their use. But Angola failed to account for more than 1 million doses, internal e-mails between WHO and its colleagues show. “How can this happen in a situation of (vaccine) shortage?” WHO’s Sergio Yactayo wrote to his colleagues at UNICEF, Doctors Without Borders, IFRC and WHO. He speculated that the shots had probably been taken by local politicians, the military or resold on the private market.

“People are very afraid,” says Dr. Ernest Mumakubwidi, who treats patients out of a tiny cement-block neighborhood clinic in Kinshasa. “They won’t be able to vaccinate everyone — that is clear.”

But Angolan officials disputed that any vaccines were missing at all.

As the disease tore through Angola and Congo in the spring, U.N. officials made a series of avoidable mistakes that stalled control efforts, including sending vaccines without syringes.

In early February, a senior U.N. official alerted other colleagues that cases were nearly tripling every week.

“WE MIGHT (FACE) A DISASTER FOR THIS COUNTRY AND THE REGION,” warned Paolo Balladelli, the U.N. coordinator in Angola.

With competing vaccine requests from worried countries, experts began to consider diluting the shot.

Dr. Bruce Aylward, who directed WHO’s Ebola response, acknowledged that more changes are still needed to enable WHO to respond quicker to complex emergencies, even though he said the agency has made substantial reforms in the past two years. “It would be a mistake to think that WHO is now ready,” he said. “I think a lot of things are better but I don’t think we’re there yet.”