LONDON — The ongoing yellow fever outbreak in Africa is serious but does not warrant being declared an international health emergency, the World Health Organization said Wednesday.

Since it was first identified in Angola last December, yellow fever has spread to Congo and is believed to have sickened more than 6,300 people and killed about 400, despite millions of doses of vaccine having been sent repeatedly to Angola.

The U.N. health agency convened an emergency committee of experts to consider the outbreak’s status on Wednesday and said after its conclusion that the increase of the mosquito-spread hemorrhagic fever appears to have slowed, with no new cases reported in Angola since June and none in Congo since July.

A WHO situation report published last week, however, stated that “the geographical extent of the outbreak in (Congo) continues to increase.”

The vaccine shortage has now become so acute that officials have begun diluting the vaccine by 80 percent to stretch the supply. A vaccination campaign in the Congolese capital of Kinshasa using this experimental partial dose began earlier this month aiming to immunize about 11 million people.

Another 20 million people in Angola were also vaccinated — although they got the full dose of the vaccine.

Health officials worry that the impending rainy season, set to begin in September, will complicate relief efforts and also increase mosquito populations, possibly causing a resurgence of cases.

“We are not out of the woods yet,” said Oyewale Tomori, chair of WHO’s yellow fever emergency committee.

Other experts said that the limited vaccine supply could prove catastrophic if more countries are affected. There are currently 6 million yellow fever vaccines in an emergency stockpile and WHO said there would be 20 million doses available by December.

“If we use up our vaccine supply and we end up with transmission in more cities, then all bets are off,” said Michael Osterholm, a specialist in infectious diseases at the University of Minnesota.

“It’s like having your headlights out when you’re driving on a road with hundreds of deer,” he said. “It’s only a matter of time before you crash.”

Dr. Peter Salama, WHO’s new chief of emergencies, said officials were in “uncharted waters” and applauded Congo for agreeing to use fractional vaccine doses.

An Associated Press investigation published earlier this month found that 1 million vaccines in Angola went missing and that WHO’s response has been plagued by logistical problems, like sending vaccines without syringes and losing ice packs needed to keep them potent.

Salama acknowledged there had been “operational problems” in efforts to slow the outbreak but said “the epidemiological response is now pointing in the right direction.”

Tomori said that what happened in Angola should be a lesson for health experts and that it wasn’t uncommon to see “vaccine wastage” during emergencies.


Jamey Keaten in Geneva contributed to this report.