A major outbreak of yellow fever in Angola, as well as two smaller flare-ups in Congo and Uganda, are now practically under control. But the World Health Organization (WHO) has urged countries to be vigilant in the event the disease erupts elsewhere as a global epidemic.

Transmitted by the same mosquito carrying the Zika virus, yellow fever is quite difficult to identify early on and yet spreads quickly. Worries involve some countries’ high vector densities as well as the bugs’ thriving in abnormal El Niño weather of the last year.

Dr. Sylvie Briand, director of the Department of Pandemic and Epidemic Diseases of the WHO, said that yellow fever cases in Angola have reached 2,267 suspected and 696 confirmed cases as of May 9. The death toll is at 292.

She added that 90 percent of the 41 confirmed cases in Congo were from Angola, while Uganda has seven confirmed illnesses in rural areas.

“What we hope is that El Niño will not be faster than we are,” Briand announced at a Geneva press briefing. “We are concerned for other countries that have high densities of mosquitoes.”

Yellow fever has no particular treatment and proves deadly in up to 50 percent of patients who develop severe jaundice in the late stages of the condition. A vaccine provides 90 percent protection against it after 10 days, which increases to 99 percent after three months.

“One injection, lifelong protection. It’s the best vaccine I know,” Briand said, adding that before immunization, yellow fever outbreaks delayed the Panama Canal construction and drove Napoleon to give up his planned conquests in North America.

Luanda, the capital of Angola, is now nearly 100 percent vaccinated, but this has consumed the world’s entire emergency stockpile. Briand warned that the vaccine supply may become stretched if new outbreaks strike in the months to come.

One risk of transmission is through Angola’s foreign oil workers, who may take the disease home with them. Portugal and China, for instance, have strong links to the African nation.

Commenting in JAMA, Dr. Daniel Lucey and Lawrence Gostin of Georgetown University urged greater WHO involvement in the stewardship of yellow fever vaccine and preventing the disease’s further spread globally. They called it likely the “last opportunity” to make sure that the agency is empowered and well-equipped in emergency preparedness and response.

Four vaccine manufacturers currently produce 60 to 70 million doses every year, but only 6 million are allotted for emergency use. Luanda consumed 7.2 million doses.

The WHO continues to coordinate with governments at risk of yellow fever coming from southern Africa. Each year, around half a million people travel back and forth from Portugal to Angola every year, with the former’s government actively pursuing vector control and enforcing traveler vaccination.

Nigeria, which suffered yellow fever deaths by the thousands in an outbreak back in the 1980s, is still deemed at risk.

For Briand, it is time to take mandatory traveler vaccination “very seriously.” She noted, however, that current vaccine supply is insufficient to address needs, and there’s also the matter of tracking population movement and whether vaccine coverage is enough to block transmissions.

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