The World Health Organization is asking for expert help charting its response to the alarming yellow fever outbreak in Angola.

It has convened an “emergency committee,” a panel of outside experts who will offer counsel to Director-General Dr. Margaret Chan.

The group holds its first meeting Thursday. Here’s what you need to know.

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What does an emergency committee do?

The committee’s first job is to advise the WHO on whether it thinks the event under study is a PHEIC — a public health emergency of international concern. (WHO insiders pronounce that acronym “fake.”)

The committee can also offer advice on whether the WHO should recommend that any affected country take specific steps to try to prevent the problem from spreading. During the recent Ebola crisis, the WHO recommended certain countries in West Africa do fever screening of departing travelers at airports.

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It can also suggest the WHO make recommendations to unaffected countries. Those often take the form of instructions not to block trade and travel from affected countries, because doing so generally just makes matters worse for the worst-hit nations and could deter them from being transparent about their problems.

Important fact: The WHO has no power to force anyone to follow its suggestions. During the West African Ebola epidemic, a number of countries stopped issuing visas to people from Liberia, Sierra Leone, and Guinea. The WHO asked them repeatedly to stop. The requests were ignored.

What’s the specific issue WHO wants help with now?

Yellow fever, a nasty disease spread by the same kinds of mosquitoes that transmit Zika, dengue, and chikungunya viruses. It’s currently rife in Angola, especially in the capital Luanda, where a large outbreak was first detected in December.

For experts who know what yellow fever can do, urban outbreaks are the stuff of nightmares. They are hard to control — large populations plus lots of breeding grounds for mosquitoes equals lots of transmission. And big cities have international airports, which can send the virus winging off to other vulnerable parts of the world in the blood streams of infected people.

Here's what you should know about the yellow fever outbreak happening in Angola right now. Hyacinth Empinado/STAT

Worryingly, the Angolan outbreak has already spread. Kenya has reported two cases in people who were in Angola; and 11 Chinese workers infected in Angola have travelled home while sick. Yellow fever has never taken off in Asia, but if it were to take root there, it would be a disaster. The region has the right mosquitoes, teeming megacities, and is a critical engine of the global economy.

The Democratic Republic of Congo, which neighbors Angola, has also reported 39 cases, including some in people who did not travel (which means they caught it at home). The capital, Kinshasa, has reported two cases. An outbreak there — a city of more than 10 million people — would also be a very bad thing.

Will the emergency committee declare the outbreak a PHEIC?

Since this system of emergency committees was established about a decade ago there have been five previous panels convened to address public health crises. In 4 of the 5 instances — the H1N1 flu pandemic, the Ebola outbreak, Zika, and setbacks in the polio eradication effort — a PHEIC was declared. The committee monitoring the fifth, Middle East respiratory syndrome, has met 10 times, but on each occasion said the disease bears watching but is not an international emergency.

Past experience can’t necessarily predict the future, but the WHO isn’t in the habit of convening emergency committees unless a situation is quite concerning, so a declaration of a PHEIC would not come as a surprise.

Duane Gubler from the Duke-NUS Medical School in Singapore — who is not on the emergency committee but who advises the WHO on yellow fever and Zika — thinks the declaration should be made. He has likened the situation to “a time bomb.”

Either way, an emergency committee could give the WHO some political cover as it tries to address some problems it is having with the yellow fever response.

People traveling to places where there’s a risk of yellow fever are supposed to be vaccinated. It’s spelled out in the International Health Regulations, a treaty to which all WHO member countries are signatories. But it’s known that in some countries, fake vaccination certificates are cheaper than the vaccine. (There have been reports of them being sold along the borders of some African countries.) And the requirement may not always be enforced. China has tens of thousands of workers in Angola, and if the 11 who went home sick with yellow fever are any indication, they aren’t all vaccinated.

That constitutes an international risk. Recommendations issued during a PHEIC carry more weight. And if the WHO stresses the need for countries to follow yellow fever vaccination rules, they are more likely to do so.

Why not just vaccinate the problem away?

The yellow fever vaccine is one of the most effective ones in the world — near-perfect protection after a single dose. But there’s a hitch. It’s not used very much because there haven’t been big yellow fever outbreaks in several decades, and cash-strapped governments in some vulnerable countries stopped seeing yellow fever as a significant threat.

Vaccine manufacturers don’t make what they can’t sell. So, currently, there are only four producers in the world, and they make between 35 million and 40 million doses a year, according to Dr. Thomas Monath, a yellow fever vaccine expert who leads infectious disease research and operations for NewLink Genetics, which is developing an Ebola vaccine.

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Some yellow fever vaccine is used in routine immunization programs in countries where the virus is a threat. Some is sold to people who are traveling to countries that require proof of yellow fever immunization to enter. And the WHO keeps some in emergency stores.

Earlier this year, the WHO emptied the stockpile and sent 6 million doses of the yellow fever vaccine to Angola. That has since been replenished; a WHO spokesperson said Wednesday the agency currently has around 6.3 million doses. But if the entire population of Kinshasa needs to be vaccinated, the emergency stockpile won’t do the job.

What can be done?

Experts like Monath have been arguing that the WHO should recommend using a smaller dose of vaccine to stretch supplies. Studies in Brazil have shown one-fifth of a dose is protective. The emergency committee may advise the WHO go down that route.

But changing the amount might hit some hurdles from national regulatory agencies. The emergency committee could advise the WHO to invoke its powers during a public health crisis to greenlight the vaccine-sparing change. Georgetown University’s Dr. Daniel Lucey and Lawrence Gostin called for such an action last week in the Journal of the American Medical Association.

Correction: A previous version of this story did not include a complete list of countries whose residents were denied visas during the Ebola crisis.