The WHO just declared a public health emergency related to Zika virus

Zika, a virus that barely bothered humans until last year, has been moving. First, it made its way from Africa to a series of tiny islands in Micronesia. Then it bounced through the Pacific Ocean to Easter Island, off the coast of Chile. From there, it was on to Brazil. Now Zika has infected people in more than 20 countries in Central and South America and the Caribbean.

The mosquito-borne virus doesn't seem to harm most of its victims. But there's increasing evidence that it can cause serious damage to the brains of fetuses and, in rare instances, devastating neurological problems in adults.

The potential harm to fetuses, though still unproven, was enough to prompt the World Health Organization to declare Zika a "public health emergency of international concern," or PHEIC, today.

The WHO doesn't issue these declarations very often, and they carry a lot of political heft — as well as economic repercussions for the countries involved. Here's what you need to know.

1) What is a public health emergency of international concern?

Countries with active Zika transmission. (CDC)

Formally, a PHEIC — pronounced "fake" — is defined as "an extraordinary event which is determined to constitute a public health risk to other States through the international spread of disease and to potentially require a coordinated international response."

In reality, it's a political tool that the WHO uses to sound the alarm about a serious disease that has caught the world off guard and put people's health in danger. It's meant to draw countries' immediate attention — to galvanize resources and stop the disease from spreading further across borders.

2) Who decides to declare a public health emergency?

Margaret Chan, director general of the World Health Organization, announcing the Middle East respiratory syndrome (MERS) is not a public health emergency. (Chung Sung-Jun/Getty Images)

When there's a looming disease crisis on the horizon, the World Health Organization convenes a panel of experts under the International Health Regulations (which are a set of laws that govern global responses to pandemics involving 196 member countries).

These experts — dubbed an "emergency committee" — meet and assess the risk posed by a disease outbreak and then advise the WHO director general (Dr. Margaret Chan) about whether to declare a PHEIC. Chan then decides whether to take action. So far, she's always followed the committee's advice, as she did today. (For a list of experts who deliberated on the Zika threat, see here.)

3) How often does the WHO declare these emergencies?

Not very often. The WHO has only declared a public health emergency three times since the International Health Regulations were enacted in 2007.

The first time was in 2009, with the outbreak of the H1N1 swine flu pandemic. The second time was in May 2014, when polio seemed to surge again, threatening the eradication effort. The third time, in August 2014, came as the Ebola outbreak in West Africa was growing out of control. Notably, the emergency committee decided not to declare the MERS virus a PHEIC.

So the Zika declaration will be the fourth PHEIC in history. It's also the first time the WHO has issued such a warning over a mosquito-borne disease.

4) Why are these declarations so rare?

These decisions are not taken lightly. First, the PHEIC is a political tool used to focus the world's attention on a health crisis. Using this declaration too often would weaken its significance.

Second, one of the key considerations in declaring a PHEIC is whether the disease threat is dire enough for countries to be forced into enacting travel and trade restrictions. These can be devastating to local economies.

Even if the WHO only warns people to limit or delay travel to affected regions (instead of outright travel restrictions), health emergency declarations are often associated with economic losses.

Because of the Ebola crisis, the World Bank Group estimated that the West African countries at the center of the outbreak — Guinea, Liberia, and Sierra Leone — lost out on about $1.6 billion in economic growth in 2015. Similarly, the South American countries hit by swine flu suffered economic losses ranging from 0.5 to 1.5 percent of their GDPs.

5) Zika doesn't even cause symptoms in most people. So why did the WHO declare a PHEIC?

In an announcement about the declaration, the WHO director general explained that it's really Zika's link with microcephaly, a condition that causes babies' brains and heads to stop growing, that prompted the PHEIC. The emergency committee considered evidence of the link in outbreaks in Brazil and French Polynesia, and thought that was enough to call for "a coordinated international response to make sure we get to the bottom of this," Chan said. (You can read about microcephaly in our explainer.)

So it's not the Zika virus itself that prompted the PHEIC, per se, but it's the virus' potential to harm newborns — even though this link isn't yet fully established or understood.

6) Beyond economic repercussions, do these declarations have any impact?

Yes and no.

Naming a PHEIC doesn't mean the countries battling an outbreak will suddenly be flooded with funds and support from the WHO. In fact, one key problem is that many WHO member states don't have the resources available to take the measures needed to wipe out a disease threat, and a public health emergency doesn't change that.

We saw this very clearly with Ebola: The three worst-affected countries also happened to be some of the poorest on the planet. Despite agreeing, under the International Health Regulations, to have robust disease surveillance systems in place and systems for sharing information about outbreaks, they mostly didn't — and that's part of the reason it took months to even identify the fact that an Ebola outbreak was happening in West Africa. (You can read more about that in an article I co-authored with Vox contributor Steven Hoffman.)

But as we saw with Ebola, the emergency declaration escalated media attention and global focus on the disease. It helped wake up the world to the gravity of West Africa's outbreak. It helped bring resources from wealthier countries into West Africa, and slowly the global effort got the outbreak under control.

So even though there's no money necessarily attached to a PHEIC, it's a powerful political tool that can get a lot of other resources moving. And that's what critics hope will happen with this Zika outbreak.

7) What will this health emergency mean for Latin America?

A health worker fumigates to prevent dengue, chikungunya, and Zika virus, at El Angel cemetery, in Lima, Peru, Wednesday, January 20, 2016. (AP Photo/Martin Mejia)

The WHO isn't recommending any restrictions on travel and trade. This puts them at odds with the CDC, which advised pregnant women to avoid travel to Zika-infected countries. "This has a political overtone," said Lawrence Gostin, a Georgetown University global expert. The lack of travel advice may have something to do with the upcoming Olympics in Rio, he added.

For now, the WHO is alerting countries to the threat of Zika and advising health officials to coordinate a public health response. This involves taking measures to strengthen surveillance of Zika cases and associated birth and neurological complications, controlling mosquito populations that carry the virus, and expediting the development of a vaccine as well as improved diagnostic tests for the virus.

A PHEIC also means the WHO will closely track and monitor the disease and issue regular media updates about the outbreak. It'll draw global attention to the disease, and probably encourage governments and health agencies in and out of Latin America to research Zika and send resources to places that need them in order to help stop the virus from traveling further. Brazil, a medium-size economy, may not need much help with Zika — but poorer countries in Latin America, like El Salvador, probably do.

Critics like Gostin will be waiting to see what concrete steps follow. "What really matters is what kind of actions happen, what strategy and funding we will see on the ground."

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