Citing “extraordinary” circumstances, the World Health Organization (WHO) today declared the international spread of wild poliovirus a public health emergency of international concern. It is just the second time that WHO has declared an emergency, saying the recent export of polio from three nations to adjoining areas could threaten global efforts to eradicate the disease.

Under the new recommendations, which are not legally binding but carry tremendous weight, the three countries deemed to pose the greatest risk of further spread—Pakistan, Cameroon, and Syria—must now ensure that all residents and long-term visitors have proof of recent polio vaccination before leaving the country. WHO is also encouraging the seven other nations known to have polio infections to implement similar measures.

This is just the second time that such an emergency has been declared under the International Health Regulations adopted in 2005; the first was during the H1N1 swine flu pandemic in April 2009.

Stopping and preventing further spread of polio requires a coordinated international response, the Emergency Committee of the International Health Regulations concluded unanimously today. If the situation is unchecked, it could result in the failure to globally eradicate polio, “one of the world’s most serious vaccine preventable diseases.”

After a banner year in 2012, when polio cases globally were at an all-time low, the situation deteriorated sharply in 2013, when 417 cases were reported. And so far this year, three of the 10 nations known to have polio have exported the virus outside their borders: Pakistan to Afghanistan, Cameroon to Equatorial Guinea, and Syria to Iraq. This spread is particularly alarming, according to the committee, because it has occurred during the “low season,” January through April, when polio transmission usually ebbs; the high season begins in May or June.

The consequences of further spread are great, the committee warned, because of the large number of currently polio-free but “conflict-torn and fragile States,” where routine immunization is low and where mounting an effective response would be especially challenging. Bruce Aylward, who heads the Global Polio Eradication Initiative and announced the decision at a press conference this morning, has said he is especially worried about spread from Cameroon to the Central African Republic.

Starting immediately, the committee recommended, Pakistan, Cameroon, and Syria must ensure that all residents have proof of polio vaccination, with either the oral polio vaccine or the inactivated polio vaccine, between 4 weeks to 12 months before leaving the country. For urgent travel, vaccination should be ensured at least by the time of departure. In a nod to the difficulty of ensuring compliance when so many refugees are fleeing the civil war in Syria, Aylward acknowledged that countries surrounding Syria already require that all registered refugees be vaccinated. Many refugees are not registered, however. Similarly, implementing these recommendations at the porous border between Afghanistan and Pakistan could be impossible.

The committee also recommends slightly less stringent measures for those countries that are currently infected but have not exported polio this year: Afghanistan, Equatorial Guinea, Ethiopia, Iraq, Israel, Somalia, and Nigeria. There, WHO says governments should “encourage” as opposed to “ensure” that all residents and long-term visitors are vaccinated before leaving the countries.

These measures should continue until at least 6 months after any reported case of wild poliovirus within the 10 countries. The Emergency Committee will meet again in 3 months to reassess the situation and decide whether tougher measures are needed.

*Correction, 5 May, 12:35 p.m.: The month that WHO declared the first public health emergency under the International Health Regulations adopted in 2005 has been corrected.