Over 20 million children will be vaccinated in Syria and neighboring nations in an attempt to halt the spread of polio, United Nations agencies said Friday.

A week ago, the World Health Organization (WHO) and UN Children’s Fund (UNICEF) announced that the region was a polio emergency, as the infectious disease can spread rapidly among children. Reuters referred to it the “largest-ever consolidated immunization response in the Middle East.” The polio outbreak is the first in Syria since 1999.

“The polio outbreak in Syria is not just a tragedy for children, it is an urgent alarm – and a crucial opportunity to reach all under-immunized children wherever they are,” Peter Crowley, UNICEF’s Chief of Polio, said in a statement. He also added that Syria’s polio outbreak should be a “stark reminder” to all countries that polio is still a threat to children anywhere.

Sona Bari, a spokeswoman for the WHO, said at a briefing in Geneva that it would take six months of vaccination campaigns to reach 22 million children. Although Syria eradicated polio 14 years ago, vaccinations have stalled in the past three years due to the civil war. “It is going to need quite an intense period of activity to raise the immunity in a region that has really been ravaged both by conflict in some parts, but also by large population movements,” she said. The Syrian outbreak has already paralyzed 10 children and may be a risk for hundreds of thousands of other children in the region to become paralyzed. Polio, or poliomyelitis, is a virus that mainly affects children under 5 years old by attacking the nervous central system. One in 200 infections leads to permanent paralysis.

According to the WHO, polio has almost been completely wiped out, but still exists in certain pockets of the world. Polio cases have decreased by 99 percent since 1988 — from more than 350,000 cases to 223 reported cases in 2012. Polio workers are often targets of attack, especially in Pakistan, where vaccination workers have been bombed or killed during campaigns. Pakistan, Afghanistan, and Nigeria are the only three countries where polio has remained endemic, due in part by militant resistance to vaccination in those areas.

Earlier this week, Syrian officials promised to vaccinate children and claimed the polio virus was brought in by rebels from Pakistan. Health officials and scientists have also warned this week that the outbreak of polio in Syria could lead to the virus entering Europe and other countries. There have been some 322 reported cases of polio this year in comparison to the 223 total last year, according to the Global Polio Eradication Initiative.