A mass polio vaccination campaign for young children will begin in the Gaza Strip on Sunday, an especially challenging effort in a war zone where hundreds of thousands of people have been displaced repeatedly, buildings and infrastructure have been destroyed and moving around is often dangerous for aid workers and civilians.

The inoculation campaign depends on brief pauses in fighting and requires coordination among Israeli authorities, humanitarian agencies, aid workers and the health ministry in Gaza, which is run by Hamas.

Israel and Hamas have both said they will abide by staggered pauses in fighting to allow aid workers to vaccinate children, and Israel has said it will not issue evacuation orders in places where vaccinations are happening.

But after nearly a year of almost nonstop fighting in the enclave, there are fears the agreement may not hold long enough to complete the two rounds of vaccinations that health authorities say are needed to prevent the spread of the disease in Gaza and beyond.

“We welcome the commitment to humanitarian pauses in specific areas, and suspension of evacuation orders for the implementation of the campaign,” Tedros Adhanom Ghebreyesus, the director general of the World Health Organization, said in a post on social media on Thursday. “But the only lasting medicine is peace. The only way to fully protect all the children of Gaza is a cease-fire.”

There are two basic, highly effective types of polio vaccine: an injection that uses dead virus, and an oral inoculation that uses a significantly weakened live strain of the virus.