A large-scale campaign to vaccinate hundreds of thousands of children against polio in the Gaza Strip launched Sunday. The three-day campaign hopes to prevent an outbreak of the virus – which was recently reported in the territory for the first time in 25 years.

Palestinian health authorities and United Nations agencies plan to vaccinate children in central Gaza until Wednesday before moving on to the more devastated northern and southern parts of the strip.

The campaign began with a small number of vaccinations on Saturday and aims to reach about 640,000 children.

The World Health Organization said Thursday that Israel had agreed to limited pauses in the ongoing fighting against Hamas to facilitate the campaign. However, in a statement late Saturday, the office of Israeli Prime Minister Benjamin Netanyahu said that “reports of a general cease-fire for the purpose of giving polio vaccines in Gaza are false.”

Instead, Netanyahu’s office said, Israel “will allow a humanitarian corridor only” to allow for the passage of polio vaccines into Gaza.

“Demarcated areas will be established that will be safe for administering the vaccines for a few hours,” Netanyahu’s office said. “Israel considers it important to prevent the outbreak of polio in the Gaza Strip, including with the aim of preventing the spread of epidemics in the entire region.”

The vaccination campaign faces a host of challenges, from ongoing fighting to devastated roads and hospitals shut down by the war. Around 90% of Gaza’s population of 2.3 million people have been displaced within the besieged territory, with hundreds of thousands crammed into squalid tent camps.

The campaign comes after 10-month-old Abdel-Rahman Abu El-Jedian was partially paralyzed by a mutated strain of the virus that vaccinated people shed in their waste, scientists say. The baby boy was not vaccinated because he was born just before Oct. 7, when Hamas militants attacked Israel and Israel launched a retaliatory offensive in Gaza.

The boy’s mother, Neveen Abu El Jidyan, told CBS News on Tuesday she has been able to do very little for her son due to the dire conditions in the camp for displaced Palestinians where they’re living.

“We haven’t given him any treatments. We live in a tent and there is no medication,” El Jidyan, 35, told CBS News.

The World Health Organization says the presence of a paralysis case indicates there could be hundreds more who have been infected but aren’t showing symptoms.

Most people who have polio do not experience symptoms and those who do usually recover in a week or so. But there is no cure, and when polio causes paralysis, it is usually permanent. If the paralysis affects breathing muscles, the disease can be fatal.

The vaccinations will be administered at roughly 160 sites across the territory, including medical centers and schools. Children under 10 will receive two drops of oral polio vaccine in two rounds, the second to be administered four weeks after the first.

Israel allowed around 1.3 million doses to be brought into the territory last month, which are now being held in refrigerated storage in a warehouse in Deir al-Balah. Another shipment of 400,000 doses is set to be delivered to Gaza soon.

Health officials have expressed alarm about disease outbreaks as uncollected garbage has piled up and the bombing of critical infrastructure has sent putrid water flowing through the streets. Widespread hunger has left people even more vulnerable to illness.

“We escaped death with our children, and fled from place to place for the sake of our children, and now we have these diseases,” said Wafaa Obaid, who brought her three children to the Al-Aqsa Martyrs Hospital in Deir al-Balah to get the vaccinations.

Ammar Ammar, a spokesperson for the U.N. children’s agency, said it hopes both parties adhere to a temporary truce in designated areas to enable families to reach health facilities.

“This is a first step,” he told The Associated Press. “But there is no alternative to a cease-fire because it’s not only polio that threatens children in Gaza, but also other factors, including malnutrition and the inhuman conditions they are living in.”