World Health Organization officials say Israel has given United Nations agencies the go-ahead to start inoculating hundreds of thousands of young Palestinian children in the Gaza Strip against polio.

A mass polio vaccination campaign to immunize more than 640,000 children under the age 10 against the crippling disease is set to begin Sunday, according to Dr. Rik Peeperkorn, WHO representative for the Occupied Palestinian Territory.

Speaking from Deir al Balah in central Gaza, Peeperkorn told journalists in Geneva Friday that Israel has agreed to a series of so-called humanitarian pauses.

“We want to emphasize without humanitarian pauses, the campaign’s delivery, which is already being implemented under incredibly complex and challenging circumstances, will not be possible,” Peeperkorn said.

“So, we welcome the preliminary commitment to these area-specific humanitarian pauses during the campaign,” he said. “We call on all parties to pause the fighting to allow children and families to safely access health facilities and for community outreach workers to get the children who cannot access health facilities for polio vaccinations.”

The campaign, which is being run by Gaza’s Ministry of Health, the WHO, UNICEF, UNRWA and partners, will be split into three, three-day phases, starting with central Gaza, followed by south Gaza, and lastly north Gaza.

The campaign will involve two doses of novel oral polio vaccine and will be given to the children in two rounds four weeks apart. The WHO says 1.26 million doses of vaccine and 500 vaccine carriers have been delivered to Gaza, noting that an additional 400,000 doses will arrive in Gaza soon.

Tedros Adhanom Ghebreyesus, WHO director-general, told journalists during an online briefing Friday that staff have prepared for the logistically mammoth undertaking.

“We have trained more than 2,100 workers and community outreach workers to provide vaccinations and inform communities about the campaign,” he said. “Our aim is to reach at least 90% vaccination coverage during each round of the campaign to stop the current outbreak and prevent the international spread of polio.”

Decisions to vaccinate Gaza’s children were made just days after a 10-month-old baby became partially paralyzed with polio, the first case of the infectious disease in the Palestinian enclave in 25 years.

Polio mainly affects young children and is spread through contact with feces or from contaminated food or water. Health officials agree that the overcrowded, unsanitary conditions under which people in Gaza are forced to live create an environment in which the poliovirus thrives.

“Due to insecurity, damage to road and infrastructure, and constant population displacement, conducting the campaign for just three days in each area is unlikely to be sufficient to achieve adequate vaccination coverage,” Tedros said.

“Vaccination coverage will be monitored throughout the campaign,” he said, “and it has been agreed that vaccination will be extended by one day wherever necessary.”

Peeperkorn said that every day a technical team will analyze the campaign to see how it is progressing and whether adjustments need to be made.

“In an ideal situation, you would go house to house. Unfortunately, that is not feasible in Gaza,” he said.

“If after three days, we see that coverage is too low, we will ask for additional days,” he added, explaining that this has been approved by the Israeli COGAT authority, which coordinates government activities in the Palestinian territories.

“We have this agreement, and I expect all the parties to stick to this agreement,” he said. “We are here. The teams are ready to move forward, and we expect that this will happen in the best possible way.”