A senior World Health Organization official said Thursday that Israel has agreed to a series of daily nine-hour humanitarian pauses for the duration of a massive polio vaccination campaign in the Gaza Strip, where the first case of the disease was found in a baby earlier this month.

“The campaign will start on the first of September in central Gaza for three days,” Rik Peeperkorn, WHO’s representative for the Palestinian Territory, told reporters in a video call from Gaza. “There will be a humanitarian pause during the vaccination for three days.”

He said they had agreed to a humanitarian pause from 6 a.m. until 3 p.m. daily during each vaccination day with COGAT, the Israeli agency that coordinates access for humanitarians in Gaza.

Peeperkorn said their teams would evaluate after the first three days whether an additional one to two days more were necessary to reach enough children in central Gaza. Then the teams would move to southern Gaza and finally northern Gaza, with each area expected to take three to five days.

More than 1.2 million doses of the polio vaccine have already been delivered to Gaza and an additional 400,000 are on the way.

The virus was detected last month in environmental samples in southern and central Gaza. At least one case has been confirmed, in an 11-month-old baby — the first case in Gaza in 25 years — raising fears of a larger outbreak.

“Israel will work with WHO and other organs to support all the campaigns to bring vaccines into Gaza,” Israel’s U.N. Ambassador Danny Danon told reporters on Wednesday.

WHO says that Gaza had a high level of vaccination coverage before the escalation of hostilities in October but that the war has disrupted routine immunizations, including polio.

Peeperkorn said at least 90% of children need to be vaccinated to stop transmission of the poliovirus, which can cause irreversible paralysis in children. The virus is spread from person to person, mainly through feces, but also through contaminated food and water. Gaza’s water, sewage and sanitation systems have collapsed during the 10-month-long conflict, and living conditions are desperate.

More than 2,000 health care workers and community volunteers will be aiming to reach 640,000 children under age 10 during the campaign with a double dose of the novel oral polio vaccine type 2. The second dose will be administered four weeks after the first one.

Peeperkorn said the humanitarian pauses are vital to allow health workers and families to reach the vaccination sites.

“We need these humanitarian pauses, and that has been very clear. There is an agreement on that, so we expect that all parties will to stick to that,” he said. “Otherwise, indeed, it is actually impossible to do a proper campaign because you will definitely not reach your 90% [coverage].”

WHO, along with the U.N. children’s agency UNICEF, and UNRWA, the agency that assists Palestinian refugees, will be implementing the vaccination campaign. There will be 392 sites across Gaza where families can take their children for the polio vaccine. Nearly 300 other mobile units will be in the field to reach those who cannot access a vaccination site.

Israel has issued 16 separate evacuation orders to Gaza residents during August, displacing more than a quarter of a million Palestinians. Peeperkorn said Israel has agreed not to issue any evacuation orders on the days the vaccination campaign is in progress.

At a U.N. Security Council meeting on the humanitarian situation in Gaza, the U.S. envoy expressed support for the vaccination campaign.

“It is especially important for Israel to facilitate access for agencies carrying out the vaccination campaign, and for it to ensure periods of calm and to refrain from military operations during vaccination campaign periods,” Ambassador Robert Wood said. “We urge Israel to avoid further evacuation orders during this period.”