A mass polio vaccination campaign began in Gaza on Sunday morning to immunize more than 600,000 children, the U.N. agency for Palestinians (UNRWA) and the World Health Organization said, as the besieged enclave faces a public health crisis with the reemergence of the highly contagious disease.

       In a voice message around noon local time, UNRWA spokeswoman Louise Wateridge said the pause in fighting held through the morning, although some gunfire was heard shortly after 6 a.m., when it was scheduled to begin.

       Hundreds “if not thousands” of people turned up at the health centers, mobile clinics and tents that Wateridge visited Sunday morning in central Gaza, she said. “It’s been a hugely positive response.”

       “It’s probably the most relieved I’ve seen children,” she added, attributing the feeling partly to the short-lived halt in fighting.

       “We hope it will continue over the next days, but what we really need is a cease-fire” that would enable the provision of more relief at a time when the humanitarian situation in Gaza is as “catastrophic as it’s ever been,” she said. “Civilians need this war to end.”

       Aid workers have warned throughout the war that Gaza was vulnerable to a reemergence of polio and the spread of other diseases. The fears were realized in July, when the poliovirus was detected in six sewage samples in central and southern Gaza, where many people have been displaced multiple times by Israel’s military offensive against Hamas and are living in squalor, in tents and bombed-out buildings.

       “There is no health infrastructure left in Gaza. There are no preventive activities,” said Javed Ali, the emergency team lead for the International Medical Corps (IMC) in Gaza. The enclave’s children have been suffering from a massive outbreak of hepatitis A as well as an epidemic of skin diseases, and a polio outbreak was “bound to happen,” he said.

       UNRWA chief Philippe Lazzarini wrote on X on Sunday morning that it was “a race against time” to reach Gaza’s children in the coming days and urged all parties to respect the “temporary area pauses.” The WHO said last week that the campaign to immunize 640,000 children under the age of 10 will unfold during limited “humanitarian pauses.”

       Israeli Prime Minister Benjamin Netanyahu’s office said Saturday that Israel will allow a “humanitarian corridor” for vaccination personnel and designate safe areas for administering the vaccines during certain hours. His statement emphasized that the pauses would be limited in time and geographic scope and did not amount to a broader end to the fighting across Gaza.

       COGAT, the Israeli military agency that oversees Palestinian territories, said Saturday it would facilitate a three-stage vaccination program, with the first stage scheduled to take place in central Gaza from 6 a.m. to 2 p.m. Sunday through Tuesday.

       In a post on X on Sunday morning, sharing photos of what it said was Nuseirat in central Gaza, UNRWA said the vaccination campaign began in the “middle area” of the Strip, where more than 200 members of the agency’s team were administrating doses to children across 28 sites.

       The vaccinations Sunday at IMC’s health facilities “went well,” Ali said, but some people were deterred by rain that fell in Gaza and had to return with their children to check on their tents. The group, which is trying to vaccinate 6,000 children, was able to reach 1,200 on Sunday, he said.

       Photographs posted on Telegram by Gaza’s Health Ministry showed lines of young children opening their mouths to receive doses from medical personnel in tents.

       Rik Peeperkorn, who heads WHO operations in Gaza and the West Bank, said in a video message that the vaccine rollout, beginning in central Gaza, was aiming to vaccinate 157,000 children under the age of 10. The campaign would continue for three consecutive days, and “if needed, we will add a day,” he said.

       Polio affects the nervous system and is most frequently transmitted by mouth or through feces. It is difficult to contain because most people who are infected either have no symptoms or mild flu-like symptoms, potentially spreading the virus unknowingly.

       A polio vaccine has existed since 1955, but a confluence of wartime factors has resulted in the disease’s return to the Gaza Strip. The conflict has ravaged the enclave’s water, sanitation and health infrastructure, and since the war began in October, the WHO said, Gaza’s polio vaccination rate has dropped from 99 percent to 86 percent.

       In August, Gazan health authorities confirmed that an 11-month-old boy had contracted polio — the first case of the disease detected there in 25 years. They warned that a number of children had also shown symptoms “consistent with polio.”

       “All his limbs are now paralyzed,” Nevin Abu al-Jidyan, the mother of the 11-month-old who contracted polio, told The Washington Post last week from her tent in Deir al-Balah in central Gaza. “The left side is more rigid. He is even unable to bend over. I cannot even help him sit.”

       A small number of infants in southern Gaza were administered the vaccine on Saturday as a symbolic start to the program.

       Vaccination is key to stopping the disease’s spread. Children under 5 are most vulnerable to polio, although anyone who is unvaccinated can contract it. One in 200 infections leads to irreversible paralysis, the WHO says, adding that among those paralyzed, between 5 and 10 percent die when breathing muscles become immobilized.

       Israeli forces recovered the bodies of six hostages from Gaza, including Israeli American Hersh Goldberg-Polin, the Israel Defense Forces said early Sunday. The bodies of two women and four men were recovered Saturday from an underground tunnel in Rafah, the statement said, adding that they were killed “shortly before” being found.

       Three Israeli police officers were shot dead near a checkpoint in the city of Tarqumiya in the occupied West Bank, Israeli police said Sunday. The violence caps a weekend of unrest in the West Bank. In the Jenin area, Israeli forces said they killed 15 militants in operations, while the local municipality accused Israeli authorities of bulldozing streets and cutting off water to swaths of the population.

       At least 40,738 ？？people have been killed and 94,154 injured in Gaza since the war started, according to the Gaza Health Ministry, which does not distinguish between civilians and combatants but says the majority of the dead are women and children. Israel estimates that about 1,200 people were killed in Hamas’s Oct. 7 attack, including more than 300 soldiers, and it says 339 soldiers have been killed since the start of its military operations in Gaza.

       Alon Rom and Heba Farouk Mahfouz contributed to this report.