A first group of children received polio vaccinations in besieged Gaza on Saturday ahead of the official start of a much-anticipated United Nations-led campaign on Sunday.

Children at the Nasser Hospital in Khan Younis, central Gaza, were among the first to receive the vital vaccines on Saturday, according to footage shown on American television.UN official said that success of the vaccination operation depends on the “humanitarian pause” in the bombardments of the enclave agreed by Israel.

After beginning in central Gaza, vaccines are set to be administered in southern Gaza and then in northern Gaza.

The campaign, which involves two doses, aims to cover more than 640,000 children under 10.

Michael Ryan, WHO deputy director-general, told the UN Security Council this week that 1.26 million doses of the oral vaccine had been delivered in Gaza, with another 400,000 still to arrive

The Ramallah-based Palestinian health ministry said earlier this month that tests in Jordan had confirmed polio in an unvaccinated 10-month-old baby from central Gaza.

Intense heat in Gaza could worsen health crisis for Palestinians, WHO warns

Poliovirus is highly infectious, and most often spread through sewage and contaminated water – an increasingly common problem in Gaza as the Israel-Hamas war drags on.

The disease mainly affects children under the age of five. It can cause deformities and paralysis and is potentially fatal.

Israel’s military campaign has killed at least 40,691 people in Gaza, according to the territory’s health ministry. The UN rights office says most of the dead are women and children.

Relentless Israeli bombardment has also caused a major humanitarian crisis and devastated the health system.