Palestinian medics administer polio vaccines to children in Gaza City. AFP

"This is a massive success amidst a tragic daily reality of life across the Gaza Strip," Tedros Ghebreyesus said on X, formerly Twitter.

Disease has spread with Gaza lying in ruins and the majority of its 2.4 million residents forced to flee their homes due to Israel's military assault — often taking refuge in cramped and unsanitary conditions.

After the first confirmed polio case in 25 years, a massive vaccination effort began on Sept.1 targeting at least 90 per cent of children under 10, aided by localised "humanitarian pauses" in fighting.

The first phase of the campaign, which first brought vaccines to children in central Gaza, then the south, and finally to the hardest-to reach north of the territory, wrapped up on Thursday.

A fresh campaign to provide a needed second dose is due to begin in about four weeks in Gaza, besieged for over 11 months.

"We admire all the health teams, who conducted this complex operation," Tedros said, also voicing gratitude to the families for turning out in droves to get their children vaccinated against polio.

Poliovirus, most often spread through sewage and contaminated water, is highly infectious. It can cause deformities and paralysis, and is potentially fatal. It mainly affects children under the age of five.

WHO has hailed that area-specific humanitarian pauses were respected, allowing the campaign to go ahead, and has urged a broader halt in fighting to help establish humanitarian corridors and the delivery of desperately-needed throughout the war-torn territory.

"Imagine what could be achieved with a ceasefire!" Tedros said.

Agence France-Presse