The necessary second round of vaccinations has been completed in central and southern Gaza, and was to begin on Wednesday in the north.
But the WHO said it had been “compelled to postpone” the bid to give 119,279 children in northern Gaza a second vaccine dose.
Israel launched a major air and ground assault in northern Gaza this month, vowing to stop Hamas militants from regrouping in the area.
The vaccination campaign was called off “due to the escalating violence, intense bombardment, mass displacement orders, and lack of assured humanitarian pauses across most of northern Gaza”, the UN health agency announced.
“The current conditions, including ongoing attacks on civilian infrastructure, continue to jeopardise people’s safety and movement in northern Gaza, making it impossible for families to safely bring their children for vaccination,” and for health workers to operate, it added.
The vaccination drive began after the Gaza Strip confirmed its first case of polio in 25 years.
The war has left most medical facilities and Gaza’s sewage system in ruins.
Typically spread through sewage and contaminated water, poliovirus is highly infectious.
It can cause deformities and paralysis, and is potentially fatal, mainly affecting children under the age of five.
The WHO says a minimum of two separate doses of oral vaccine are needed to interrupt poliovirus transmission, requiring 90 percent of all children aged under 10 to be vaccinated in a given community.
As in the initial round of vaccinations last month, the second round was divided into three phases, helped by localised “humanitarian pauses” in the fighting: first in central Gaza, then in the south and finally the north.
Each phase was to take up to four days.
The WHO warns that immunity levels are lower if the second dose is given more than six weeks after the first.
The UN agency said the approved area for humanitarian pauses in the north had been cut down to Gaza City alone, meaning many children would have missed the second dose.
This would “seriously jeopardise efforts to stop the transmission of poliovirus in Gaza”, it noted.
Since the second round of the campaign began on October 14, some 442,855 children under 10 have been vaccinated in central and southern Gaza, with coverage at 94 percent.
“It is imperative to stop the polio outbreak as soon as possible, before more children are paralysed and poliovirus spreads further,” the UN health agency said, adding, “It is crucial therefore that the vaccination campaign in northern Gaza is facilitated through the implementation of the humanitarian pauses.”
More than 560,000 children below the age of 10 in Gaza received their first doses of the vaccine in September, according to the WHO.
The war in Gaza began with Hamas’s October 7, 2023 attack on Israel, which resulted in the deaths of 1,200 people, mostly civilians.
Israel’s retaliatory offensive has killed more than 42,700 people in Gaza, also mostly civilians, according to figures from the Hamas-run territory’s health ministry which the UN considers reliable.
