Geneva, Switzerland - The World Health Organization said Wednesday it will send more than one million polio vaccines to war-torn Gaza after the virus was detected in the Palestinian territory's wastewater.

WHO chief Tedros Adhanom Ghebreyesus said health workers need free movement within Gaza to administer the vaccines, saying that a ceasefire, or at least a few "days of tranquillity," was essential to protect Gaza's children with routine vaccinations.

"WHO is sending more than one million polio vaccines which will be administered in the coming weeks," he told a press conference.

"The detection of polio in wastewater in Gaza is a tell-tale sign that the virus has been circulating in the community, putting unvaccinated children at risk."

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No clinical cases have yet been detected.

Andrea King, from the WHO's global health cluster team, said the vaccination campaign would be a "huge logistical challenge."

"It's vaccines as well as the associated cold chain supplies that are needed to enter Gaza... as well as the micro-planning within Gaza," she told the press conference.

"The hope is that if everything lines up, these will arrive in time for the planned vaccination dates later this month, the first round to start on August 17."

On July 30, the health ministry in Hamas-run Gaza declared the Palestinian territory to be a "polio epidemic zone," blaming the reappearance of the virus on Israel's military offensive since the October 7 Hamas attacks and the resulting destruction of health facilities.

The ministry said the virus was detected in wastewater samples taken in the Khan Yunis region in the south of the strip, as well as in areas of central Gaza.

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

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The wild version of the virus is now only endemic in Afghanistan and Pakistan, but an oral vaccine that contains small amounts of weakened but live polio still causes occasional outbreaks elsewhere.

United Nations agencies said that such vaccine-derived type-2 poliovirus had been detected in the Gaza sewage samples.

Oral polio vaccine replicates in the gut and can be passed to others through fecal-contaminated water – meaning it won't hurt the child who has been vaccinated but could infect their unvaccinated neighbors in places where hygiene and immunization levels are low.

The Israel-Gaza war was triggered by Hamas' unprecedented October 7 attack on Israel, which resulted in the deaths of 1,198 people, mainly civilians, according to an AFP tally based on Israeli official figures.

Palestinian militants also seized 251 hostages, 111 of whom are still held in Gaza, including 39 the Israeli military says are dead.

Israel's retaliatory military campaign in Gaza has killed at least 39,677 people, according to the Hamas-run health ministry, which does not break down civilian and militant deaths.