The World Health Organization has raised alarm saying it was very likely that polio has already infected people in the Gaza Strip.

The development could prove to be a setback for global efforts to eradicate the disease, a WHO spokesperson said Tuesday.

Gaza's Health Ministry declared a polio epidemic across the Palestinian enclave late Monday after samples of the virus were found in sewage. It has not announced any human cases.

Polio is now endemic only in Pakistan and Afghanistan, but more than 30 countries are still listed as subject to outbreaks, including Gaza's neighbors Egypt and Israel. Any country risks a return of polio if outbreaks are not contained with mass vaccinations.

The WHO's Christian Lindmeier told a U.N. press briefing that people had probably already been infected in Gaza but that detecting cases can be difficult since most cases of the potentially deadly viral disease are asymptomatic.

"Having vaccine-derived poliovirus in the sewage very likely means that it's out there somewhere in people," he said. "So the risk of (it)... spreading further is there and it would be a setback definitely (for global efforts)."

He said an investigation and risk assessment was underway in Gaza.

Poliomyelitis, which is spread mainly through the fecal-oral route, is a highly infectious virus that can invade the nervous system and cause paralysis and death in young children.

Cases of polio have declined by 99% worldwide since 1988 thanks to mass vaccination campaigns and efforts continue to eradicate it completely.

Ayadil Saparbekov, the WHO's head of health emergencies in the occupied Palestinian territories, said Tuesday that genomic sequencing conducted by the U.S. Centers for Disease Control and Prevention (CDC) in Atlanta indicated that the samples had close genetic linkage with each other and to samples found in Egypt in late 2023.

"We do consider that there is a high risk of spreading of... (the) vaccine-derived poliovirus in Gaza," he said.

Mass vaccination needed

WHO and the UNICEF children's agency will send a joint team to Gaza on Thursday to begin collecting human samples, with clear recommendations expected within coming days on how to address the threat, including possibly for a mass vaccination campaign.

The WHO has already sent more than 1 million vaccines to prevent children from being infected in Gaza. Israel's military, which is carrying out a genocidal war on the Palestinian territory, said last week it would start offering polio shots to its soldiers there.

James Elder, spokesperson for the U.N. children's agency, said that more than nine months of conflict had led to a drop in polio vaccination rates from 99% to 89%. He voiced concerns about vaccines reaching people in need, given humanitarian access constraints into and within the enclave.

"The mass displacement, the decimation of health infrastructure, the horrendously insecure operating environment, they will make it much, much more difficult (to do vaccinations), hence putting more and more children at risk."

Israel, which vets goods entering Gaza and is in charge of granting security clearance for aid convoys within it, blames U.N. inefficiency for aid delays.

Gaza's health ministry says nearly 40,000 people, mostly women and children, have been killed in Israel's brutal military offensive.

Polio is not the only disease that risks spreading in Gaza. Humanitarian workers say the real death toll including those killed by disease is likely much higher given high case numbers of Hepatitis A, dysentery and other diseases among people displaced by the conflict.

"I'm extremely worried about outbreaks happening in Gaza," Saparbekov said, pointing to the "very dire situation with water sanitation".

When it comes to polio and other diseases, he warned "it will be very difficult for the population to follow the advice to wash their hands and drink safe water" when living in shelters with one toilet for 600 people and extremely limited access to clean water.

"With the crippled health system, lack of water and sanitation, as well as lack of access of the population to health services ... this is going to be a very bad situation," he warned.

Crippled health care

Only 16 of Gaza's 36 hospitals are even partially functional, according to WHO, which on Tuesday lamented "a new mass casualty influx" to the Nasser hospital following deadly Israeli strikes in Khan Younis.

Saparbekov warned of a dire lack of blood units, medical supplies and hospital beds.

Before the war began there were 3,500 hospital beds available in Gaza. Today there are 1,532, he said, adding that only 45 of Gaza's 105 primary health care facilities were functioning.

At the same time, WHO said as many as 14,000 people are in dire need of being evacuated from Gaza for medical treatment.

Amid such a "crippled health system," Saparbekov warned that Gaza could soon see more people dying of different infectious diseases than from injury-related diseases.