Variant type 2 poliovirus (VDPV) has been isolated from six environmental (sewage) samples in the Gaza Strip – collected from two different collection sites in the southern city of Khan Younis as well as Deir al Balah, further north,  WHO confirmed Friday.

The variant poliovirus strains detected in all six wastewater samples, collected in late June,  are genetically linked to each other, said the WHO-hosted Global Polio Eradication Initiative (GPEI), in a news release.

“It is important to note that virus has been isolated from the environment only at this time; no associated paralytic cases have been detected,” the GPEI statement noted. Even so, it added that, “WHO considers there to be a high risk of spread of this strain within Gaza, and internationally, particularly given the impact the current situation continues to have on public health services.”

The oral polio vaccine (OPV) that has brought the wild poliovirus to the brink of eradication provides better immunity in the gut, which is where polio replicates. But the vaccine virus is also excreted in the stool. And in communities with low-quality sanitation, this means that it can be spread from person to person and actually help protect the community.

However, in communities with low immunization rates, as the virus is spread from one unvaccinated child to another over the course of 12-18 months, it also can mutate and take on a form that can cause paralysis just like the wild poliovirus. This mutated poliovirus can then spread amongst undervaccinated children in the same communities – as cases of vaccine-derived poliovirus (VDPV).

Immunization rates have declined sharply in Gaza – while clean water is scarce and garbage piles up

Routine immunization rates in the occupied Palestinian territory (oPt), including Gaza and the West Bank, were estimated at 99% before the start of the conflict on 7 October 2023, when Hamas forces invaded some two dozen Israeli communities, killing 1200 people mostly civilians and taking 240 people hostage.  By end 2023, vaccination rates had declined sharply to 89%, according to the latest WHO-UNICEF routine immunization estimates. But the data does not separately address Gaza, which has suffered widespread destruction of its health infrastructure, along with over 38,000 deaths and nearly 90,000 injuries over the course of the nine month war.

Inside Gaza, currently only 16 out of 36 hospitals are partially functional and 45 out of 105 primary health care facilities are operational, contributing to reduced immunization rates. In addition, garbage has piled up, drinking water sources have been damaged or destroyed by the war, and sewage water is pervasive across the enclave in the wake of the breakdown in the enclave’s sewage treatment plant.  And that is in addition to pervasive malnutrition and undernutrition – increasing infectious diseases of all kinds, including but not limited, to polio.

Virus crosses borders – and conflict zones

Responding to the alert, Gaza’s Hamas-controlled Ministry of Health called for “an immediate halt to the Israeli aggression, providing usable water, repairing sewage lines, and ending population crowding in places of displacement.”

Israel’s Ministry of Health also confirmed the presence of the virus in samples it had taken from Gaza. The samples “raise concerns about the presence of the virus in this region,” the Ministry said.

Vaccine-derived polio virus has also plagued Israeli communities in recent years, where it has been circulating widely in sewage for some time, occasionally striking in under-vaccinated communities. In March 2022, a four-year-old girl from Jerusalem was diagnosed with the paralytic disease – the first case in the country since 1989.  Her case was also determined to be a vaccine-derived form of the virus.  In March 2023, four more children in northern Israel were diagnosed with the asymptomatic cases of the virus – they, too, counted among the estimated 150,000 Israeli children who are unvaccinated – mostly living in ultra-Orthodox Jewish communities.

“Unfortunately, it is no surprise that there is polio in Gaza sewage,” Professor Nadav Davidovitch, director of the School of Public Health at Ben Gurion University, was quoted as saying in Israeli media.

“Concerted effort is needed to see that everyone is vaccinated in Gaza, including Israeli soldiers, hostages, and especially the babies born in Gaza,” he said. “We must have regional collaboration in order to stop the spread of the disease.”

Image Credits: Global Polio Eradication Initiative, UNRWA .