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The World Health Organisation has this week set out plans to administer 1.2 million polio vaccine doses in Gaza in an effort to prevent an outbreak that could exacerbate the current humanitarian crisis.
Two rounds of mass vaccination – targeting hundreds of thousands of children under the age of eight – are to begin this month after the detection of poliovirus in sewage.
Healthcare facilities and sewage systems in Gaza have suffered severe damage after months of bombardment by Israel following the attacks on October 7 last year, raising the dangers of disease spreading.
Here we consider the risks facing Gaza and ask experts whether there is a likelihood of people suffering paralysis as a result of the spread of polio.
What is polio and what has been found in Gaza?
Typically passed on through the faecal-oral route – such as by eating or drinking something contaminated with faeces – poliomyelitis is a viral condition that, in most cases, does not cause symptoms.
A minority of infected people have headaches and other mild effects, but about one in 200 suffers irreversible paralysis. Five to 10 per cent of such paralysis cases result in death when breathing becomes affected.
It was revealed last month that poliovirus had been detected in a number of sewage samples collected from two governorates in Gaza in June. The WHO says it could have been in circulation since last year.
This viral matter is not wild poliovirus, but instead is circulating vaccine-derived poliovirus (cVDPV), which appears to be spreading within the Gazan population.
With many areas in Gaza severely overcrowded, and with health facilities and sewerage damaged, conditions are ripe for the spread of polio.
cVDPV originates from the oral polio vaccine (OPV), which contains a weakened form of the virus that, in populations where immunisation rates are low, can spread between people.
"When vaccine-derived polio circulates unhindered, after a certain length of time there’s concern it would mutate back to become fully virulent," said Paul Hunter, a professor in medicine at the University of East Anglia in the UK.
"Then you would start to see a big rise in cases of paralytic poliomyelitis."
Vaccine-derived poliovirus can circulate even in developed nations when vaccination rates fall, such as in New York, where it caused the paralysis of a man in 2022.
Huge progress has been made against wild poliovirus, especially since the launch, in 1988, of the Global Polio Eradication Initiative, with cases having dropped by 99 per cent.
Only Pakistan and Afghanistan are still affected by the wild poliovirus, while in Africa, for example, cVDPV is the problem.
What vaccination programme is planned in Gaza?
The WHO aims to have two vaccination rounds of 600,000 doses for children under eight in Gaza, but has admitted that administering this could be difficult logistically.
The organisation is reportedly securing approvals to enter Gaza to undertake the programme and has called for a ceasefire so it can operate safely.
Prof Hunter, who helped to prepare a report for the WHO on infection control in Gaza during the conflict, said the situation there was "turning into one of the biggest humanitarian catastrophes" of the region in recent times.
Separate to the vaccination in Gaza, in May the Palestinian Ministry of Health announced it was carrying out a round of vaccinations using the OPV for children aged under five in Jerusalem and Bethlehem.
While there is a small risk with the OPV that it could mutate, become virulent again and spread in populations where vaccination rates are not high enough, it is easier to administer than the inactivated polio vaccine (IPV), which is given by injection. Also, the OPV offers stronger protection against the spread of the disease than the IPV does.
In developed nations the polio vaccine used is typically the IPV, which is given as an injection in the arm or leg.
In the US, for example, the IPV has been used since 2000, while the UK moved from the OPV to the IPV four years later.
Should other countries be worried?
Prof John Oxford, emeritus professor of virology at Queen Mary, University of London, said there was a danger that polio could spread outside Gaza.
"It’s very worrisome indeed," he said. "You cannot escape the fact it’s a risk to neighbours and even further afield. It’s a huge dilemma and humanitarian crisis. We don’t want polio coming back anywhere in the world."
Given the risks to Israel, Prof Hunter said the country should allow vaccination efforts to take place for reasons of self-interest, as well as for the sake of the Gazan population.
"If paralytic poliomyelitis started spreading in Gaza, the risk is that it spreads more regionally to Israel and Egypt," he said.
"If it happened in Gaza, it would be absolutely appalling for the Palestinians but it would present a risk for neighbouring countries, including Israel.
"Israel has a responsibility to allow this vaccination to go ahead not only for humanitarian reasons, but to protect its own population."
Separate to the WHO’s planned vaccination of Gazan children, the Israeli military last month announced it would give polio boosters to its soldiers in Gaza.
Dr Hamid Jafari, director of the WHO polio eradication programme, said a fast international response to contain the outbreak could reduce the risk of a wider health crisis.
“There is a clear and present danger and risk of spread internationally of this virus, and the sooner we can respond, the faster we can begin to reduce that risk,” Dr Jafari told The National.
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“Our risk assessment tells us there is a very high risk of spread to surrounding countries, particularly Egypt, Lebanon, Syria and Jordan.
“Under-vaccinated pockets are everywhere, including in Israel.
“It's very concerning. Polio is the only ongoing public health emergency of concern under the international health regulation.
“This requires a regional response and what that means is an initial focus on Gaza to stop the outbreak.”
Travel restrictions within the enclave have made it increasingly difficult for health workers to interact with communities in dire need of care.
The polio virus has been detected in sewage in two governates, from several sites in Deir al Balah and in Khan Younis.
The WHO and Unicef are working closely with the Ministry of Health to get the OPV programme up and running.
The initial plan was to start the campaign on August 17 but that has been delayed by another week to August 24.
Maintaining the cold chain storage required for the integrity of vaccine doses is another considerable challenge.
“Most likely polio has been circulating perhaps as long as since September last year,” said Dr Jafari.
“The conditions for polio virus transmission and the force of infection in the present situation in Gaza are extremely high.
“People are getting constantly displaced and they're coming into contact in crowded conditions.
“Gaza had very high immunisation rates and childhood immunisation, but they are falling.
“These conditions are perfect for the virus to circulate very efficiently and risk of spread to other parts of the occupied territories.
“The focus of our response is Gaza, but we are also preparing other countries of the region for a regional response, to increase their surveillance and preparedness.
“The only way to stop this outbreak is a series of mass vaccination campaigns.”
