The first round of an international campaign to vaccinate children in the Gaza Strip against polio ended in mid-September, with some 560,000 children receiving their first dose during a humanitarian pause in the war in the Strip.
Alarm bells were sounded in August when a 10-month-old boy became partially paralyzed by the polio virus in the Gaza Strip. The case made history as the first confirmed case of polio in the Strip in 25 years, and provided damning evidence to public health agencies that the poliovirus previously detected in wastewater in central and southern Gaza was, in fact, circulating among the Strip’s residents.
While independent monitors are currently verifying the exact percentage of children vaccinated in the campaign, the first round appears to have achieved the goal needed for herd immunity: a minimum of 90 percent of all children under the age of 10. Aid workers distributed the vaccine in difficult conditions, during a brief nine-hour period of peace and following mass displacement. But the work will need to be repeated to eradicate the virus. Children need two doses of the vaccine to be effectively protected against polio.
Supporting science journalism
If you enjoyed this article, please support our award-winning journalism by subscribing. By purchasing a subscription, you will help ensure the future of influential stories about the discoveries and ideas shaping the world today.
Ideal conditions for polio
Paul A. Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia, says the urgency of the campaign is driven by the “uniquely terrifying” nature of polio: The virus primarily infects children under the age of 5, leaving 1 in 200 with lifelong or fatal paralysis. There is no treatment that can reverse such paralysis. Only vaccination can prevent the disease. In communities that cannot achieve high levels of herd immunity, the virus spreads easily and quickly.
Before the war in Gaza began, 99 percent of the population was vaccinated against polio. Israeli bombing of the region has left two-thirds of the country’s hospitals unable to function and forced some 2 million Palestinians from their homes. As of early September, vaccination rates were low at 86 percent.
It’s hard to estimate how many people in Gaza have the virus; the majority show no symptoms, and of those who do, a quarter mistake fatigue, fever and headache for a cold or flu. Paralysis, the disease’s most visible sign, is rare, and reports of a 10-month-old baby who cannot sit and three children with suspected polio-related muscle weakness may be “the tip of a much larger iceberg,” Offit says.
In unsanitary conditions, the chances of catching the virus are almost certain. In Gaza today, clean water is scarce, sewage accumulates in the streets, and shelters provide just one toilet for every 700 residents. The poliovirus, a gastroenteritis infection, spreads best through contact with feces. In addition to contaminated surfaces and unclean food and water, the virus can also spread from person to person through sneezing and coughing, a route of infection that could play a major role in cramped refugee camps.
Those at highest risk of paralysis are children born after Gaza’s health system collapsed, says Roland W. Sutter, a now-retired epidemiologist with the Global Polio Eradication Initiative (GPEI). These infants are likely to have missed some or all of their routine vaccinations, but some may still have retained varying levels of their mothers’ protective antibodies against the virus from the time they were in the womb.
A successful campaign in Gaza is crucial not only for the children of Gaza, but also for children in neighboring regions and around the world. Currently, only two countries have regular outbreaks of polio: Afghanistan and Pakistan. But in recent years, even areas long considered polio-free have seen resurgences of the virus due to uneven vaccination coverage. Between 2021 and 2022, Mozambique and Malawi reported a total of nine cases. London also had a small outbreak in 2022. And in the same year, a 20-year-old man in Rockland County, New York, became the first case of paralytic polio in the United States in almost a decade. These outbreaks speak to the enormous challenges facing polio eradication efforts, Sattar emphasizes. “If we don’t succeed everywhere, children won’t be safe anywhere,” he says.
Following the path of the virus
Indeed, the arrival of this particular strain of type 2 poliovirus in the Gaza Strip reflects a series of failures to contain the pathogen elsewhere. A related strain was last seen in Egypt and is believed to have crossed the Gaza border as early as September 2023. The Egyptian strain itself emerged as a by-product of a flawed outbreak response and is a so-called vaccine-derived virus, a pathogen that is produced when traces of a particular polio vaccine reach a large, unprotected population.
The widely available oral polio vaccine (OPV) uses a live weakened virus that recipients can shed in their faeces. When the weakened virus is transmitted from host to host, it can slowly revert to a dangerous form that attacks the nervous system. This is rare, with vaccine-associated paralysis occurring once every 2.7 million doses. However, wild polio cases have declined dramatically over the past 30 years, and the vaccine-derived virus is now the predominant cause of disease, sometimes accounting for hundreds of cases per year.
Few other vaccines use a live weakened virus, and even those that do not cause epidemics like OPV does. The use of OPV carries unique threats that make it controversial. For example, OPV played a key role in quelling the polio epidemic in the United States in the 1960s, but it caused an average of nine cases of paralysis per year through 1989 and about six per year through the 1990s. In 2000, the United States adopted strict use of inactivated poliovirus vaccine (IPV), which is administered by injection.
However, oral vaccines have unique advantages that make them attractive in a variety of global contexts. They are cheap and easy to produce, and whereas IPV is administered by health care workers trained in sterile injection procedures, OPV can be administered by anyone, with a single dose requiring only a few drops in the mouth. In addition, the vaccine induces a stronger immune response than IPV, not only preventing disease but also stopping the spread of the virus. The same mechanism by which the vaccine reduces the number of polio cases also makes herd immunity easier to achieve in low-resource communities. Children vaccinated with OPV often transmit the weakened virus to their family members, who also become immune.
Of course, the GPEI knows that to eradicate polio, it will eventually have to phase out the use of OPV. So in 2016, the effort tried a trial and error, with what’s known as a “switchover”: switching the global supply of trivalent OPV, which protects recipients against all three types of wild poliovirus, to a bivalent version that triggers an immune response to only types 1 and 3. Because type 2 had last been seen in 1999, the researchers reasoned that removing it would eliminate the possibility of vaccine-derived type 2 cases. Their logic was sound, but the global implementation of the strategy backfired, says Kimberly M. Thompson, founder of the nonprofit Kid Risk and a disease modeler at GPEI. Vaccine-derived type 2 virus lurked quietly in some communities, and after the switch, small outbreaks of vaccine-derived cases suddenly emerged, preventing global polio eradication partners from eradicating it as planned. As a result, cases of vaccine-derived type 2 polio, like the one in the Gaza Strip, have increased more than tenfold since 2016.
There are no easy solutions
In 2020, the World Health Organization approved emergency use of a new vaccine, new oral polio vaccine type 2 (nOPV2), which aims to lower the chance of vaccine-derived type 2 cases. In this new formulation, researchers tweaked the genetic code of the live attenuated virus of type 2 OPV, generating a strain that is 80 percent less likely to mutate and become dangerous. Raul Andino Pavlovsky, a virologist at the University of California, San Francisco, who helped design nOPV2, believes the product is a necessary improvement over the decades-old vaccine. It is this one that is being used in the current vaccination campaign in Gaza. But even nOPV2 can revert to a paralytic form, Thompson noted, so it cannot serve as a “silver bullet.” In fact, the virus circulating in Gaza itself has evolved from previous uses of nOPV2, a World Health Organization spokesperson told Scientific American.
The complex factors that caused this epidemic have reignited the debate about how best to eradicate polio. Some epidemiologists have suggested reversing the switch and returning routine immunization to trivalent OPV. Others say the public health system could combine bivalent OPV with nOPV2, or that the GPEI could develop a safer trivalent OPV vaccine like nOPV2. A vocal minority argues that the benefits of using OPV don’t outweigh the risks. “It’s insane to use an unsafe product,” Offit argues. “I have no choice but to believe that if we come together, we can go out into communities and vaccinate them with IPV.” He and others are especially hopeful about early efforts to create a microneedle patch product that sticks on the skin. (IPV must be administered by injection, because otherwise the inactivated virus would enter the bloodstream and not fight off the poliovirus on its way to the spine and brain. If ingested orally, the inactivated virus could be destroyed by the harsh environment of the stomach.)
For now, Gaza public health officials are focused on preparing for the next vaccination round, scheduled for late September or early October. Many are optimistic that the campaign will meet its 90 percent goal—that is, as long as peace holds during a scheduled ceasefire in the war. But failure to achieve high vaccination rates could not only lead to further spread of the current virus strain across Gaza and neighboring countries, but could also lead to the emergence of new vaccine-derived virus strains. For that reason, the GPEI will be closely monitoring virus concentrations in wastewater and polio cases in the eastern Mediterranean region in the coming months.
Stopping the spread of the virus would be a real victory for the children of Gaza and for this effort. If the campaign is successful, polio eradication partners will have worked at “unprecedented scale and speed,” said Richard Pieperkorn, WHO representative in the occupied Palestinian territories, in a recent press release. But as the war continues, Gazans will continue to struggle to access the medical care they need. Children are especially vulnerable to vaccine-preventable diseases, such as measles, cholera, and pneumonia. Many children are also expected to die from hunger and bombings.
“The people of Gaza have a lot to worry about besides polio,” Andino Pavlovsky said.
