A Palestinian child is vaccinated against polio, amid the Israel-Hamas conflict, in Deir Al-Balah in the central Gaza Strip, Sept. 4.
The World Health Organization (WHO) has set about vaccinating hundreds of thousands of children in Gaza in response to the Palestinian enclave's first polio case in 25 years.
The WHO last month reported one confirmed case, an almost 1-year-old boy whose limbs have been paralyzed by the illness. In addition, health officials found a variant of the virus in wastewater samples taken from central Gaza in June, indicating that the virus is circulating in the community.
"The principal cause is low vaccination rates among children," said Bruce Gellin, adjunct professor of medicine at Georgetown University School of Medicine and former president of global immunization at the Sabin Vaccine Institute in Washington. "Because this is a very infectious virus, when it comes to a community with many who have not been vaccinated and have no immunity, it can cause an outbreak, just as a spark can ignite a fire in a dry forest."
Here's what you should know about the disease and its potential impacts beyond Gaza as the mass vaccination campaign continues.
What is polio, how's it transmitted and how does it affect the body?
Polio, known formally as poliomyelitis, is a highly infectious viral disease spread through food or water that has been contaminated, most often by feces from an infected person. Though it is less common, the virus can also be spread through droplets released when someone who is infected sneezes or coughs, according to the Centers for Disease Control and Prevention.
The disease gets into the intestines and can migrate into the nervous system, where it can cause paralysis. One of every 200 infections results in permanent paralysis. As many as 10% of the patients who become paralyzed die when breathing muscles become immobilized.
Why is there a resurgence of the virus in Gaza? What is being done to stop it?
The polio outbreak is a likely consequence of the war in Gaza. Before the war, 99% of Gaza's 2.2 million residents were vaccinated against polio, according to the WHO. Since the war began, however, that rate has dropped to about 86%. To prevent outbreaks, vaccine coverage in a community should be about 95%, said Rupali Limaye, an associate professor at Johns Hopkins Bloomberg School of Public Health.
The type of polio in Gaza is known as vaccine-derived poliovirus and can occur when the live weakened virus used in the oral polio vaccine is shed in feces. The virus has been detected in wastewater and was able to infect unimmunized or under-immunized children. This allowed the virus to mutate back to a more dangerous form that causes illness and paralysis.
Health officials are now immunizing children in Gaza with a novel vaccine that contains live weakened virus. The vaccine they're using has been "developed for outbreak settings," said Gellin, who served as deputy assistant secretary for health and director of the U.S. National Vaccine Program Office at the U.S. Department of Health and Human Services from 2002 to 2017. "It is genetically more stable and less likely to mutate to a strain that can cause paralysis."
Is the surge of polio in Gaza a threat in the US?
The threat to the United States is only in communities that have low vaccination levels. This was the case in July 2022, when New York announced that an unvaccinated man in Rockland County had contracted the first case of polio in nearly a decade.
As public pushback against vaccines has grown in recent years, scientists are concerned a polio outbreak would be more difficult to contain.
"Think about it like a fire," Gellin said. "If the ground is relatively wet, a spark isn't going to take off. But if it's kind of dry, that spark has a chance of taking off. The spark in this case is the vaccine-derived virus that normally doesn't cause any problems."
In 2000, the United States stopped using the oral polio vaccine that contains live but weakened polio virus. The current vaccine is made using dead polio virus. The dead virus "is still going to boost your immune system," Limaye said. "Even if you get a vaccine with inactivated virus, it's protection against" the polio that can cause paralysis.
How many people get polio each year in the US and worldwide?
Polio's peak in the United States was in 1952, when the virus attacked the brains and spinal cords of some 20,000 people, killing 3,145, according to a description in the National Library of Medicine. But the disease, which once sparked panic, has largely vanished from the United States since the discovery of an effective vaccine in 1955 and the advent of mass vaccinations.
President Franklin D. Roosevelt became paralyzed after contracting polio in 1921.
There have been no cases of paralytic polio in the United States that were acquired from the wild virus since 1979. In 2022, there was a case of paralysis caused by exposure to the oral polio vaccine.
According to the CDC, Afghanistan and Pakistan identified 12 total cases of wild polio in 2023. Those were the only cases worldwide. The number of vaccine-derived cases of poliovirus worldwide decreased from 881 in 2022 to 524 in 2023.
The CDC recommends that children receive four doses of the inactivated vaccine to offer the maximum protection against severe disease.
Has there been an effort to eradicate the disease worldwide?
"There is a massive program. Since 1988, there has been a goal of eradication," Gellin said.
The Global Polio Eradication Initiative involves national governments, the WHO, UNICEF, the CDC, as well as Rotary International, the Bill & Melinda Gates Foundation and Gavi, the Vaccine Alliance. The effort has succeeded in eradicating polio by more than 99%.