The first case of polio in the U.S. since 2013 has shaken New York State, particularly because it occurred in an area where many people are not vaccinated against the disease.

Rockland County recently announced that a young adult living in the area had been partially paralyzed by polio. The poliovirus has been eliminated in the U.S. and most countries for decades; the infected person is believed to have caught the virus from an international traveler. Three weeks after the case was announced, the New York State Department of Health said the virus had been found in samples of wastewater from New York City, following detection in samples from Rockland and nearby Orange County since May. Only 60 percent of people in Rockland County and 59 percent of those in Orange County are vaccinated against polio, compared with nearly 80 percent statewide.

“It is relatively unexpected and unfortunate that we have a case of paralysis from a completely preventable disease like polio,” says Ananda Bandyopadhyay, deputy director for polio at the Bill & Melinda Gates Foundation. Public health officials are particularly worried because Rockland County and other areas of New York that have large Orthodox Jewish communities experienced a massive measles outbreak in 2019. The outbreak was attributed to the large number of unvaccinated people in the areas.

A polio outbreak may be harder to spot than measles: much like COVID, many polio cases are asymptomatic. Only one in four people who catch the virus develop coldlike symptoms—which likely would not be suspected to be polio—and one in 200 infected people develop paralysis. Scientists in London who routinely monitor the city’s wastewater for pathogens also found polio in its sewage this year. This allowed public health officials to mount a quick response, including offering booster shots for children ages one through nine. But in the U.S., where most jurisdictions do not monitor wastewater as broadly, cases are typically found only after someone becomes paralyzed.

A Terrifying Disease

Before the first vaccine for polio was developed in the 1950s, it was a terrifying disease. Although it could strike anyone, the ease with which the virus spread meant that most people encountered it as children and either developed immunity to it or became paralyzed. Many of those infected in a massive 1952 outbreak had to spend anywhere from weeks to decades in “iron lungs”—negative-pressure ventilation machines that allowed them to breathe. Some of them who are living today use a wheelchair, and a few still require an iron lung.

As a result of widespread vaccination and public health awareness efforts, the World Health Organization (WHO) declared polio eradicated in the Americas in 1994. “The success in terms of ending polio in the United States is one of the great American stories,” says Kimberly Thompson, a public health expert and president of the nonprofit Kid Risk in Orlando, Fla. Since then the wild virus has been eradicated in every country except Afghanistan and Pakistan. “Many people had no idea it was still around,” she adds.

That means that if today’s parents aren’t afraid of polio, they may decide it’s not important to get their children vaccinated against it. “For each new set of parents, it’s a question of how they view it,” Thompson says. “But the choices people make for their kids are lifelong” unless those children get vaccinated as adults.

Even polio-free countries need to realize they’re not risk-free, Bandyopadhyay says. “We’ve known all along that a virus like polio is essentially a plane ride away as long as it is still there in some corner of the world,” he says.

The unusual characteristics of the oral polio vaccine (OPV) present a further threat. If you were born after 2000 in the U.S., you most likely received an injectable inactivated polio vaccine (IPV). But before that year, the country had used OPV, which contains a weakened version of the virus. People who receive this vaccine excrete the weakened virus in their waste for several weeks, meaning that others who come into contact with it can become inoculated as well. Because of this, nearly everyone who was born before 2000 has some immunity to the virus.

There is a dark side to that secondary exposure, however. In extremely rare cases, the weakened virus reactivates and infects either the vaccinated person or an unvaccinated person around them—which is what happened in New York State. Such “vaccine derived” cases account for all modern cases of polio in countries where the disease is eradicated.

In 2000 this risk, which results in about 1,000 polio cases around the world each year, prompted the U.S. to exclusively switch to IPV, which cannot become reactivated. But because IPV does not spread to others, unvaccinated people in countries that use it have no immunity at all. Nevertheless, people who have received this injectable vaccine can still acquire and carry the poliovirus and spread it to others, despite having no symptoms themselves.

OPV is easier to store and administer, so it is still used throughout much of the world. The WHO has been encouraging countries to switch away from the most problematic form of this oral vaccine since 2016.*

That effort has backfired somewhat because IPV rollout has been slow in many countries, meaning that they continue to use the OPV or do not vaccinate at all. Thompson says that when most people in a community are vaccinated, cases of vaccine-derived polio are extremely rare. But when relatively few people are vaccinated, the poliovirus in the oral vaccine has a chance to evolve back into a relatively normal virus—and to spread like one.

As a result, the world has seen about 2,500 more cases of vaccine-derived polio since 2016 than public health experts had expected. And in 2020 the WHO granted emergency authorization to a new oral polio vaccine (nOPV2) that does not reactivate because it is more resistant to mutations. Over the past two years, tens of millions of children in Africa have received the new vaccine.

But polio is unlikely to be completely eradicated anytime soon. Plummeting child vaccination rates resulting from the COVID pandemic have not helped matters: in 2021, for instance, tens of millions of children worldwide missed out on routine vaccinations for diseases, including polio and measles. In July WHO officials said that if these trends continue, more widespread outbreaks of these diseases could occur.

And in an undervaccinated community such as the New York areas with large Orthodox Jewish populations, high numbers of children could be susceptible. Public health officials in New York have launched vaccination campaigns aimed at reaching these communities, including bulletins in Yiddish.

There is nothing in Orthodox Jewish law or tradition that prohibits vaccination, says Blima Marcus, an Orthodox Jewish nurse in Brooklyn, N.Y. In fact, rabbis and community leaders encourage it. The reason for the low vaccination rates, she says, is that antivaccine activists tend to target tight-knit communities where their message will spread largely unchallenged. For instance, activists descended on Rockland County amid the 2019 measles outbreak in an attempt to counter provaccine public health messaging. “We have a group of passionate, fierce antivaccination activists who throw all their energy and passion behind their cause, and it’s met by silence because the rest of us are just doing our jobs,” Marcus says.

Without opposing arguments, the activists’ messages—including the discredited notion that vaccines cause autism—tend to spread quickly among concerned parents. “People want to do what’s right; it’s a community [of people that are] very reliant on each other,” Marcus says. “These pockets of deep mistrust and misinformation are much wider because of these close social connections.” She says that rumors are now circulating in her community that the person infected with polio, who now uses a wheelchair, didn’t actually have polio and was paralyzed for other reasons, despite physicians having confirmed the infection.

But Aaron Glatt, an infectious disease physician at Mount Sinai South Nassau on Long Island who is also a rabbi, says that a case of polio might wake people up to the dangers of the disease and the importance of vaccination. “People have short memories. They don’t remember how dangerous it was in the 1950s,” he says. “[People] were terrified of it. I hope we don’t have to get back to that level of illness.”

*Editor’s Note (8/23/22): This sentence was edited after posting to clarify the description of the World Health Organization discouraging the use of a type of oral polio vaccine since 2016.