After decades of absence, polio has reemerged in the United States.

In late July, a young unvaccinated man living in Rockland County, New York, became the first case of paralytic polio in the U.S. in nearly a decade. Soon after, state officials announced they’d found poliovirus in the wastewater in that county, neighboring Orange County and New York City, confirming that the virus that once maimed thousands of children each year was back.

Most Americans haven’t had to think about it since naturally occurring versions of the virus were eliminated in 1979 through mass vaccination campaigns, meaning wild polioviruses are no longer spreading in the United States. Worldwide, safe and effective polio vaccines have driven wild-type polioviruses from nearly all countries and prevented nearly 30 million cases of paralysis since 1960.

But now, a primary driver of that success — the weakened version of polioviruses within some vaccines — is causing some problems of its own, threatening to erode decades of progress. The story of why is complicated.

Grid broke down how incomplete global eradication efforts collided with vaccine hesitancy and pandemic-related declines in routine childhood immunization to spark the biggest resurgence of poliovirus the U.S. has seen in decades, and what might happen next.

Why is this happening?

The most obvious explanation for the reemergence of polio in Rockland County is that only about 60 percent of residents are vaccinated against polio. But the seeds of the current moment can be traced to incomplete efforts to eradicate polioviruses from the globe with vaccines.

Historically, three strains of wild-type polioviruses have spread among humans, called types 1, 2 and 3. These viruses, which infect the intestinal lining, primarily spread through contact with the stool of an infected person. Most unvaccinated people who contract the virus don’t get sick at all or have mild illness, and about 25 percent will get mild flu-like symptoms. But for every 2,000 infections, somewhere between one and 10 people will experience paralysis, depending on the strain, as the virus travels from the gut to the brain or spinal cord. In the U.S., polio cases peaked in the late 1940s and early 1950s, disabling more than 35,000 people each year. Globally, cases peaked in the 1980s, when polioviruses disabled more than 300,000 to 400,000 kids annually.

To understand this outbreak, you need to understand how polio vaccines work

Two types of vaccines developed in the mid-20th century have drastically improved the situation, eliminating wild-type polio from the U.S. and bringing the world to the brink of eradication. Both vaccines have strengths, but key differences between them help explain the current situation.

Inactivated poliovirus vaccine, or IPV, was developed first by Jonas Salk and is currently the only polio vaccine used in the U.S. It prevents paralysis, but it doesn’t fully block infection. It contains “dead” poliovirus delivered by injection, spurring the production of antibodies against poliovirus in the bloodstream, which prevents the virus from wreaking havoc in the brain or spinal cord. But much like how covid vaccines don’t fully prevent SARS-CoV-2 from infecting the nose, IPV doesn’t fully block polioviruses from infecting the gut, meaning people inoculated with IPV can still transmit poliovirus.

Oral poliovirus vaccine, or OPV, was developed several years after IPV. It’s cheaper and easier to administer than IPV, and has become the main tool in global eradication. OPV contains a live but weakened version of up to all three strains of poliovirus. When swallowed, “you have something like an infection, the vaccine virus replicates and stimulates robust immunity,” in both the gut and blood, preventing paralysis and infection, said Kim Thompson, a polio expert at Kid Risk, a nonprofit research outfit.

While the vaccine-virus in OPV is working within a person, it can get excreted through feces and spread to others, especially in areas with poor sanitation. That proved powerful, in a good way, during the early days of global eradication campaigns, since vaccinating just a fraction of people could eventually immunize an entire community.

Since 1988, a global vaccination campaign, fueled primarily by OPV and nearly $20 billion in funding, has eradicated types 2 and 3 wild polioviruses from the globe. Type 1 wild-type poliovirus remains endemic in Pakistan and Afghanistan.

But the power of that live, attenuated virus comes with a cost, as about 1 of every 6 million doses causes paralysis in children who receive OPV or unvaccinated close contacts. (Because of this risk, the U.S. stopped OPV use in 2000.) Additionally, the attenuated vaccine virus can regain its ability to paralyze through evolution as it spreads through unvaccinated individuals, becoming what’s known as a vaccine-derived poliovirus.

“It’s only when you have low enough [vaccine] coverage that the strains can just keep circulating and they evolve to be like wild viruses capable of causing disease,” Thompson said. This evolution can also occasionally happen within an immunocompromised individual, she said.

Over the last 15 years, vaccine-derived polioviruses have become a larger and larger problem, causing thousands of cases of vaccine-derived polio, largely in low- to middle-income countries.

It turns out that most of these cases stem from the type 2 strain, which was declared eradicated from the wild in 2015. Over the last 15 years or so, thousands of cases of vaccine-derived polio have popped up in the developing world. And because paralysis occurs in such a small fraction of infection, this trend suggests a much larger wave of transmission.

The Global Polio Eradication Initiative tried to address this problem by telling countries to switch to a version of OPV without type 2 in 2016. By removing the most problematic, and now unnecessary, strain from the source, public health leaders hoped vaccine-derived polio cases would peter out.

For the switch to work, mass vaccination campaigns had to be undertaken just before removing type 2, to build up immunity, and then all countries needed to switch at once, said Walter Orenstein, a vaccine researcher at Emory University who consulted on the switch. “That didn’t happen as well as we’d hoped for.”

Instead, insufficient campaigns left large pockets of susceptibility in many countries that have only grown since, as more children are born without any immunity to the type 2 vaccine-derived virus. Since the switch, more than 2,200 cases of vaccine-derived paralytic polio have been reported in 36 countries.

“The bottom line is that since 2016, type 2 cases and transmission have basically surged in some populations,” Thompson said. “And that just, of course, increased the risks that an importation might make it into the United States or into London.”

The virus is exploiting pockets of lower vaccination

On the whole, the United States is well-protected against polioviruses, with over 90 percent of the population vaccinated. If you underwent the normal childhood vaccine schedule, you are very likely well protected against paralytic polio.

But community protection across the U.S. has eroded in recent years for two major reasons: pandemic-related declines in childhood vaccinations and pockets of vaccine hesitancy.

“A lot of children missed vaccine appointments due to covid,” said Jay Varma, a physician and epidemiologist who specializes in infectious diseases at Weill Cornell Medical School. In Rockland County, for instance, vaccination coverage among kids aged under 24 months declined by 7 percent from 2020 to 2022, to 60 percent. Additionally, Varma said, “I think you’re also seeing the impact of the anti-vaccine movement.”

These growing pockets of susceptibility have given imported vaccine-derived polioviruses a foothold in the U.S.

The 20-year-old who caught polio in June and suffered paralysis lived in Rockland Country, a suburb of New York City that has polio immunization rates that range from about 37 percent to 62 percent, depending on the ZIP code. Low measles vaccination rates among communities of Hasidic Jews who live in the area helped sustain a large measles outbreak in 2018 that nearly cost the U.S. its measles elimination status. And vaccine hesitancy generally has grown in recent decades, Varma said, fueled in part by disinformation online.

Since paralytic polio is relatively rare, a single case is a sign that “likely hundreds or thousands have already been infected,” said Varma, meaning the virus has been circulating for months. Based on wastewater samples in and around New York City, the Centers for Disease Control and Prevention said the virus started circulating in the region as early as April.

Unvaccinated individuals provide more opportunity for the virus to spread, but it’s also possible that people who received the IPV vaccine (which in the U.S. includes everyone born after 2000) are contributing too. “The fact that most of our young people in the U.S. are only protected by IPV means that they might participate relatively more [in transmission] than people who have OPV protection,” Thompson said, though they wouldn’t get sick and likely don’t transmit the virus as readily as an unvaccinated person. To what extent that’s happening is unclear, but the predominance of IPV vaccination in the U.S. could be a factor in how widely the virus spreads.

There is a best-case scenario — but it’s far from inevitable

What happens next in the United States is hard to predict, but the primary determinant will be how many people get vaccinated.

If vaccination rates don’t tick back up, “the worst-case scenario is that this virus spreads readily among unvaccinated people, primarily through children, through many different communities,” Varma said.

People unknowingly infected with the virus could travel to other areas with lower vaccination rates, seeding new chains of transmission.

Vaccinated people will be well protected from symptomatic disease. But if the poliovirus’ new range continues expanding, “it will infect anybody who comes across it who is not immune, and roughly 1 in 2,000 people will develop paralysis,” said Gregory Poland, head of the Mayo Clinic’s Vaccine Research Group. Even people who recover from symptomatic polio can develop recurrent symptoms years later, a sort of “long polio syndrome,” said Poland. In addition to unvaccinated people, some immunocompromised individuals and infants under 2 who can’t yet be vaccinated will be at risk, something Americans have not had to seriously worry about for decades.

“This won’t just be a blip of a public health problem that will go away,” Poland said. “Because of low vaccination rates, it will continue to be a problem.”

Such an outcome is not inevitable, however. “The best-case scenario is that we just have this one case, as tragic as it is,” Thompson said. In that scenario, the circulating virus that’s popping up in wastewater would gradually disappear, unable to gain a strong foothold. Vaccinating as many unprotected people as quickly as possible will help tip the scales toward that outcome, she said.

What should we do now?

In London, where vaccine-derived poliovirus has been detected in wastewater since late June, vaccination campaigns have already begun. Health authorities are urging all children aged 1 to 9 to get a dose of the IPV vaccine, whether it’s a booster dose or just to catch up. “Even though they haven’t found children with paralysis, there is a risk,” said David Heymann, an epidemiologist at the London School of Hygiene and Tropical Medicine, and robust vaccination campaigns are key to minimizing that risk.

In the U.S., the Rockland County Department of Health launched a vaccination effort in late July, though it hasn’t significantly improved IPV coverage, according to the CDC. Reaching vaccine-hesitant people will require more targeted campaigns.

“There needs to be a tremendous vaccine drive done in collaboration with all the members of the Orthodox community and any other community in which there are high rates of under-vaccination,” said Varma. “We know many of these people aren’t up to date because they’re concerned about the safety of these vaccines, so the only way to reach these people is through trusted community messengers.”

Such messengers played an important role in previous outbreaks of measles and hepatitis A in the area. “Rabbis were very helpful in those instances in rallying their congregations toward acceptance of vaccines, and those outbreaks ceased,” said Poland.

If vaccine-derived poliovirus circulation continues to expand, there’s a chance that the IPV vaccine may not be a powerful-enough tool to slow transmission and countries might turn back to OPV, as Israel did in 2013 when IPV proved insufficient to contain an outbreak there, said Thompson. Such a campaign would come with risk of vaccine-associated paralytic polio, she said: “It would not be a simple thing, in my opinion, to put OPV back into circulation in the U.S.”

In 2020, the World Health Organization granted emergency-use listing to a new OPV vaccine, which uses a modified virus that’s less likely to spark continued transmission. But this vaccine remains in relatively low supply and is not authorized in many countries, including the U.S.

Whatever tools the U.S. uses, convincing enough people to use them will be the main challenge, said Poland. As consensus around public health issues continues to fracture, that’s increasingly difficult and may hamper efforts to control a virus we once had under wraps.

“We have a case of polio, in the third decade of the 21st century, in the richest country in the history of mankind with more healthcare, more education, more of everything,” Poland said. “The anti-vaccine and the vaccine-hesitant movement is a very, very dangerous one.”

Thanks to Lillian Barkley for copy editing this article.