After decades of absence, polio has resurfaced in the United States.
In late July, an unvaccinated young man living in Rockland County, New York, became the first case of paralytic polio in the US in nearly a decade. Soon after, state officials announced that they had found poliovirus in the sewage of this county, which neighbors Orange County and New York City, confirming that the virus that once maimed thousands of children each year had returned.
Most Americans haven’t had to think about it since wild 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 eradicated wild-type poliovirus from nearly every country and prevented nearly 30 million cases of paralysis since 1960.
But now one of the main drivers of that success, the weakened version of polioviruses in 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 vaccines and pandemic-related declines in routine childhood immunization to lead to the biggest resurgence of polio the United States has seen in decades, and what could happen next.
Why does this happen?
The most obvious explanation for the resurgence 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 back to incomplete efforts to eradicate polioviruses from the world with vaccines.
Historically, three strains of wild-type poliovirus, called types 1, 2, and 3, have spread among humans. These viruses, which infect the lining of the intestine, are spread primarily through contact with the feces of an infected person. Most unvaccinated people who get the virus do not get sick or have a mild illness, and about 25 percent will have mild flu-like symptoms. But for every 2,000 infections, 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 United States, polio cases peaked in the late 1940s and early 1950s, incapacitating more than 35,000 people each year. Globally, cases peaked in the 1980s, when polio viruses disabled more than 300,000 to 400,000 children each year.
To understand this outbreak, you need to understand how polio vaccines work
Two types of vaccines developed in the mid-20th century have dramatically improved the situation, eliminating wild-type polio from the US and bringing the world to the brink of eradication. Both vaccines have strengths, but key differences between them help explain the current situation.
The inactivated polio vaccine, or IPV, was first developed by Jonas Salk and is currently the only polio vaccine used in the US. Prevents paralysis, but does not completely block infection. It contains “dead” poliovirus given by injection, stimulating the production of antibodies against the poliovirus in the bloodstream, which prevents the virus from wreaking havoc on the brain or spinal cord. But just as covid vaccines do not completely prevent SARS-CoV-2 from infecting the nose, IPV does not completely block polioviruses from infecting the gut, meaning that people inoculated with IPV still they can transmit the poliovirus.
The oral polio vaccine, or OPV, was developed several years after IPV. It is cheaper and easier to administer than IPV, and has become the main global eradication tool. OPV contains a live but weakened version of up to three strains of poliovirus. When swallowed, “you get something like an infection, the vaccine virus replicates and stimulates robust immunity,” both in the gut and in the blood, preventing paralysis and infection, said Kim Thompson, an expert in polio by Kid Risk, a non-profit research team.
While the OPV vaccine virus works inside a person, it can be excreted through the feces and spread to other people, especially in areas with poor hygiene. This proved powerful, in a good way, in the early days of global eradication campaigns, as vaccinating just a fraction of people could immunize an entire community.
Since 1988, a global vaccination campaign, driven primarily by OPV and nearly $20 billion in funding, has eradicated wild poliovirus types 2 and 3 from the world. Wild-type poliovirus type 1 remains endemic in Pakistan and Afghanistan.
But the power of this live, attenuated virus comes at a cost, with about 1 in 6 million doses causing paralysis in children receiving OPV or unvaccinated close contacts. (Because of this risk, the US stopped using OPV in 2000.) Also, the attenuated vaccine virus may regain its ability to paralyze through evolution as it spreads through unvaccinated individuals, becoming what is known as vaccine-derived poliovirus.
“It’s only when you’re low enough [vaccine] coverage that strains can continue to circulate and evolve to be like wild viruses capable of causing disease,” Thompson said. Such evolution can also occasionally occur in an immunocompromised individual, he said.
Over the past 15 years, vaccine-derived polioviruses have become an increasing problem, causing thousands of cases of vaccine-derived poliomyelitis, largely in low- and middle-income countries.
It turns out that most of these cases come from the type 2 strain, which was declared eradicated from the wild in 2015. Over the past 15 years or so, thousands of cases of vaccine-derived polio have appeared 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 sought to address this problem by telling countries to switch to a type 2-free version of OPV in 2016. By eliminating the most problematic, and now unnecessary, strain at the source, leaders of public health expected that polio cases resulting from the vaccine would die out.
For the switch to work, mass vaccination campaigns had to be carried out just before type 2 was eliminated, to boost immunity, and then all countries had to switch at the same time, said Walter Orenstein, a vaccine researcher at the Emory University who consulted on the change. “That didn’t go as well as we hoped.”
Instead, insufficient campaigns left large pockets of susceptibility in many countries that have only grown since then, as more children are born without any immunity to the vaccine-derived virus type 2. Since the switch, more have been reported of 2,200 cases of vaccine-derived paralytic poliomyelitis in 36 countries.
“The bottom line is that since 2016, type 2 cases and transmission have basically increased in some populations,” Thompson said. “And that, of course, increased the risks that an import could reach the United States or London.”
The virus is exploding lower vaccination bags
The United States is generally well protected against poliovirus, with more than 90 percent of the population vaccinated. If you have undergone the normal childhood vaccination schedule, you are most likely well protected against paralytic polio.
But community protection in the United States has eroded in recent years for two main reasons: pandemic-related declines in childhood vaccinations and vaccine hoardings.
“A lot of kids missed their vaccination appointments because of covid,” said Jay Varma, an infectious disease physician and epidemiologist at Weill Cornell Medical School. In Rockland County, for example, vaccination coverage among children younger than 24 months decreased by 7% from 2020 to 2022, to 60%. In addition, Varma said, “I think you’re also seeing the impact of the anti-vaccine movement.”
These growing pockets of susceptibility have given way to imported vaccine-derived polioviruses in the US
The 20-year-old who contracted polio in June and was paralyzed lived in Rockland Country, a New York City suburb that has polio immunization rates ranging from 37 percent to 62 percent, depending on the zip code. Low measles vaccination rates among Hasidic Jewish communities living in the area helped sustain a large measles outbreak in 2018 that nearly cost the US its measles-free status. And vaccination coverage has generally grown in recent decades, Varma said, fueled in part by online misinformation.
Because paralytic polio is relatively rare, a single case is a sign that “probably hundreds or thousands have already been infected,” Varma said, meaning the virus has been circulating for months. Based on sewage samples in and around New York City, the Centers for Disease Control and Prevention said the virus began circulating in the region as early as April.
Unvaccinated people provide more opportunities for the virus to spread, but people who received the IPV vaccine (which in the US includes everyone born after 2000) may also contribute. “The fact that most of our youth in the US are only protected by IPV means they could be relatively more involved [in transmission] than people who have OPV protection,” Thompson said, even though they wouldn’t get sick and probably wouldn’t spread the virus as easily as an unvaccinated person. It’s unclear to what extent this is happening, but the prevalence of IPV vaccination in the US could be a factor in the extent of the spread of the virus.
There is a best-case scenario, but it is far from inevitable
What will happen next in the United States is difficult to predict, but the main determinant will be how many people get vaccinated.
If vaccination rates don’t increase, “the worst-case scenario is that this virus spreads easily 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,…