Skip to Main Content

Depending on how young you are, your mother may remember polio. Your grandmother, though, would likely shudder at the mere mention of the word, which for years evoked images of iron lungs, of paralyzed children, of summers when fevers struck fear in parents’ hearts.

But if you are in your 20s, 30s, 40s, or even 50s, it’s possible you don’t know much at all about the disease caused by a trio of viruses that have long been vanquished from North America and most of the developed world.

That said, it may be time for a crash course in polio.


There have been recent signs that we may be forced to pay attention again to a plague we thought was safely in the rear-view mirror. A vaccine-derived polio case was detected in Rockland County, north of New York City — the first such U.S. case in a decade. Health officials reported the discovery of vaccine viruses in London sewage.

If we’re lucky, these incidents won’t lead to additional transmission of vaccine-derived polioviruses in the U.S. and the U.K. But it’s also possible Rockland County could see local transmission. London may find children paralyzed by the polio viruses circulating there. And other places that haven’t experienced polio in decades may also experience introductions of vaccine-derived viruses. As polio expert Kim Thompson, president of the nonprofit Kid Risk, told STAT last week, “There’s just a lot more polio going around than there should be.”


What follows is a primer on a virus that, 22 years after it was supposed to have been eradicated, continues to evade efforts to stamp it out.

The basics

Poliomyelitis — polio for short — is the disease caused by polioviruses. Polioviruses are enteroviruses, which means they infect by attaching to cells in the intestinal tract.

There were once three types of polio viruses, types 1 through 3. If you read about polio science, you might see those viruses described as “wild type” polio viruses, meant to distinguish them from the viruses in one type of polio vaccine. (More on this later.)

The Global Polio Eradication Initiative has rid the world of types 2 and 3 wild type polioviruses. Only type 1 viruses remain.

Nearly three-quarters of people infected with polio won’t feel sick at all; a quarter will have flu-like symptoms that will resolve on their own. But for every 1,000 people infected, somewhere between one and five will develop neurological illness — meningitis or paralysis. Some of those people die, others will have some form of permanent paralysis.

Some people who contract polio but who recover will go on later in life to develop “post-polio syndrome” — an onset of muscle weakness or even paralysis that can appear decades after the initial polio illness.

The United States used to record thousands of cases every year; in the 1940s, cases started to surge and peaked at nearly 60,000 in 1952.

The eradication effort

In 1980 the World Health Organization declared that smallpox had been wiped from the face of the earth thanks to a 13-year international effort.

The success of the smallpox campaign led a coalition of global health partners to set their sights on another target, polio. Like smallpox, polioviruses only infect people; they don’t have an animal reservoir. If they did, the prospect that they could be repeatedly reseeded into the human population from that reservoir would have disqualified them as an eradication target. Monkeypox, for example, may be controlled but it can’t be eradicated, because it exists in small mammals in parts of Africa.

In 1988, four partners — the WHO, the U.S. Centers for Disease Control and Prevention, the United Nations’ Children’s Fund (UNICEF), and the service group Rotary International embarked on a program to eradicate polio. (Later, the Bill and Melinda Gates Foundation signed on as a partner.) The goal was to get the job done by 2000.

The job is not yet done.

Despite decades of work, conducted to the tune of nearly $19 billion and counting, wild polioviruses continue to circulate, though in low numbers in a small corner of the world. Two neighboring countries — Pakistan and Afghanistan — have never stopped polio transmission. So far in 2022, 13 children have been paralyzed by polio in those two countries.

In February health officials announced that a child in Malawi had been paralyzed in late 2021 by type 1 viruses that had somehow made their way from Pakistan to East Africa. Another neighboring country, Mozambique, has also found a type 1 polio case since, raising questions about whether declines in routine vaccination rates triggered by the Covid-19 pandemic have set some African countries up for a reintroduction of wild polio. Emergency vaccinations campaigns were launched this spring across four countries in East Africa.

Wild poliovirus in Africa is a setback, but it may not be the biggest problem facing the eradication program. The biggest challenge is vaccine-derived polio.

About polio vaccines

Back in the 1950s and early 1960s, when families were frantic about the risk polio posed to children, Jonas Salk and Albert Sabin were hailed as heroes when they successfully developed polio vaccines.

Salk’s vaccine, which was licensed in 1955, was an injectable one. It contains killed or inactivated viruses, hence its nickname — IPV.

Sabin’s vaccine, which followed in 1960, was an oral vaccine, one that contained live but weakened type 1 polioviruses. A trivalent version, targeting all three polioviruses, was licensed in 1963. Initially OPV, as it is known, was administered by dripping a dose onto a sugar cube, which a child would eat. Nowadays vaccinators use eyedroppers to squeeze a couple of drops of vaccine into the mouths of children in the parts of the world where it is still used.

Both vaccines protected children from paralysis if they are infected with polioviruses. Both have substantial but differing strengths.

IPV is safer than OPV, because it uses killed viruses. The United States has used it exclusively since 2000 for that reason.

OPV is far cheaper than IPV and it doesn’t require syringes, an added saving. Unlike IPV, it can be administered by trained volunteers; IPV must be delivered by a health care worker, a significant hurdle in some parts of the world.

Another major advantage of OPV: It creates immunity in the gut. Children vaccinated with it are less likely to shed — and spread — polioviruses via their stools if they become infected.

They do, though, shed vaccine viruses after being vaccinated. In places where hygiene is poor, those vaccine viruses can make their way from a vaccinated child to unvaccinated ones. That passive vaccination means polio vaccinators in countries where OPV is used don’t need to reach every child in household or a neighborhood.

The big ‘but’

Actually, there are a couple.

In rare cases, the oral vaccine will paralyze a child who receives it, or a child who was a close contact of a vaccinated child. This is called vaccine-associated paralytic polio.

A more common problem is something known as vaccine-derived polio — the type of polio detected in Rockland County.

As mentioned earlier, the viruses Sabin modified for his vaccine can spread from child to child. If they continue to find children they can infect — if they passage through enough intestinal tracts — they can regain the power to paralyze. This phenomenon is seen in places where there are too many unvaccinated children.

Since the mid-aughts, cases of vaccine-derived polio have increased, spreading from Africa to other parts of the developing world.

In a bid to reduce the risk of such cases, in 2016 the polio program told countries to stop using trivalent oral polio vaccine and move to a version that protected against just type 1 and type 3 polio. Type 2 polio had been eradicated at that point and the type 2 vaccine viruses were most likely to trigger chains of vaccine-derived polio. It was thought a coordinated withdrawal of type 2 oral polio vaccine would solve the type 2 vaccine-derived cases. Integral to the plan were instructions to vaccinate all children around the globe with IPV before the withdrawal, to build up a wall of immunity against type 2 viruses.

The move, known as “the switch,” failed. It wasn’t sufficiently coordinated and there were shortages of IPV. In the years since, cases of type 2 vaccine-derived polio have mushroomed and have now been reported in 36 countries.

This year alone, 182 people, mostly children, in 14 countries have been paralyzed by vaccine-derived polioviruses; 176 of those, including the Rockland County case, were caused by type 2 viruses.

The fix

A new oral polio vaccine, the first in decades, has been developed to combat the type 2 vaccine-derived viruses. It is known as nOPV2, short for novel oral polio vaccine type 2.

Like Sabin’s OPV, this new vaccine contains live, weakened virus. But it was carefully engineered to try to avoid the Achilles’ heel of his vaccine. It is believed the weakened viruses in the new vaccine will be far less likely to regain the power to paralyze.

It has begun to be used in a number of countries, but there have been supply constraints that are supposed to be resolved in the next few months.

So where does that leave us now?

A thing to know about vaccine-derived polio: Scientists who study the genomes of polioviruses can distinguish wild-type polio from vaccine-derived viruses, but the human body can’t. To the gut and the central nervous system, both cause the same damage. Or as the CDC says on its website: “there is no clinical difference.”

So while the Global Polio Eradication Initiative keeps separate tallies of wild polio cases and vaccine-derived polio cases, in a community where there are unvaccinated people, the vaccine viruses pose the same risks as wild polio, and they can ignite an outbreak.

Rockland County has lots of unvaccinated children and young adults, as the country witnessed a couple of years ago when it was the epicenter of a large measles outbreak that almost cost the United States its measles-free status. This could be ripe terrain for vaccine-derived polioviruses.

“We need to treat this as if it were a wild virus,” Walter Orenstein, a polio expert at Emory University, said in response to the detection of the Rockland County case. “And it argues all the more of our need to continue our efforts around the world to get rid of polio, so we don’t have to worry about this happening.”

Create a display name to comment

This name will appear with your comment