The global campaign to eradicate polio has been incredibly successful except in one key way: It hasn't actually eradicated polio.

Some prominent scientists who've spent their entire careers battling polio are now saying it's time to rethink the multi-billion dollar fight against the disease.

When the Global Polio Eradication Initiative was launched in 1988, the goal was to extinguish polio by the year 2000. At the time, polio was still paralyzing hundreds of thousands of people a year, and some cases were even fatal.

In the first three months of this year there have been 15 cases, total, in the entire world.

Even such a relatively small number of cases, say some specialists, shows that the goal of wiping out the disease may be elusive.

Countries, particularly in Africa, were declared polio-free only to have polio pop up again years, even decades, later. Last year, the United States, which had officially eliminated polio in 1979, had a case of the paralytic disease. The virus has also been found in sewage samples in London, Finland and Jerusalem, all places where it supposedly was extinguished decades ago.

Where the cases are

The U.S. case and most of the current outbreaks in Africa (which was declared a polio-free continent in 2020) are linked to vaccine-derived strains of the virus. These strains were originally part of oral polio vaccine. They were shed in human stool from those who'd been inoculated and mutated in the wild, regaining strength and becoming just as dangerous as the original virus.

Health officials were aware that the virus in the oral vaccine could mutate, says Paul Offit, director of the Vaccine Education Center at the Children's Hospital, Philadelphia, and a professor of pediatrics at the University of Pennsylvania School of Medicine.

Despite this concern, the oral vaccination campaigns proceeded. "We were seduced by the fact that it was cheap. It caused contact immunity. It was easy to give," Offit says. "And so we thought, 'We can eliminate this this disease in the world.' "

Last year there were only 30 cases of wild polio detected while there were nearly 800 cases of vaccine-derived polio. Of the 15 cases reported in the first quarter of 2023, 14 are from strains of the virus that mutated from the oral polio vaccine used in lower-income nations.

"We released the dragon and the dragon was the circulating vaccine-derived polio viruses," Offit says.

So in an ironic twist, countries are now launching emergency vaccination campaigns to protect children from outbreaks linked to earlier vaccination campaigns.

How vaccine-derived polio spreads

Public health officials knew early on in the eradication effort that the oral polio vaccine could spread far beyond the child who was initially vaccinated. This in fact was one of the benefits of the polio drops. It spread like other viruses in the community and could immunize unvaccinated kids in the neighborhood who were exposed as kids pooped out – or "shed" — the vaccine into sewage. In places with poor sanitation, the sewage mixes with drinking water. When unvaccinated kids consume that water, they gain immunity.

But the virus spreading in waste water can also cause paralytic disease. That is most likely what happened last year in the case of an unvaccinated man in New York being paralyzed by a vaccine derived strain of the virus. Offit says this case is concerning and shows that vaccine-derived polio virus is probably widespread even in the United States.

People who've been vaccinated with the injectable form of the polio vaccine, as have most in the U.S., can still carry and spread the virus to unvaccinated individuals.

"For every 2,000 people who are infected with this strain, one will be paralyzed. So he clearly is the tip of a much bigger iceberg," Offit says. The virus was also found in wastewater samples in Rockland County, N.Y. near where the man lived. Now the CDC has committed to check sewage systems nationwide to see if there is more polio virus circulating in the United States. "I would be surprised if there weren't," Offit predicts.

Despite the attention the New York polio case attracted last year, most of the current outbreaks are in low-income countries which use the less-expensive oral polio vaccine and where vaccination rates are often incredibly low. In some places only 5% of kids are fully up to date on the childhood immunizations.

In a perfect world, health experts say they'd like everyone would get the more expensive polio shot which is incapable of mutating into a dangerous, circulating virus.

But Aidan O'Leary, who heads up the Global Polio Eradication Initiative (GPEI) at the World Health Organization, points out that the polio elimination effort is happening in a world that's far from perfect.

The worst outbreaks of vaccine-derived polio right now, he says, are in parts of Nigeria, Somalia, the Democratic Republic of Congo and Yemen. Last year there were 461 cases of paralytic polio in the DRC — more than in any other country in the world.

"As you're probably aware these locations are fairly complicated places to operate," says O'Leary. "There's political instability, insecurity and protracted, complex humanitarian emergencies." He says these hot spots are the "engines" driving the outbreaks of vaccine-derived polio globally.

In many of these remote, conflict-ridden parts of the world, O'Leary says the options for polio vaccination are very limited. "The health systems are kind of, I would say, skeletal in nature. Childhood immunization rates can be as low as 5%."

Mass immunization campaigns with the more expensive yet safer injectable vaccine, he says, are nearly impossible in some of these places. There simply isn't the infrastructure or trained medical staff to carry out such campaigns.

"In places like northern Yemen, in places like south central Somalia, the essential immunization systems are suboptimal and the issue is about building [public health] capacity," he says. "But these are not quick fixes, not over three years, not over five years, not over ten years. It takes a long, long time to really make the investments to strengthen these health systems."

And if polio is left unchecked in some of these places, it not only affects local kids but these areas become reservoirs from which the virus spreads around the world.

The debate over anti-polio strategies

"Do we continue to review and adjust tactics? The answer is, of course, yes," O'Leary says. In fact a new version of the oral polio vaccine that is less likely to mutate was introduced in 2021. There are attempts in many places to mix the oral and the injectable vaccines in hopes of providing broader protection from the virus.

"But the backbone of our approach is really this house-to-house coverage [with oral vaccine], O'Leary says, "It has been demonstrated over time is that this is the best means of ensuring that we're able to reach each and every child. And that is fundamentally the key to actually achieving the goals we've set for ourselves."

Konstantin Chumakov disagrees. He's the former associate director for research at the U.S. Food and Drug Administration's Office of Vaccines and has been involved in the global effort polio against polio for decades. "If we keep doing the same thing, expecting different results, that's a recipe for failure," he says.

Zulfiqar Bhutta, the founding director of the Institute for Global Health and Development at the Aga Khan University in Pakistan, also believes that the house-to-house campaigns can't go on forever.

"We have had these campaigns for such a long time," Bhutta says. "Remember, the global eradication program started in 1988 and people kept asking, why are you coming every few months with these vaccine doses that we have already had and then kids are getting the paralytic polio despite the vaccine."

There's another reason to rethink the goal of no more vaccines: If human immunity to polio disappears, says Chumakov, the virus could become an incredibly dangerous biological weapon.

"I don't envision the circumstances where we can stop vaccinating, and say, 'Okay, there's no more polio. Forget it.' Just for one simple reason: Polio can be made in a lab in two weeks," he says. More than 20 years ago researchers at the State University of New York, Stony Brook, cooked up infectious polio from scratch using publicly available data.

One of the fundamental goals of the Global Polio Eradication Initiative is to wipe the polio virus off the face of the earth (similar to the way smallpox was eradicated) and then do away with polio immunization programs entirely, says Chumakov. But because the virus can be made in a lab, this goal is unrealistic, he says. He believes that "you cannot stop" polio immunizations: "No, they will have to continue forever, everywhere, indefinitely."

"And there's no reason to stop," adds Chumakov, who is now an adjunct professor at George Washington University and the University of Maryland. But there is a different way to administer the doses. he says. Along with colleagues from the Global Virus network, Chumakov co-authored an article in the New England Journal of Medicine in February, arguing that resources being used to attack polio should instead be folded into programs that beef up all routine childhood immunization programs globally.

Bhutta says polio vaccine — both oral and injectable in an ideal world — should be given along with other childhood immunizations

O'Leary at GPEI says that they fully support rolling polio into routine childhood health programs in places where the health-care systems have the capacity to do it. The problem is that there are still places in the world where that isn't possible. And polio finds those places. It settles in. It multiplies.

"Viruses always find a way," adds Offit from the Children's Hospital of Philadelphia. "Their goal is to be transmitted from one person to the next to the next. And they'll do whatever they can to get there, whether it's polio or SARS-CoV-2 or the common cold virus or influenza virus. They will do what they have to do to survive. And they're smarter than we are."