Something momentous happened in the history of polio eradication in April 2016: Over a period of 2 weeks, 155 countries and territories started to use a new version of Albert Sabin’s classic oral polio vaccine (OPV) that no longer protected against one of the three types of poliovirus. Type 2 virus had been eradicated by then, and the only remaining type 2 polio cases were touched off by the live virus in the vaccine itself. Dropping the type 2 component from the vaccine would end those cases as well, the thinking went.

But “the switch,” as this global move has become known, became “an unqualified failure,” according to an unusually blunt draft report commissioned by the Global Polio Eradication Initiative (GPEI) that is now open for public comments. Unexpectedly, vaccine-derived poliovirus type 2 has continued to circulate after the switch, paralyzing more than 3300 children. And GPEI has spent more than $1.8 billion trying to quash these outbreaks, mostly in Africa. Those numbers are certain to increase until the polio program finds a way to deal with the problem it inadvertently—and with the best of intentions—created.

“It is about time someone publicly declared the switch a failure, given the obvious management and leadership errors,” says Kimberly Thompson, who heads Kid Risk, Inc., a nonprofit that has long modeled the consequences of various polio vaccine options.

The authors of the report wanted it to catch the program’s attention. “We need to recognize that these are paralyzed children in some of the hardest places in the world,” says Natalia Molodecky, a consultant for the Task Force for Global Health. “These are very real consequences of the program.” But, she adds, the switch was a very difficult operation to get right. “This is a real-life example that we can learn from to guide future action.”

“We are trying to call a spade a spade,” says her co-author Roland Sutter, who, as the head of polio research at GPEI until 2020, was one of the architects of the switch. “I hope the report will be a wake-up call for the program,” Sutter says. “Polio eradication has been my life’s work and ambition, and I would really like to see it succeed.” He still thinks it’s doable.

What the switch was intended to do, reduce vaccine-derived type 2 cases to zero, “clearly didn’t happen,” acknowledges Aidan O’Leary, director for polio eradication at the World Health Organization (WHO), one of six core partners in GPEI. “The [case numbers] speak for themselves. The key is what we do about it.”

OPV, made of live,  weakened polioviruses, is the most effective vaccine for eradicating polio because it induces strong immunity in the gut and spreads through the stool of immunized children, protecting even those who don’t get the vaccine drops. OPV has helped bring down polio cases by more than 99.9% since the eradication program began in 1988. The “wild” poliovirus is now cornered in Afghanistan and Pakistan, where it has paralyzed four children so far this year.

But in rare instances, the weakened OPV viruses can regain their ability to paralyze and start to circulate among susceptible children, sparking new polio outbreaks in areas where vaccination rates are low. That’s why the endgame of the eradication initiative calls for stopping all use of OPV once the wild virus is gone and introducing vaccine made of inactivated virus (IPV), which can’t revert, into routine immunization.

Planned after wild type 2 poliovirus had been eradicated, the switch—which the report calls “the largest coordinated public health effort in history”—was like a trial run for that ultimate goal. GPEI created a stockpile of monovalent vaccine targeting only type 2 to respond to the inevitable but, GPEI thought, small, vaccine-derived type 2 outbreaks that would initially occur. After a few years, the reasoning was, the world could stop worrying about type 2.

Some countries were indeed able to quash type 2 outbreaks with the monovalent vaccine. But in Africa, the “worst-case scenario” quickly materialized, the draft report says: Type 2 outbreaks began raging out of control as an increasing number of children who were no longer receiving the type 2 virus in routine immunization became susceptible to it. Between 3 and 4 years after the switch—“the point of no return,” the report says—cases jumped from 84 in seven countries to 548 in 21 countries. The number of cases has increased about 10-fold since 2015.

How could that happen? The draft report says the monovalent vaccination campaigns GPEI launched in response to outbreaks were too limited geographically, came too late, and often didn’t reach enough children. Because these outbreaks occurred in places where routine immunization rates were low, there was no backdrop of protective immunity.

Another factor, the authors say, is the “inability or unwillingness of GPEI leadership to recognize the seriousness of the evolving problem and take corrective action.” A strategy committee with representatives from GPEI’s core partners—WHO; UNICEF; the U.S. Centers for Disease Control and Prevention (CDC); Rotary International; the Bill & Melinda Gates Foundation; and GAVI, the Vaccine Alliance—governs by consensus. A key example of its inaction, Molodecky says, is that the evaluation was commissioned only in August 2023, more than 7 years after the switch. “Having a formal review at year three … would have enabled the program to make course corrections,” she says.

An underlying problem is that GPEI has long regarded vaccine-derived viruses as less dangerous or of secondary importance compared with wild virus, Sutter says, even though they’re the same biologically and epidemiologically: “There’s only poliovirus. It paralyzes kids.” When wild type 1 poliovirus jumped from Pakistan to Malawi and Mozambique in 2022, GPEI was on it immediately, quickly snuffing out the outbreak. If GPEI had applied the same urgency to vaccine-derived virus, “we would be in a different place,” Sutter says.

Now the goal is to stop the ongoing outbreaks. Faster, broader responses will help, as will a novel type 2 polio vaccine that is far less likely to spark outbreaks, and new vaccines are in development. The report also calls for the broader use of IPV, which doesn’t stop transmission but prevents paralysis, in routine immunization and outbreak control. Boosting routine immunization in vulnerable populations is key.

As for the future cessation of all OPV, the lessons from the switch are “unambiguous,” the authors write: It should not be tried until GPEI has not only eradicated the wild poliovirus, but also stopped the persistent transmission of vaccine-derived viruses. “It would be better to take the time, get it right, than to rush, and fail spectacularly,” the draft report says.

The authors end with an unusual epilogue for a technical report, saying there’s a “moral imperative” for the partners in GPEI to provide more rehabilitation and education to the thousands of children who have been paralyzed by vaccine-derived polio-virus type 2. “I do feel we have a responsibility, myself as well, to take care of these children,” Sutter says. The authors also call on GPEI to “take a hard look” at how to lower the risk to polio workers and their guards, more than 100 of whom have been assassinated in the past dozen years—most of them in Pakistan—and whether their families receive enough monetary compensation. The longer eradication takes, the more such murders may occur, the authors say.

O’Leary, who notes the draft is still not finalized, says GPEI will consult with its management and oversight groups, outside experts, and WHO member states on how to proceed with OPV cessation, making sure lessons from the switch are incorporated. Decisions need to be made by early 2025.

The report is “a hard read,” says John Vertefeuille, director of CDC’s global immunization division and a member of the strategy committee that governs GPEI. But, he says, “I also think it will lead us to forging very effective paths to finish the job” of eradication.