EExperts have long understood that a new polio vaccine designed to minimize the risks associated with the oral polio vaccine manufactured by Albert Sabin could also end in the problem it created , is bypassed. It is now clear that the theoretical risk is a real one.
The Global Polio Eradication Initiative announced Thursday that six children in the Democratic Republic of Congo and one in Burundi have been paralyzed by viruses from the new vaccine, dubbed the novel oral polio vaccine, or nOPV2. (The “2” signals that the vaccine targets type 2 poliovirus.) In addition, five environmental samples collected from Burundi contained what is known as type 2 circulating vaccine-derived poliovirus, or cVDPV2.
“We are disappointed,” said Ananda Bandyopadhyay, associate director for technology, research and analytics on the polio team at the Bill and Melinda Gates Foundation, a partner in polio eradication efforts. “Any outbreak of this nature is disappointing.”
The Gates Foundation is one of half a dozen partners in the Global Polio Eradication Initiative. The others include the World Health Organization; UNICEF, the United Nations Children’s Fund; the Centers for Disease Control and Prevention; Gavi, the Vaccine Alliance; and the service club Rotary International.
Bandyopadhyay and the polio eradication initiative itself were quick to point out that this turn of events was not unexpected. The live polioviruses used in oral vaccines are manipulated to eliminate their ability to cause paralysis. Children who receive these vaccines shed live virus in their stools. In environments with poor sanitation and hygiene, the viruses can be transmitted from child to child and indirectly vaccinate children who have failed to reach immunization teams — a trait that has made Sabin vaccines the workhorse of polio eradication.
But if the viruses spread long enough, they can regain the ability to paralyze — a problem that prompted the polio program to abandon the use of the oral type 2 vaccine in 2016 in a bold and ultimately failed attempt known as “the change” to stop spreading type 2 viruses through the Sabin vaccines.
The injectable polio vaccine, developed by Jonas Salk and used in wealthy countries like the United States, contains no live virus and therefore does not induce paralysis. But while it prevents paralysis, it can’t stop the transmission of polioviruses — wild-type or vaccine-derived — making it less useful in countries where vaccine-derived viruses are spreading.
In recent years, the nearly 35-year-old effort to rid the world of polio has managed to reduce wild-virus infections to low levels. Last year, just three countries — Pakistan, Afghanistan and Mozambique — reported 30 cases. So far this year, only one case has been detected in a child in Afghanistan.
However, as the fight against wild viruses has gained ground, the use of the oral vaccine has led to chains of transmission of the vaccine-derived viruses. In 2022, nearly 800 children or young adults in about two dozen countries became ill with paralytic polio after being infected with one of the vaccine viruses in the Sabin vaccines. Among them was an unvaccinated young man in New York State, the first case of polio in that country in nearly a decade.
Of the three original polio strains — types 2 and 3 were eradicated, only type 1 remains — the portion of Sabin vaccines that target type 2 viruses triggers the vast majority of vaccine-caused polio cases.
A few years ago, the novel oral vaccine against type 2 viruses was developed with support from the Gates Foundation. It went into operation in mid-March 2021 – two years ago. Since then, 590 million nOPV2 doses have been administered in 28 countries.
The seven cases of paralytic polio, which stem from two strains of vaccine-derived viruses, are far fewer than would likely have occurred if those hundreds of millions of doses had been the Sabin vaccine, Bandyopadhyay said. An analysis by the Gates Foundation’s polio team suggested there would have been 30 to 40 new strains of vaccine type 2 viruses in that period, rather than two, he said.
Other experts agreed that it is important to put the finding in context.
“I’m not worried. It’s a much better tool than it used to be,” said Walter Orenstein, a polio expert at Emory University.
“It’s not perfect,” he said of the new oral vaccine. “But given its rarity, hopefully it will be able to get the job done. At least not generating many such bursts.”
Kim Thompson, president of the nonprofit Kid Risk and a mathematical modeler who has been working to eradicate polio for decades, said this event just showed the world what was believed about the new oral vaccine was actually true.
“This possibility has always been in sight. In fact, this is just the proof of concept that nOPV2 can lose the attenuated mutations and behave like other live polioviruses, especially in populations where it does [vaccine] Coverage is low,” she said.
But Thompson is concerned that with low immunity to type 2 polio, even less frequent vaccine virus outbreaks will exacerbate a problem that the polio program is struggling to contain.
“The reality is that since transmission is happening in these low-coverage areas and this immunity gap that exists, there is more room for these viruses. That’s part of the challenge here, figuring out what to do to stop type 2,” she said.