Cases of polio from a new oral vaccine were reported for the first time

EExperts have long understood that the new polio vaccine, developed to minimize the risks associated with Albert Sabin’s oral polio vaccine, may also cause the problem it created to be ignored. It is now clear that the theoretical risk is real.

The Global Polio Eradication Initiative announced Thursday that six children in the Democratic Republic of Congo and one in Burundi have been paralyzed by a new vaccine called the new oral polio vaccine, or nOPV2. (“2” means the vaccine targets type 2 polioviruses.) In addition, five environmental samples collected from Burundi contained so-called type 2 circulating vaccine-derived polioviruses, or cVDPV2s.

“We are disappointed,” said Ananda Bandyopadhyay, associate director of technology, research and analytics at the Bill and Melinda Gates Foundation Polio Group, a partner in the polio eradication project. “Any outbreak like this is disappointing.”

The Gates Foundation is one of half a dozen partners in the Global Polio Eradication Initiative. Others include the World Health Organization; UNICEF, United Nations Children’s Fund; Centers for Disease Control and Prevention; Gavi, Vaccine Alliance; and 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 remove their paralytic ability. Children who receive these vaccines shed live viruses in their feces. In settings with poor sanitation and hygiene, viruses can be transmitted from child to child and effectively indirectly vaccinate children not reached by vaccination teams. This feature has made Sabin vaccines the workhorse of polio eradication.

But if the viruses circulate long enough, they can regain the ability to paralyze — a problem that led to the end of the type 2 oral polio vaccine in 2016 in a bold and ultimately failed attempt known as a “switch” to spread type 2 viruses from Sabin vaccines.

The injectable polio vaccine designed by Jonas Salk, used in wealthy countries such as the United States, does not contain live viruses and therefore does not cause paralysis. But while it prevents paralysis, it cannot stop the spread of polioviruses — wild-type or vaccine-derived — making it less useful in countries where vaccine-derived viruses are prevalent.

In recent years, the nearly 35-year effort to rid the world of polio has succeeded in reducing the number of 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 identified, in a child in Afghanistan.

But as the fight against wild viruses has gained ground, the use of the oral vaccine has spawned chains of transmission of vaccine-derived viruses. In 2022, nearly 800 children or young adults in about twenty countries contracted 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 country’s first case of polio in nearly a decade.

Of the three original polio strains—types 2 and 3 have been eradicated, only type 1 remains—the Sabin vaccine that targets type 2 viruses triggers the majority of vaccine-derived polio cases.

A few years ago, a new oral vaccine targeting type 2 viruses was developed with the support of the Gates Foundation. It was introduced in mid-March 2021 – two years ago. Since then, 590 million doses of nOPV2 have been administered in 28 countries.

The seven cases of paralytic polio from the two strains of vaccine-derived viruses are far fewer than would likely have occurred if the hundreds of millions of doses had been the Sabin vaccine, Bandyopadhyay said. An analysis by the Gates Foundation’s polio team suggests that there would have been 30 to 40 chains of type 2 vaccine viruses during that time, rather than two, he said.

Other experts agreed that it is important to put the observation in context.

“I’m not worried. It’s a much better tool than we had before,” said Emory University polio expert Walter Orenstein.

“It’s not perfect,” he said of the new oral vaccine. “But because of its rarity, hopefully it will be able to do its job. At least it won’t create many outbreaks like this.”

Kim Thompson, president of the nonprofit Kid Risk and a mathematical modeler who has worked on polio eradication for decades, said this event just shows the world that what was assumed about the new oral vaccine is actually true.

“This possibility has always been in the cards. And really, this is just proof of concept that nOPV2 can lose the weakened mutations and behave like other live polioviruses, and especially in populations where [vaccine] coverage is low,” he said.

But Thompson worries that because immunity to type 2 polio is low, even fewer outbreaks of the vaccine-borne virus will add to the problem the polio program is trying to contain.

“The reality is that because we have infections in these areas with low coverage and this immunity gap … there is more room for these viruses. That’s part of the challenge here is figuring out what to do to stop Type 2,” he said.

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