EExperts have long understood that a new polio vaccine developed to try and minimize the risks associated with Albert Sabin’s oral polio vaccine could also get around the problem it was created. 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 viruses from the new vaccine, referred to as new oral polio vaccine, or nOPV2. (The “2” indicates that the vaccine targets type 2 polioviruses.) In addition, five environmental samples collected in Burundi contained what are known as type 2 circulating vaccine-derived polioviruses, or cVDPV2s.
“We are disappointed,” said Ananda Bandyopadhyay, deputy director for technology, research and analytics in the Bill and Melinda Gates Foundation’s polio team, a partner in polio eradication efforts. “An outbreak like this is disappointing.”
The Gates Foundation is one of six partners in the Global Polio Eradication Initiative. The others are the World Health Organization; UNICEF, the United Nations Children’s Fund; the Centers for Disease Control and Prevention; Gavi, the Vaccine Alliance; and the Rotary International service club.
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 engineered to eliminate their ability to paralyze. Children who receive these vaccines shed live viruses in their stool. In environments where sanitation and hygiene are poor, the viruses can be passed from child to child, effectively indirectly vaccinating children who have not reached the vaccination teams – a feature that has made the Sabin vaccines the workhorse of polio eradication made.
But if the viruses spread long enough, they can regain the ability to paralyze — a problem that led the polio program to stop using the type 2 oral vaccine in 2016, in a bold and ultimately unsuccessful effort known as “the switch”, to stop spread of type 2 viruses from the Sabin vaccines.
The injectable polio vaccine, designed by Jonas Salk and used in affluent countries such as the United States, does not contain live viruses and therefore does not cause 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 succeeded in bringing the number of infections with wild viruses to a low level. Last year, just three countries – Pakistan, Afghanistan and Mozambique – reported 30 cases. Only one case has been discovered so far this year, in a child in Afghanistan.
But as the battle against wild viruses gains ground, the use of the oral vaccine has led to chains of transmission of the vaccine-derived viruses. By 2022, nearly 800 children or young adults in about two dozen countries will develop paralytic polio after being infected with one of the vaccine viruses from the Sabin vaccines. Among them was an unvaccinated young man in upstate New York, the country’s first case of polio in nearly a decade.
Of the three original strains of polio — types 2 and 3 have been eradicated, only type 1 remains — the portion of Sabin vaccines that target type 2 viruses cause the vast majority of vaccine-derived polio cases.
A few years ago, with support from the Gates Foundation, the new oral vaccine against type 2 viruses was developed. It was commissioned 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, which stem from two chains of vaccine-derived viruses, are much less likely than would have happened 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 might have been 30 to 40 new chains of type 2 vaccine viruses in that period, instead of two, he said.
Other experts agreed that it’s important to put the finding in context.
“I am not alarmed. It’s a much better tool than we used to have,” 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 do its job. In any case, don’t cause many outbreaks like this.”
Kim Thompson, president of the nonprofit Kid Risk and a mathematical modeller who has worked to eradicate polio for decades, said this event just shows the world that what was believed about the new oral vaccine is, in fact, true .
“This possibility has always been there in the cards. And this is really just the proof of concept that the nOPV2 can lose the attenuated mutations and behave like other live polioviruses, and especially in populations where [vaccine] coverage is low,” she said.
But Thompson is concerned that given low levels of immunity to polio type 2, even less frequent outbreaks of vaccine-derived viruses will compound a problem the polio program is trying to contain.
“The reality is that as we have transmission in these areas of low coverage and this immunity gap that exists…there is more room for these viruses to disappear. That’s part of the challenge here, to figure out what to do to stop type 2,” she said.