Global Polio Eradication Falters in the Final Stretch

"Fighting these flare-ups will mean difficult decisions in the coming year."

In this news article, Leslie Roberts reports on vaccine issues related major setbacks that the decades-long campaign to eradicate polio suffered in 2019. While the effort lost ground in 2 of the remaining polio-endemic countries, Afghanistan and Pakistan, Roberts focuses on the situation in Africa, where 196 children in 12 countries were paralysed by vaccine-derived polio virus type 2, a strain of the virus derived from the live oral polio vaccine (OPV) that has regained its virulence and ability to spread.

Roberts explains that, for decades, OPV contained a mix of 3 weakened polio viruses, all 3 of which have the potential to revert to more dangerous versions. Wild serotype 2 was last sighted in 1999, so in 2016, as a first step in the "endgame" to eradicate polio, all 155 countries using OPV took part in "the switch" - replacing the trivalent version with a bivalent one without the type 2 component. Some type 2 outbreaks would inevitably occur for several years, the Global Polio Eradication Initiative (GPEI) partner organisations realised, but those would be addressed with a live, monovalent vaccine targeted against type 2 (mOPV2). Most of today's outbreaks stem from mOPV2 responses to previous ones, and the risk of outbreaks around the globe is ratcheting up, because millions of children born since the switch have little or no immunity to type 2 virus.

The World Health Organization (WHO)'s Michel Zaffran, who leads the GPEI, indicates there is room to make better use of mOPV2 by detecting outbreaks sooner, getting money and vaccines to countries earlier, and reaching more children. But hopes are pinned on a novel OPV (called nOPV2) that could both trigger an immune response and be unlikely to regain virulence. At the time of this writing, Phase II studies were underway in Belgium and Panama. GPEI is also pushing for an Emergency Use Listing, a WHO mechanism that would enable the programme to deploy the vaccine while it collects more data.

Roberts explains that, if nOPV2 doesn't work or vaccine-derived outbreaks occur before it is ready, the GPEI might have to resurrect the trivalent live vaccine, which would reintroduce immunity against type 2 in young children while maintaining protection against serotypes 1 and 3. Such a move "would be an enormous blow to the polio program and to international public health," according to Nicholas Grassly, a modeler and epidemiologist at Imperial College London. Roland Sutter, former director of polio research at the WHO, agrees that this would be a hard decision to communicate, given the major global effort that went into persuading countries to switch to the bivalent vaccine in the first place (the switch).

At the end of the day: "No vaccine can stop polio if it doesn't get into children's mouths, program leaders and their advisers caution - and that has been a long-standing problem anywhere the virus, vaccine-derived or wild, still circulates. The polio eradication program has been struggling with complacency, fatigue, resistance, and poor planning - all human issues that technology can't fix."