In August, the World Health Organization (WHO) certified the African region free of wild polio. This immense achievement, years in the making, is a testament to the power of vaccines.
Vaccination is how the world eradicated smallpox—the only disease successfully eradicated so far. And vaccination has gotten us 99 percent of the way there with polio, but the end of wild polio will not necessarily mean the end of the disease.
Reaching the brink
Most of the world’s polio progress is owed to oral polio vaccine (OPV). Since the launch of the Global Polio Eradication Initiative—spearheaded by WHO, Rotary International, US Centers for Disease Control and Prevention, UNICEF, Bill & Melinda Gates Foundation, and Gavi, the Vaccine Alliance—18 million people are walking today who would have otherwise been paralyzed by polio, thanks to OPV.
Unlike inactivated polio vaccine, which primarily protects the person vaccinated, OPV protects the individual and prevents person-to-person transmission. It’s also inexpensive and easy to deliver with minimal training.
OPV does carry a miniscule risk—because it uses a weakened or “attenuated” version of live poliovirus to elicit an immune response, remnants of the live virus can be shed from individuals recently vaccinated with OPV. In areas with insufficient vaccination coverage, the attenuated virus can then circulate among susceptible individuals.
Though rare, if this transmission goes on long enough in under-immunized populations—usually about 12 months—the weakened virus can genetically change and revert to its virulent state, giving rise to what is called “vaccine-derived” poliovirus.
Even rarer is reversion in the vaccine itself, causing disease in the vaccine recipient or in close, non-immune contacts. How rare? One case for every 2.7 million doses—roughly four times less likely than being struck by lightning.
Though OPV has been highly effective, the existence of vaccine-derived cases means that—once all wild polio strains are eliminated—the world will eventually have to withdraw OPV to achieve full eradication.
It is key to remember that, if a population is fully immunized, it will be protected against both wild and vaccine-derived poliovirus. Until now, doubling down on strong OPV coverage has been our best hope of hedging against new polio outbreaks. But PATH and our partners are working on new solutions.
The last mile to eradication
OPV has been so effective in reducing the burden of wild polio, these rare vaccine-derived cases now outnumber wild cases.
To accelerate global progress toward eradication, PATH and our partners have been advancing research on novel oral polio vaccine (nOPV) candidates since 2011. The primary focus has been polio type 2, since the circulation of this strain is the most common cause of vaccine-derived cases.
nOPV2 works the same way as OPV, but studies have demonstrated that the weakened virus in nOPV2 is less likely to revert to a virulent state that causes disease.
Due to the urgency to deploy a vaccine less likely to seed new outbreaks, WHO recently approved nOPV2 for initial use deployment under its Emergency Use Listing procedure.
This time-limited listing, applied during other health emergencies like Ebola and Zika, will enable rapid availability of nOPV2 to high-risk regions based on the strong safety and immunogenicity data gathered to date in Phase 1 and 2 clinical trials.
In parallel, PATH and our partners continue to gather clinical data to support product licensure and WHO prequalification of nOPV2 to help ensure long-term access to the vaccine. And we’re advancing clinical trials of nOPV1 and nOPV3 candidates for the potential replacement of the current OPV that protects against these strains.
For only the second time in human history, total eradication of a deadly disease is within reach. Paired with existing outbreak tools and strategies, nOPVs have the potential to get us there faster.