Alarming Polio Outbreak Spreads in Congo, Threatening Global Eradication Efforts

"[T]he endgame is proving to be much more complicated than eradicating the wild virus." - Michel Zaffran, Global Polio Eradication Initiative (GPEI), World Health Organization (WHO)

This news piece explains the causes and ramifications of the polio outbreak in the Democratic Republic of the Congo (DRC), an economically poor country with a weak surveillance system and issues of insecurity that can impede efforts to reach children with the oral polio vaccine (OPV). First detected in June 2017, the outbreak has, as of this writing, paralysed 29 children across the country, with one case reported on the border with Uganda.

As is explained here, the outbreak is caused not by the wild virus still endemic in Afghanistan, Pakistan, and Nigeria, but by a rare mutant derived from the weakened live virus in the OPV. As OPV campaigns have driven the wild virus to near-extinction, these circulating vaccine-derived polioviruses (cVDPVs) have emerged and could spiral out of control, setting eradication efforts back years. For a short time after vaccination with OPV, the weakened live virus can spread from person to person, boosting immunity even in those who didn't receive the polio drops. But in countries such as the DRC where many children have not been vaccinated, the virus can continue circulating for years, accumulating mutations until it reverts to its dangerous form.

In 2016, poliovirus type 2 had been eradicated in the wild, which meant that every type 2 virus originated from the vaccine itself. In April of that year, the 155 countries still using the trivalent vaccine replaced it with a bivalent vaccine with the type 2 component removed. It was clear that for a few years some type 2 outbreaks would still occur - either those that had started before "the switch" but had not been detected or those caused by the last use of trivalent OPV. To fight these outbreaks, GPEI created a new monovalent OPV type 2 (mOPV2), which can only be released with the approval of the director-general of WHO. If mOPV2 is used judiciously and sparingly, it can stop an outbreak without starting a future one, but speed is of the essence, because population immunity to the type 2 virus is waning now that it has been removed from the vaccine.

In the worst case - if type 2 expands across Africa, or if case numbers rise exponentially - OPV2 would have to be reintroduced into routine immunisation. This would mean that the switch failed and that the eradication quest could be delayed and made even more expensive. But the GPEI's Michel Zaffran is quoted here saying that type 2 cVDPVs "can be managed. The only question is for how much longer," he says. "I have yet to see anything that makes me think eradication is not possible."