Polio Eradication: New Progress toward an Elusive Target

Published - Written by Rena Xu

The quest to eradicate poliovirus has been ongoing for more than 20 years, but today, despite a 99% reduction in the annual incidence of paralytic poliomyelitis, eradication remains a largely unattained goal.

This is not for a lack of trying. The Global Poliomyelitis Eradication Initiative (GPEI) of the World Health Organization (WHO) has spent more than $8 billion over the last two decades on the quest for eradication. As this week’s Perspective by John F. Modlin of Dartmouth Medical School explains, eradicating poliovirus has proven challenging on a number of fronts.

One major concern is the emergence of circulating vaccine-derived polioviruses (cVDPVs), viruses that have reverted to virulence and, like wild-type viruses (WPVs), can cause ongoing chains of transmission and paralytic disease.

A study by Jenkins et al., published in this week’s issue, examined a large outbreak of type 2 cVDPV. The outbreak occurred in Nigeria due in part to poor immunization rates with the trivalent oral poliovirus vaccine (OPV), leading to a large segment of the population being non-immune to polioviruses, and included multiple independent lineages of type 2 cVDPV. Type 2 WPV was eradicated globally in 1999.

Significantly, the Jenkins study suggested that cVDPVs may be just as virulent and transmissible as WPVs. It also showed that circulating cVDPVs can be controlled by vaccinating with the trivalent OPV, a finding that has implications for future attempts to control or eradicate poliovirus. NEJM deputy editor Dr. Lindsey Baden states, “These data highlight an important concern associated with the global OPV vaccine strategy – the emergence of virulent transmissible poliovirus strains; however, large-scale vaccination campaigns can be effective at addressing this untoward event.”

Another growing concern is the affordability of vaccines. Given the emergence of cVDPVs, the use of live OPVs will need to be carefully considered globally, both in the design for the end-game regarding eradication and subsequently if poliovirus eradication is achieved. The inactivated poliovirus vaccines (IPVs) will become an important tool in these circumstances for maintaining immunity. Currently, however, IPVs are a lot more expensive and complicated to administer and so may not be affordable for many developing countries. The weighted average purchase price per dose when purchased by the United Nations Children’s Fund (UNICEF) is $3 for the IPV, compared to $0.15 for the trivalent OPV.

A study by Mohammed et al., also published in this week’s issue, showed that one-fifth the standard dose of IPV, when delivered intradermally using a needle-free device, can achieve seroconversion to all poliovirus serotypes comparable to that induced by the standard dose delivered intramuscularly. The study findings point to a potential dose-sparing strategy that can significantly reduce the cost of IPV immunization. These reduced costs are still higher than the cost of OPVs, and the proposed strategy will need to be evaluated against other approaches to cost reduction, such as decreasing the number of doses or using adjuvants. But as Dr. Baden states, “This study puts hard data points on what is possible and represents an important step toward developing an affordable IPV.”

Given the risk of cVDPVs and the difficulties of implementing large-scale vaccinations in many regions, do you think eradication is a realistic goal?

In resource-constrained environments, how do you think governments should prioritize poliovirus eradication versus competing demands for health care funds?