May 8, 2020 will mark the 40^th anniversary of the groundbreaking declaration by the World Health Assembly that smallpox—a disease that had plagued humanity for over two millennia—had been completely eradicated following the vaccination efforts of the World Health Organization (WHO). This year also marks the 1^st anniversary of another less publicized milestone: the eradication of the second of three strains of Polio virus, thanks to another herculean campaign by the WHO, in combination with other public health organizations. But even as the campaign for the elimination of Polio draws closer to the finish line, there is the ever-growing danger of a global resurgence of the virus.

The History

The Global Polio Eradication Initiative (GPEI) began in 1988, decades after the introduction of the first Polio vaccine by Jonas Salk in 1955, followed by development of a more easily administered oral vaccine by Albert Bruce Sabin in 1961. Poliovirus was chosen as the next focus for global eradication after the success of smallpox for two reasons: its prevalence and severity in children, and its unique virus properties which make it well-suited for elimination via vaccination.

For a virus to be completely eradicated through prevention, there must be certain natural hosts and reservoirs of the virus, as well as an ability for the virus to survive outside of a host for an extended period of time. Poliovirus is unique because it can only naturally infect humans as a host. It is not carried in healthy individuals for an extended period of time, and it cannot survive more than two months outside of a host. The combination of these traits make vaccination against the virus highly effective, and a vaccinated population can act as a barrier to prevent spread between two non-immunized populations. Essentially, vaccinating every susceptible child against Poliovirus and maintaining the immunization for just two months will rob the virus of a viable host, killing it off completely. The GPEI has eliminated two of the three strains of Poliovirus worldwide and has provided vaccination coverage in nearly every country in which Poliovirus was still endemic at the start of the initiative. With only a few dozen reported cases of Type 1 Poliovirus annually, it seems as though this vaccination effort will soon join that of the smallpox campaign as a worldwide success.

The Problem

The final barrier to Poliovirus elimination is twofold:

1. The remaining countries in which the virus is still endemic are particularly difficult for the initiative organizations to reach
2. The nature of the vaccine has resulted in the emergence of a new vaccine-derived form of Poliovirus.

Pakistan and Afghanistan are two countries that recently reported cases of Polio. These nations are incredibly difficult for charitable or non-profit organizations to access due to limited infrastructure, as well as political unrest and social conflict. The fight to overcome these barriers is ongoing, but until the GPEI has access to these final reservoirs of Poliovirus, the risk of resurgence remains.

The length of the Polio vaccination effort has also led to a new danger to the eradication effort: the emergence of a vaccine-derived Poliovirus strain. The oral Poliovirus vaccine contains live attenuated virus, which results in a strong immune response and thus highly effective immunity against the natural virus but also presents the (extremely rare) possibility of viral mutation into an infectious form, should the attenuated virus be allowed to circulate in a poorly-vaccinated population for an extended amount of time. This situation has now occurred in several countries that have participated in the GPEI.

The Solution?

While the emergence of vaccine-derived Poliovirus is concerning, we have an effective solution: an efficient, high-coverage vaccination with multiple rounds of high-quality immunizations. The unreachable Poliovirus reservoirs in the Middle East are a pressing and difficult issue. Until these populations can be vaccinated, there is a risk of infected individuals from these endemic countries reaching and infecting unvaccinated individuals from countries in which Poliovirus is no longer present. GPEI efforts in these countries must therefore continue indefinitely with each generation to ensure lasting immunity against Poliovirus. Not only is this extremely costly, but the popularity of anti-vaccination campaigns in several developed nations where Polio vaccination is still the norm has placed even more individuals at risk should an outbreak occur. The resolution of this complex problem lies more within the geopolitical sphere than the GPEI. The solution will depend upon the spread of true and accurate information regarding the Poliovirus vaccine and an increased emphasis on the importance of the eradication effort to those with the power to effect appropriate change.

By Ellie Lesko, PhD Candidate

Sources

The World Health Organization @ https://www.who.int/

The Centers for Disease Control and Prevention @ https://www.cdc.gov/

Polio Global Eradication Initiative @ http://polioeradication.org/