The Crier

Beating Polio

Wild poliovirus has been driven to the brink of extinction, but significant challenges to its eradication still exist

Eugene Morrow · The Empiricist · Mar 06, 2007

The World Health Organization’s latest report on global polio eradication should make even the most cynical public health officials smile. The report puts the number of polio-endemic countries at just four, the lowest number ever. Last year, there were a mere 1,951 polio cases worldwide. This is a decrease of over ninety-nine percent since the Global Polio Eradication Initiative began in 1988. Polio is on the brink of eradication. Not since smallpox was obliterated has there been such cause for celebration.

The campaign against polio has thrived since the late ’80s. Cheap, effective vaccines have made near-heroic vaccination rates possible. First world investment has ensured comprehensive and widespread surveillance of the disease, and superb coordination of supplementary immunizations has helped quash outbreaks in the most at-risk areas. By the late ’90s, such accomplishments led the CDC to predict that polio would be extinct by 2003.

But beating the disease has proved harder than expected. In recent years, previously polio-free countries have experienced outbreaks. This raises doubts about the current campaign’s feasibility. The problem can be traced to the eradication effort’s workhorse: oral poliovirus vaccine.

Polio is on the brink of eradication. Not since smallpox has there been such cause for celebration.

Oral poliovirus vaccine consists of live attenuated virus — strains of polio bred to be harmless. They work by creating symptom-free infections in their recipients. They can be administered orally, while conferring robust immunity. And they can be transmitted person to person, just like wild polio. This transmission was considered a blessing in the early days of eradication; it has since proven to be a curse.

Over the last decade, vaccine-derived viruses have been one of the greatest obstacles to eradication. In 2005, four cases of polio were identified among unvaccinated children in a Minnesota Amish community, despite the fact that wild polio has been absent from the U.S. since the ’60s. The culprit was a vaccine-derived poliovirus that had mutated and recovered its bite.

Oral poliovirus vaccine replicates and evolves just like wild poliovirus. This means that, in some instances, vaccine strains can still cause symptomatic infection.

Ideally, immunization should stop after eradication. But discontinuing immunization would leave the world’s population vulnerable to vaccine-derived outbreaks. What’s more, places like India’s Uttar Pradesh and Bihar have yet to halt wild polio transmission. Despite unprecedented vaccine coverage, such regions still have robust transmission rates.

Ideally, immunization should stop after eradication. But discontinuing immunization would leave the world’s population vulnerable to vaccine-derived outbreaks.

High population density and poor sanitation pose problems in these areas. But so do diseases that out-compete the vaccine virus. Prior infection with viruses similar to polio keeps the tame vaccine strains from establishing immunity. Therefore, vaccination in these areas is much less effective.

These challenges are not insurmountable. The World Health Organization has already started using type-specific vaccines, which, despite protecting against only one of the three polio strains, are more efficient than their multi-strain cousins. By targeting areas with high single-strain transmission rates, type-specific vaccines can significantly reduce the burden of disease.

The WHO is also coordinating a post-eradication halt to immunization. If executed correctly, this would slash the risk of vaccine-derived outbreaks. The greatest obstacle, it seems, is complacency.

As polio becomes ever more rare in the developed world, funding is getting scarcer. The WHO expects a funding gap of $400 million in 2007. This is cause for concern. Cases like the Minnesota outbreak should remind us of the need for continued vigilance. Polio is teetering on the edge of annihilation. It would be a shame if nearly two decades of work were spoiled on the eve of victory.