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A last push to eradicate polio

February 1, 2011 By Gayathri Vaidyanathan This article courtesy of Nature News.

Funding gap persists as agencies and organizations attempt to wipe out the tenacious virus.

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Some 99% of wild poliovirus has been eradicated, but it clings on in a few places. The last endemic hot spots are the conflict-ridden front lines of Pakistan and Afghanistan, areas of India and Nigeria — and governments and charities are scrambling to eliminate it entirely.

Bill Gates, the Microsoft co-founder and co-chair of the Bill & Melinda Gates Foundation headquartered in Seattle, Washington, announced in his annual letter yesterday his commitment to eradicate polio by 2012, by giving the vaccine to all children under five in poor countries. The initiative is led by the Gates Foundation and the World Health Organization's (WHO) Global Polio Eradication Initiative (GPEI), which includes among other organizations Rotary International, a non-profit foundation headquartered in Evanston, Illinois.

Nature examines the challenges that remain before the virus can be wiped out.

Why is polio so much harder to eradicate than smallpox?

The virus causes symptoms of paralysis in fewer than 1% of the people it infects. The rest are silent carriers. The oral polio vaccine is not as effective as the smallpox vaccine — three or more doses are needed for protection.

How prevalent is the wild virus today, country by country?

The number of cases in India dropped to 42 from 741 recorded in 2009, the result of a vaccine that is able to target two strains of polio virus simultaneously. Nigeria reported only 20 cases, down from 388 the previous year. Parents here, mainly in the conservative Muslim community, who once worried the oral polio vaccine would sterilize their children are now allowing immunization, thanks to the support of religious leaders in the country.

The front lines of the battle to eradicate the virus are now the tribal belt of Pakistan and the bordering foothills of Afghanistan. Pakistan had 144 cases last year, up from 89 in 2009, though the actual number of cases is probably higher.

The virus also migrates from the four endemic hotspots to other countries. There are outbreaks in Chad, Sudan, Angola and the Democratic Republic of Congo that have been going on for several months.

Short-lived outbreaks also occur, with Tajikistan reporting 458 cases last year. The most surprising outbreak occurred in the Republic of Congo in November. Although the virus normally infects children, a sudden surge primarily hit young men between 15 and 25 years old. The mortality rate was also higher than 40%, which is roughly double that seen in unimmunized groups, says Neal Nathanson, associate dean at the University of Pennsylvania School of Medicine in Philadelphia. The outbreak is suspected to have caused 179 deaths, according to the WHO.

What are the challenges in targeting polio in Pakistan?

Owais Ahmed Ghani, the governor of the North West Frontier Province where most of the polio cases were recorded last year, says implementation is his biggest challenge.

Ongoing conflict means around one-quarter of the children in the province are inaccessible to health workers, he said at a press conference yesterday in New York. Parents who fear the vaccines will sterilize their children have denied access to another 0.6%, he says. His administration is in talks with the army to start immunization programmes in remote and conflict-ridden places.

Why does polio crop up in places previously declared polio-free?

The poliovirus is remarkably adept at finding pockets of people who are not immunized, leading to outbreaks, according to Nathanson. No matter how thorough immunization campaigns are, it will be difficult to reach everyone.

In the case of the Republic of Congo, polio infected a group who were presumably not immunized during the years of the civil war in the late 1990s. The US Centers for Disease Control and Prevention (CDC) and the WHO are investigating this hypothesis. Other questions remain, such as why the mortality rate was so high and why young men were affected more than young women.

Could there be other unvaccinated pockets? Why is it not okay to stop vaccinations now that cases have declined by 99%?

There are likely to be other pockets, says Mark Pallansch, chief of the enterovirus section at the CDC's National Center for Infectious Diseases in Atlanta, Georgia. This makes it essential that the virus be stamped out in the four endemic countries to prevent it from being carried elsewhere1. The WHO is working on mathematical modelling and reassessing its surveillance methods to identify unimmunized populations.

How much money is still needed to wipe out the virus?

Around US$9 billion has already been spent on eradication attempts since the campaign began. The GPEI has estimated it will need a further $1.86 billion between 2011 and 2012. The organisation has already raised over $1.1 billion but still need to raise another $720 million in order to complete its vaccination programme.

There has been some good news in the past week. The Gates Foundation announced an additional $102 million for the effort at the World Economic Forum in Davos, Switzerland. The crown prince of Abu Dhabi pledged $50 million to vaccinate children for polio and other illnesses, and the UK government has doubled its commitment to $60 million, as long as other countries match its contribution 5 to 1. But that is some way short of plugging the gap.

What happens if and when the wild virus is 100% eradicated?

Complete eradication does not spell the end for polio. Once the wild virus is eliminated, the weakened living virus present in the oral vaccine will persist. These are called "vaccine derived polio viruses" or VDPVs, and in rare cases they have reverted back to virulence and caused outbreaks. Between 2005 and 2009, Nigeria witnessed 2922 cases of polio caused by VDPVs rather than the wild virus.

In the post-polio era, VDPVs will be the new target. Vaccinations would need to switch from the oral polio vaccine to the inactivated polio vaccine, which contains dead virus, according to Pallansch.


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