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Polio: Eradication

In this focus

The final hurdles in eliminating polio from the last affected countries, and hopes for a polio-free future.

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Introduction

The final hurdles to polio eradication are in Africa and Asia, where six countries remain endemic for wild-type polio (Egypt, Niger, Nigeria, Afghanistan, India and Pakistan). In Nigeria, disruption to polio immunisation campaigns caused by unfounded fears of vaccine contamination with HIV and contraceptives, led to 789 cases of polio in 2004 - the highest of any country. Another 12 countries, previously free of the disease, saw new polio cases 'imported' from Nigeria, including Chad, Burkina Faso, Ghana, Todo and, more recently, Guinea and Mali.

Despite the set-back, the WHO and other GPEI partners - Rotary International, UNICEF and the Centers for Disease Control and Prevention - remain optimistic that polio eradication is still possible in 2005, and are intensifying their efforts to reach and vaccinate all children.

"These countries have never been more ready to do the job than they are now. Things are at the maximum as far as going in and finishing the job," says WHO's global co-ordinator, David Heymann.

The overall trend looks good - the numbers have plummeted from 350,000 polio cases globally in 1988 when GPEI began its campaign, to 1263 in 2004 (as of 15 March 2005). Indeed, GPEI hopes to see zero cases in Asia in 2005 - another milestone.

The next few months will be crucial. The 'almost there' mantra has been repeated several times, and is in danger of losing its lustre. The last pockets of virus remain the toughest to eradicate due to the difficulty of immunising sufficient numbers of children in affected countries. But only when eradication is achieved can the world focus on the 'end-game' of withdrawing live oral polio vaccine from use - to protect against vaccine associated polio paralysis - and to ensure that polio never returns.

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Polio: biology and disease

How the poliovirus causes paralysis and death

Polio is a contagious viral disease that can strike at any age, but mostly affects children under the age of five. The poliovirus enters the body via the mouth, multiplies in the throat and gut, enters the blood stream and, unless impeded by antibodies, invades the spinal cord and destroys motor neurons. Most patients have no symptoms, while around five per cent experience fever, sore throat, nausea and vomiting, and one in every 200 becomes irreversibly paralysed, usually in the legs. There is no cure, only protection through vaccination.

Poliovirus is a relatively small virus (27 nanometres in diameter) with genetic material consisting of ribonucleic acid, or RNA. Research continues into the molecular events underpinning infection, in the hope of developing new drugs against this and related viruses.

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Outbreaks

Polio spreads from person to person via infected saliva and stools. Although most countries have eradicated wild-type virus it continues to circulate in South Asia and Africa, fuelled by conditions of poor hygiene. Newborns in every country need vaccinating to protect against polio outbreaks which can occur either when an infected person arrives from another country - imported cases - or when the live OPV virus mutates back to a virulent form. Reversion to virulence is rare, occurring in two to four per million babies vaccinated, but the risk remains all the while OPV is in use.

The danger of OPV was particularly highlighted by outbreaks of vaccine-associated paralytic polio occurred in areas that were previously polio-free: Madagascar (four cases in 2002), Hispaniola (22 cases in 2000 - 2001) the Philippines (three children in 2001) and, upon retrospective analysis, Egypt (at least 32 cases in 1988). To avoid the risk of VAPP, many countries in Western Europe and North America, as well as Australia, New Zealand, and Guam have now switched to using the more expensive inactivated polio vaccine (IPV). Several other countries in Eastern Europe, Middle East as well as Bermuda are using a combination of both OPV and IPV. In the remaining endemic regions of South Asia and Africa, OPV remains the vaccine of choice for its ease of administration and effectiveness.

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Vaccines

What's in today's polio vaccines, their actions and drawbacks

The two main types of polio vaccine used today are based on those developed by Jonas Salk and Albert Sabin. The Salk 'inactivated polio vaccine' (IPV), first declared safe and effective on 12 April 1955, contains all three types of poliovirus killed by treatment with formaldehyde. It elicits protective antibodies in the blood that stop wild type virus infection spreading from the gut to the central nervous system. IPV therefore provides excellent individual protection against paralytic polio, but because it provides only weak gut immunity, it is less effective than the oral polio vaccine (OPV) at preventing the spread of wild poliovirus from person to person.

The Sabin OPV contains live attenuated virus (all three types), is easily swallowed and yet does not cause disease. OPV confers strong gut immunity that limits wild virus multiplication and reduces its spread from person to person. This explains why mass vaccination campaigns with OPV can rapidly stop person-to-person transmission of wild poliovirus.

For a few months, OPV virus is shed into the stools of recently immunised children, and in areas where hygiene and sanitation are inadequate this can result in immunisation of close contacts. OPV is cheaper to produce than IPV and easier to administer as drops by mouth, and can be given by volunteers without the need for healthcare workers, needles, or syringes. OPV remains the vaccine of choice in areas where wild poliovirus is endemic, but has been replaced with IPV in many countries where polio has been eradicated.

Standard OPV consists of all three poliovirus types (trivalent OPV) while a new monovalent type 1 OPV, funded by the Bill and Melinda Gates Foundation, provides a boost in immunity against type 1 poliovirus.

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End game

The strategy to phase out vaccines after polio eradication, and ensure that the disease never returns.

The Global Polio Eradication Initiative (GPEI) hopes to achieve its goal of interrupting wild poliovirus transmission worldwide by the end of 2005 or soon after, with the help of millions of volunteers and vast publicity campaigns to reach and vaccinate all children. In 2008 GPEI hopes to certify the world polio-free, and to ensure that polio never returns. As part of the 'end-game' strategy, many countries have replaced OPV with IPV so as to avoid polio outbreaks caused by circulating OPV strains , and others plan to do the same.

But precisely how long IPV use should continue remains an open question. Some experts are concerned that although most individuals stop excreting OPV virus three to six months after vaccination, there are a few individuals with a rare form of immunodeficiency - agammaglobulinemia - who continue to harbour and excrete OPV virus for many years. Furthermore, the existence of thousands of laboratory stocks of wild type polio, and the possibility - although unlikely - that bioterrorists might one day release a laboratory sample, or create a synthetic version of poliovirus, means that the world could never be sure that polio was gone forever.

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Controversies

Polio vaccines have been mired in controversy since their creation in the 1950s, with accusations of unsafe preparation, and contamination with the monkey virus SV40, and the AIDS-causing human immunodeficiency virus, HIV.

Nature has published key papers that dispel concerns about contamination with HIV, providing reassurance for the final push towards polio eradication.


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Produced with support from: March of Dimes Produced with support from: Rotary International
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