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Polio: A crippler that is down but not out
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Today, most of us take for granted immunization against a disease that struck fear into the hearts of parents everywhere in the middle of the 20th century. Polio is finally on the cusp of eradication, but it has required an enormous effort by public health professionals the world over.
In 1988, the World Health Assembly (WHA), the annual meeting of WHO, voted to launch a global goal to eradicate polio. Before 1988, wild poliovirus (WPV) was endemic in more than 125 countries on five continents, paralyzing more than 350,000 children per year, or approximately 1,000 children per day. Since 1988, as a result of an alliance titled the Global Polio Eradication Initiative (GPEI), more than 2 billion children have been vaccinated against polio because of the cooperation of more than 200 countries and 20 million volunteers, and an international investment of more than $6 billion.
GPEI uses a four-pronged polio eradication strategy, which includes: 1) achieve high infant immunization rates with four doses of oral polio vaccine (OPV containing all three serotypes, tOPV) in the first year of life, 2) conduct mass immunization campaigns, vaccinating all children younger than 5 years not reached by the health system; these campaigns are known as supplemental immunization activities (SIAs), 3) implement a widespread surveillance network to detect all polio cases and circulating WPV, and 4) conduct targeted SIAs in communities or areas with continuing polio transmission, known as mop-up campaigns. By 2007, through application of GPEI’s strategy, transmission of WPVs had been interrupted, at least transiently, in all but four countries (Nigeria, India, Pakistan and Afghanistan, the endemic countries), polio cases had been reduced to fewer than 2,000 per year, and one of the three serotypes of WPV, type 2, had been eradicated (last detected case in Aligarh, India in 1999).
However, since 2007, progress has stalled. Both remaining types of WPV (1 and 3) are in circulation in the same four endemic countries, and repeated WPV exportations from Nigeria and India have caused outbreaks in many countries. Several of these exportations have led to persistent transmission of WPV in countries such as Chad and Angola that were previously polio-free due to low population immunity. Each of the remaining areas with WPV transmission presents difficult challenges to stopping WPV transmission and eradicating polio. The main reasons for continuing transmission are “failure to vaccinate” in most of the world and “vaccine failure,” particularly in Northern India.
Poor sanitation conditions and dense crowding in parts of Northern India provide an ideal environment for rapid and efficient WPV transmission, thus raising the level of population immunity needed to stop transmission. In addition, OPV appears to be less effective in Northern India in inducing immunity in children than in other parts of the world. The reasons for this are unclear. Monovalent vaccines have been developed that induce better serotype-specific immune responses than tOPV. However, focus, for example, on use of type 1 monovalent OPV reduces cases from type 1 disease but allows children susceptible to type 3 to accumulate and fuel resurgence of type 3 polio cases.
In Nigeria, Pakistan and countries such as Chad and Angola, low infant immunization rates combined with failure to reach a high proportion of the target population during SIAs leave large numbers of children unvaccinated and, therefore, susceptible to WPV.
In Afghanistan, Pakistan, Sudan and Somalia, ongoing or intermittent conflict prevents vaccination teams from reaching children in conflict zones, allowing WPV transmission to persist in these areas.
To overcome the remaining barriers to polio eradication, GPEI has developed a new strategic plan, which takes an aggressive approach to addressing key risks.
The plan places significant emphasis on innovation to tackle the remaining challenges. For example, the rapid introduction of bivalent OPV, which includes antigens of the two remaining types of WPV (1 and 3), has proven to induce immunity better to those two types than the traditional trivalent OPV. Research is also either being conducted or being planned to evaluate the effect of measures to reduce the force of infection, such as improving hygiene, water quality, and sanitation, evaluating the effect of adding inactivated polio vaccine (IPV) to supplement OPV, determining whether children at least 5 years of age are participating in sustained transmission and, if so, potentially include them in future SIAs, and whether agents that decrease diarrhea, such as zinc, will also lead to better vaccine virus replication in the gut and a better immune response.
From the U.S. perspective, it is important for Americans to continue to support polio eradication. Through the U.S. government and donations through Rotary International, Americans have been the largest contributors to polio eradication, and the continued support of Americans will be critical to ensuring sufficient resources are available to finish the job.
In addition, as WPV continues to circulate in the world, it is important to ensure protection of all children by vaccinating them with four doses of IPV and to maintain vigilance by assuring all cases of acute flaccid paralysis are properly worked-up to rule out polio.
Although there has not been any indigenous polio in the United States, polioviruses are only a plane ride away. In 2005, four children in Minnesota were found to be excreting polioviruses, albeit of vaccine origin. Polio should always be considered in the differential diagnosis of any child who presents with acute flaccid paralysis. Because only about one in 200 people infected with polio develops paralysis, a history of travel to a polio endemic area may not be elicited. The child may have been infected through silent transmission in a chain that started with international travel. Cases of possible polio should be reported to local health authorities, who can assist physicians in assuring the right specimens are collected (polio is confirmed through detection of virus in the stool) and sent to appropriate laboratories.
The world is taking the final steps toward eradicating polio, and we are on the verge of protecting all children from getting paralyzed by this dreadful virus. It is no time to let down the guard. In fact, it is ever more important for Americans, including parents and health providers, to do their part to help finish the job.
Apoorva Mallya is a Program Officer of Immunization Programs, Vaccine Delivery, and the Global Health Program at the Bill & Melinda Gates Foundation.
Walter Orenstein, MD, is an Infectious Diseases in Children Editorial Board member and the Deputy Director for Immunization Programs, Vaccine Delivery, and the Global Health Program at the Bill & Melinda Gates Foundation.