FACT SHEET Polio vaccination and challenges to eradication Poliomyelitis (polio) cases have decreased by more than 99 percent since 1988, when the Global Polio Eradication Initiative was launched.1 The disease remains endemic in just four countries: Afghanistan, India, Nigeria, and Pakistan. In the last decade, however, eradication efforts have been hindered by uneven vaccination coverage, viral mutations, and variable vaccine effectiveness.1, 2 Polio transmission
ŪŪ Polio is a viral disease transmitted through feces, saliva or throat mucous (fecal-oral and oraloral routes) that an cause permanent, debilitating paralysis.3 Initial infection generally occurs at a young age through close contact with infected individuals. Poor sanitation and hygiene aid the spread of the disease. ŪŪ Less than one percent of those infected with poliovirus develop paralysis.3 Most experience no symptoms, and unknowingly pass the virus on to others. Strong surveillance is essential, as detection of even one case of paralytic polio corresponds to several hundred infections in the community. ŪŪ There are three serotypes of poliovirus (type-1, -2, and -3). Immunity to one serotype does not protect against the other serotypes. The virus The vaccines The disease
Vaccine-derived poliovirus (VDPV) and vaccine-associated paralytic polio (VAPP) ŪŪ The weakened virus in OPV can revert to a virulent form, VDPV, capable of causing paralytic polio. If vaccination coverage is low in the community where this mutation occurs, VDPV can begin to circulate (cVDPV) in the same manner as WPV. Between 2000 and 2010 there were 493 cases of cVDPV reported; nearly 70 percent were from Nigeria.4 cVDPVs accounted for about 3 percent of all reported polio cases during this time period. ŪŪ Extremely rarely, or about once for every 2.5 million doses administered, OPV causes paralysis in a vaccinated child or a close contact in an adverse event referred to as VAPP.3 ŪŪ Normally, individuals vaccinated with OPV shed the weakened vaccine virus for one to three weeks. Immunocompromised people, however, may shed the virus for years, increasing the chance of mutation and a return to virulence as VDPV.2 This is referred to as immunodeficiency-related VDPV (iVDPV).
OPV vs. inactivated polio vaccine (IPV) ŪŪ Despite the risks of VAPP and VDPV, OPV is considered preferable to IPV in areas with active polio transmission, high risk of importation, or high risk of transmission should importation occur.2 • OPV vaccination reduces polio transmission by inhibiting reproduction of WPV in the intestinal tract. Because IPV does not induce intestinal immunity, IPV-vaccinated children infected with WPV continue to shed WPV in their stool. Though they themselves will not contract polio, they continue to transmit WPV to others. • OPV provides protection not only to the vaccinated individual, but also to his or her close contacts. People vaccinated with OPV shed the weakened form of the virus contained in the vaccine. Family members and close contacts are exposed to this weakened virus, which boosts their own immunity to poliovirus. IPV does not confer this benefit. • OPV is less expensive and more easily administered than IPV. Because IPV is delivered by injection, it must be given by trained health workers. Vaccine-related challenges to eradication Vaccine coverage. Conflicts in Afghanistan and Pakistan have hindered vaccination efforts, resulting in gaps in vaccine coverage that have allowed poliovirus to continue to circulate.4 In Nigeria, community resistance to vaccination resulted in low coverage for a number of years. Though this problem has largely been overcome, outbreaks resulting from cases exported from Nigeria are still occurring in many countries. Vaccine coverage has declined in some areas that have not experienced polio cases in many years, allowing outbreaks to occur when a person traveling from an endemic area reintroduces the disease. In 2010, more than 80 percent of new polio cases occurred in non-endemic countries.4, 5 VDPV. Even after interruption of WPV transmission, cVDPV will continue to emerge as long as OPV is used. This is illustrated by the case of type-2 poliovirus. No type-2 WPV cases have been detected since 1999.1 However, type-2 cVDPV was responsible for the majority of cVDPV outbreaks in the last decade.4 iVDPV poses a long-term challenge, as it could lead to the reintroduction of cVDPV years after the cessation of OPV. Vaccine effectiveness. OPV effectiveness has been found to vary by the type of vaccine and by region.1, 2 The original OPV is a trivalent vaccine that protects against type-1, -2, and -3 poliovirus. OPV targeting a single type of poliovirus has been found to be more effective. Use of monovalent OPV, however, has in some areas resulted in alternating outbreaks of type-1 and type-3 polio.1 In 2009, a bivalent OPV that protects against both type-1 and type-3 poliovirus was introduced. It has been shown to have similar effectiveness to monovalent OPV. OPV effectiveness has been found to be lower in India than in other areas. In 2010, 83 percent of Indian children that contracted polio had received at least three doses (a full course) of OPV.6 The reasons for this are not well understood, but are probably related to the high prevalence of other intestinal pathogens.
References 1. World Health Organization. Global Polio Eradication Initiative Strategic plan 2010-2012; 2010. 2. World Health Organization. Polio vaccines and polio immunization in the pre-eradication era: WHO position paper. Weekly epidemiological record. 2010; 85: 213-28. 3. Heymann DL, editor. Control of communicable diseases manual with addendeum to influenza data: updated April 18, 2006. 18 ed. Washington, DC: American Public Health Association; 2004. 4. Global Polio Eradication Initiative. Circulating vaccine-derived poliovirus (cVDPV) 2000-2011. 2011 [cited 2011 March 28]; Available from: http://www.polioeradication.org/Dataandmonitoring/Poliothisweek/Circulatingvaccinederivedpoliovirus.aspx 5. World Health Organization. Wild Poliovirus 2000-2011. 2011 [cited 2011 March 28]; Available from: http://www.polioeradication.org/Portals/0/Document/Data&Monitoring/Wild_poliovirus_list_2000_2011_22Mar.pdf 6. Centers for Disease Control and Prevention. Progress Toward Poliomyelitis Eradication -- India, January 2009--October 2010. Morbidity and Mortality Weekly Report. 2010; 59(48): 1581-5.
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Poliomyelitis (polio) cases have decreased by more than 99 percent since 1988, when the Global Polio Eradication Initiative was launched. Th...