Poliomyelitis: Progress Toward Global Eradication
Abstract & Commentary
By Mary-Louise Scully, MD, Sansum-Santa Barbara Medical Foundation Clinic, Santa Barbara, CA
Dr. Scully reports no financial relationships relevant to this field of study.
Synopsis: Polio remains endemic in only 4 countries, Afghanistan, India, Nigeria, and Pakistan, but importation cases are still occurring in 10 countries that were previously considered polio-free. Ongoing strategies to interrupt the local transmission in endemic areas such as India are critical for successful worldwide eradication.
Source: CDC. Progress Toward Poliomyelitis Eradication—India, January 2005-June 2006. MMWR Morb Mortal Wkly Rep. 2006;55:772-776.
As of June 25, 2006, India had reported 60 polio cases for 2006; 57 being wild poliovirus type 1 (WPV1) and 3 wild poliovirus type 3 (WPV3). Of these cases, 46 were from Uttar Pradesh (UP) state, 12 from Bihar state, one from Madhya Pradesh state bordering UP, and one from Jharkhand state bordering Bihar. Intense local transmission continues in western UP, where routine vaccination coverage with 3 doses of trivalent oral polio vaccine (OPV) is estimated at only 38%. The high population density, large birth cohort, poor sanitation, and high population mobility make this area of India especially challenging.
Unfortunately, 3 times as many cases of polio were reported in India during the first half of 2006 compared to 2005. However, polio transmission is now secondary to only one circulating WPV1 genetic cluster. In addition, the geographic distribution of WPV1 in India has decreased, as no new WPV cases have occurred in the southern Indian states, West Bengal, or in the western states of Maharashtra or Rajasthan. Delhi, despite its close proximity to UP, has not reported a WPV case as of June 2006.
The India Expert Advisory Group on Polio Eradication has implemented several strategies to improve supplementary immunization activity, including the use of monovalent oral poliovirus vaccine type 1 and monovalent oral poliovirus vaccine type 3 vaccines in areas of high transmission. The monovalent vaccines are felt to provide greater immunity to the specific WPV type than would the same number of doses of trivalent OPV. Trivalent OPV continues to be used in the routine childhood immunization program of India.
Commentary
In 1988 when the Global Polio Eradication Initiative began, 125 countries reported cases of paralytic polio. By 1994, the WHO Region of the Americas (36 countries) was certified polio free, followed by the WHO Western pacific Region (37 countries including China) in 2000, and lastly the WHO European Region (51 countries) in June of 2002. Polio was reported on 5 continents in 1988, and now is restricted to parts of Africa and south Asia.
As of 2006, the 4 countries currently considered endemic for polio are Nigeria, India, Afghanistan, and Pakistan. Egypt and Niger were removed from the list in February of 2006, after 12 months without indigenous WPV transmission. Nigeria has had the most cases (687 cases) thus far in 2006 (see Table).1 In 2005, 3 large outbreaks, secondary to importation of Nigeria's polioviruses into previously polio-free countries, occurred: Yemen (478 cases), Indonesia (303), and Somalia (185). Overall in 2005, 94% of polio cases worldwide were secondary to viruses that originated in northern Nigeria.2
Despite the success of OPV towards global polio eradication, concern is emerging regarding documented cases of paralytic poliomyelitis secondary to vaccine-derived polioviruses (VDPVs). Antigenic and molecular methods can now identify VDPVs from stool specimens submitted within the global Polio Lab Network. The very success of global polio eradication in eliminating the transmission of wild polio viruses and, therefore, natural immunity enables the emergence and circulation of VDPVs if routine vaccination coverage lapses.3 Several outbreaks of VDPVs have occurred in OPV-using countries since 2000: Haiti/Dominican Republic, the Philippines, Madagascar, and Indonesia. Recently, the first polio outbreak in over a decade, and the first ever VDPV cases, occurred in China.4 In September 2005, the first VDPVs within the United States since 2000 were identified in an immunocompromised infant and 3 children in an Amish community in Minnesota, whose members were mostly unvaccinated.5
Although rare, a recent case of vaccine-associated paralytic polio occurred in an unvaccinated traveler.6 A 22-year-old woman went to OPV-using country (Costa Rica) and stayed with a family as part of a university-sponsored study-abroad program. She developed a nonspecific febrile illness that progressed to bilateral areflexic lower extremity weakness and respiratory failure. The initial diagnosis considered was Guillain-Barre Syndrome (GBS) but electrodiagnostic studies were not consistent with GBS, and stool specimens were subsequently positive for Sabin-strain poliovirus types 2 and 3. An infant within the Costa Rican extended household had received the first dose of OPV shortly after the woman arrived. The woman had never been vaccinated with either OPV or IPV because of a religious exemption.
The United States and other high income countries have changed from OPV to inactivated polio vaccine (IPV) for routine polio immunization, thereby eliminatin the risk of paralytic poliomyelitis secondary to VDPVs. A present, this is not an option for many countries because of a substantial difference in cost (IPV $2.00-$3.00/dose versus OPV $0.03/dose), but efforts are underway to prepare vaccination policies for the OPV-cessation era. At the 2005 meeting of the Advisory Committee on Polio Eradication, the Global Polio Eradication Initiative and the WHO were encouraged to continue further investigation of newer products for use in the post-OPV era, such as fractional doses of IPV and Sabin-strain IPV.7
Although the risk of acquiring polio during travel is low, travelers to countries where polio is endemic or still occurring should be vaccinated, according to the current recommendations.8 Adults who have already received a primary series with either OPV or IPV should receive IPV before departure. Based on available data, adults do not need more than a single lifetime booster dose with IPV.
References
- Global Polio Eradication Initiative. Available at www.polioeradication.org/casecount.asp. Accessed: August 29, 2006.
- CDC. Progress Toward Interruption of Wild Poliovirus Transmission — Worldwide, January 2005-March 2006. MMWR Morb Mortal Wkly Rep. 2006;55:458-462.
- Dowdle W, Kew O. Vaccine-Derived Polioviruses: Is It Time to Stop Using the Word "Rare"? J Infect Dis. 2006;194:539-541.
- Liang X, et al. An Outbreak of Poliomyelitis Caused by Type 1 Vaccine-Derived Poliovirus in China. J Infect Dis. 2006.194:545-551.
- CDC. Poliovirus Infections in Four Unvaccinated Children — Minnesota, August-October 2005. MMWR Morb Mortal Wkly Rep. 2005;54:1053-1055.
- CDC. Imported Vaccine-Associated Paralytic Poliomyelitis — United States, 2005. MMWR Morb Mortal Wkly Rep. 2006; 55:97-99.
- CDC. Brief Report: Conclusions and Recommendations of the Advisory Committee on Poliomyelitis Eradication — Geneva, Switzerland, October 2005. MMWR Morb Mortal Wkly Rep. 2005;54:1186-1188.
- Prevots DR, et al. Poliomyelitis Prevention in the United States. Updated Recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR Recomm Rep. 2000;49:1-22; quiz CE1-7.
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