Progress Toward Interruption of Wild Poliovirus Transmission --- Worldwide, January 2006--May 2007

Progress toward global polio eradication continued during 2006 and the first 5 months of 2007, although the number of countries where wild poliovirus (WPV) transmission has never been interrupted remained at four (Afghanistan, India, Nigeria, and Pakistan) (1--4). Continuing challenges included intense WPV circulation in northern India during 2006, low vaccination coverage with oral polio vaccine (OPV) during supplemental immunization activities (SIAs)* in Nigeria, and security problems preventing access to children during SIAs along the Afghanistan-Pakistan border. Programmatic strategies to address these challenges consisted of large-scale use of type 1 monovalent oral polio vaccine (mOPV1) (5), targeted programs (e.g., cross-border synchronization of polio campaigns) to reach more children through SIAs, and introduction of new laboratory procedures to confirm cases more rapidly. This report summarizes these strategies and overall progress toward global polio eradication.

Routine OPV Vaccination

Routine vaccination remains an integral component of the polio eradication initiative. Global routine vaccination coverage for infants with 3 doses of OPV was estimated at 78%^† in 2005, the most recent year with fully reported data, and was similar to the 3-dose OPV coverage reported in 2004 (81%). Estimated routine coverage varied among World Health Organization (WHO) regions in 2005: 63% in the South-East Asian, 69% in the African, 84% in the Eastern Mediterranean, 87% in the Western Pacific, and >90% in the European and Americas regions. In the four polio-endemic countries, 3-dose OPV coverage was estimated at 77% in Pakistan, 76% in Afghanistan, 58% in India, and 39% in Nigeria; however, lower coverage has been reported in areas with ongoing polio transmission (e.g., northern Nigeria and the northern Indian states of Uttar Pradesh and Bihar).

SIAs in 2006

In 2006, 187 SIAs (86 national immunization days [NIDs], 84 subnational immunization days [SNIDs], and 17 mop-up rounds) with OPV were conducted in 36 countries, using a total of 2.12 billion OPV doses. Doses were delivered to 375 million children aged <5 years. Use of mOPV1 increased from 22% of all administered doses in 2005 to 46% in 2006, reflecting the programmatic shift in campaign strategy (5). A total of 58 (31%) of the 187 SIAs were conducted in the four polio-endemic countries: 17 each in India and Pakistan and 12 each in Afghanistan and Nigeria. Of the remaining 2006 SIAs, 81 (43%) were conducted in 13 countries where WPV cases were reintroduced through importation in 2006, and 48 (26%) were conducted in 19 countries with no WPV-confirmed cases in 2006 as a precaution against poliovirus importations.

To improve SIA quality, new approaches were used in the four polio-endemic countries in 2006. In mid-2006, Nigeria initiated a strategy of offering other vaccines (i.e., measles and diphtheria and tetanus toxoids and pertussis vaccine) and health interventions (i.e., bednets and deworming medication) in addition to OPV during SIAs, which were renamed "immunization-plus days" (2). The proportion of "zero-dose" children^§ in northern states decreased from approximately 50% at the end of 2005 to an average of 20% by the end of 2006. In India, in response to an outbreak in 2006, the National Polio Program increased the number of large-scale SIAs in districts with the highest polio risk (western Uttar Pradesh and Bihar), using mainly mOPV1 and concentrating on improving coverage among children aged <2 years. To reach migrating families, Afghanistan implemented a new multipronged approach that included cross-border synchronization of polio campaigns with Pakistan.

Acute Flaccid Paralysis (AFP) Surveillance

The quality of AFP surveillance is monitored by three performance indicators: 1) the rate of AFP cases not caused by WPV (i.e., the nonpolio AFP rate; target for certification: more than one case per 100,000 persons aged <15 years); 2) the proportion of AFP cases with adequate stool specimens^¶ (target for certification: >80%), and 3) the proportion of stool specimens processed in a WHO-accredited laboratory (target: 100%). In 2006, each WHO region maintained sensitivity of AFP surveillance to detect paralytic polio cases at certification-standard levels (Table). Globally, AFP case reporting increased 10%, from 62,434 cases in 2005 to 68,576 cases in 2006, mainly as a result of increased reporting from India, Nigeria, and Pakistan. In 2005, the global Advisory Committee on Polio Eradication (ACPE) endorsed a new minimum operational target nonpolio AFP rate of two cases per 100,000 persons aged <15 years for all polio-endemic countries and countries at high risk for WPV importation (6). All four polio-endemic countries and 12 of the 13 (i.e., all except Kenya) countries in which polio was reintroduced in 2006 reached this new operational nonpolio AFP target rate in 2006.

Global Polio Laboratory Network

In 2006, WHO fully accredited 97% of the 145 global poliovirus network laboratories, which together analyzed approximately 135,000 fecal samples. In late 2006, the laboratory network evaluated and began adoption of a new testing strategy that will reduce poliovirus confirmation time by 50%, from 42 days to 21 days. The new approach uses previously available technologies for poliovirus identification in a new testing sequence that generates results more rapidly.** The network has established a goal to increase to >75% (compared with 58% to date in 2007) the percentage of fecal samples tested from polio-endemic regions in laboratories with capacity for both virus isolation in cell culture and intratypic differentiation (i.e., identification of viruses as either wild or vaccine like) by mid-2008.

WPV Incidence

As of May 30, 2007, a total of 1,997 polio cases had been reported worldwide for 2006 (Table, Figure 1), essentially unchanged from the 1,979 cases reported in 2005. Although 53% of cases in 2005 were the result of polio importations and outbreaks in previously polio-free countries, 6% of cases in 2006 were in countries where polio was reintroduced through importation. As of May 30, 2007, a total of 183 WPV cases with onset of paralysis in 2007 had been reported, less than half the 452 cases reported during the same period in 2006 (Figure 2).

Nigeria. In 2006, Nigeria reported 1,123 WPV cases, compared with 830 cases in 2005. The incidence of new cases decreased in the second half of 2006, with one third of all 2006 cases reported after June 2006. The number of affected states decreased from 21 (57% of the 37 states in Nigeria) in 2005 to 18 states (49%) in 2006. Approximately 60% of 2006 cases were reported from three states in northern Nigeria (Jigawa, Kano, and Katsina). As of May 30, 2007, a total of 90 cases with onset in 2007 had been reported from Nigeria, compared with 371 reported for the same period in 2006.

India. An outbreak originating in western Uttar Pradesh in 2006 resulted in the reintroduction of polio in areas of India that had been polio free and 10 times as many polio cases in 2006 as in 2005 (676 cases versus 66 cases). Of the 676 cases, 648 were poliovirus type 1 (WPV1) and 28 were type 3 (WPV3); 73% were in children aged <2 years. As of May 30, 2007, India had reported 55 polio cases with onset in 2007, of which 31 were WPV1 and 24 were WPV3. Western Uttar Pradesh had reported one WPV1 case in 2007. WPV1 continues to circulate in other parts of Uttar Pradesh and Bihar.

Pakistan and Afghanistan. Although 40 polio cases were reported in Pakistan in 2006 compared with 28 in 2005, approximately 80% of districts were polio-free in 2006. Transmission has remained confined to a few known virus reservoirs, largely along the Afghanistan-Pakistan border. By May 30, 2007, eight WPV cases (three WPV1 and five WPV3) with onset in 2007 had been reported in Pakistan.

Afghanistan reported 31 cases in 2006, compared with nine cases in 2005. Most of Afghanistan remains polio-free, except for continued transmission in the Southern Region, where a new WPV1 outbreak started in 2005 and peaked during mid-2006. Although the last outbreak-associated case was reported in September 2006, two WPV1 cases with onset in 2007 indicate ongoing low-level WPV1 transmission in the Southern Region.

Other countries. Ten of the 26 countries where polio has been reintroduced since 2003 reported polio cases in the second half of 2006 (7). As of May 30, 2007, WPV circulation continued in five countries where polio was reintroduced (Angola, Democratic Republic of the Congo, Ethiopia, Myanmar, and Somalia); four additional countries (Cameroon, Chad, Nepal, and Niger) bordering polio-endemic areas continued to experience sporadic importations.

Reported by: Polio Eradication Group, World Health Organization, Geneva, Switzerland. Div of Viral Diseases and Global Immunization Div, National Center for Immunization and Respiratory Diseases, CDC.

Editorial Note:

The global incidence of polio was unchanged from 2005 to 2006. Although the number of polio cases from importations decreased, the number of cases in the four polio-endemic countries increased from 2005 to 2006 because of low SIA coverage in Nigeria, intense virus circulation in certain high-risk districts in northern India, and security-related access problems in Afghanistan-Pakistan border areas. However, programmatic strategies developed to address these challenges, including use of mOPV1 with its greater efficacy against WPV1 compared with trivalent OPV (5), have had an impact on polio transmission in the four polio-endemic countries, as suggested by the decrease in the number of WPV1 cases in early 2007.

In Nigeria, implementation of immunization-plus days reduced the proportion of zero-dose children by roughly 30%, indicating that more children are being reached and vaccinated for the first time. India responded to a WPV1 outbreak by increasing the number of large-scale SIAs in the highest-risk districts of western Uttar Pradesh and Bihar, using mainly mOPV1, and concentrating on improving the coverage among children aged <2 years. Polio program staff members in Afghanistan and Pakistan implemented synchronized cross-border polio campaigns, ensuring simultaneous and comprehensive coverage of children in transit through the border areas. Although these strategies have positively affected polio transmission in high-risk countries, ongoing program evaluation and adaptability to changing circumstances will be crucial for progress to continue during the remainder of 2007 and early 2008.

In February 2007, a meeting was held at WHO headquarters in Geneva, attended by envoys of the heads of state of the four polio-endemic countries and by major polio-eradication partners. Agreement was reached regarding the technical feasibility of polio eradication and the economic benefits of eradication compared with a polio-control program. The national technical advisory bodies of the polio-endemic countries subsequently convened in May and early June 2007 to review the latest epidemiologic and programmatic data and to further refine tactics to vaccinate all children with OPV during the second half of 2007. WPV1 transmission has been curtailed substantially in the polio-endemic countries. With global collaboration and sustained commitment, the world can achieve global polio eradication.

References

1. CDC. Progress toward interruption of wild poliovirus transmission---worldwide, January 2005---March 2006. MMWR 2006;55:458--62.
2. CDC. Progress toward poliomyelitis eradication---Nigeria, January 2005--July 2006. MMWR 2007;56:278--81.
3. CDC. Progress toward poliomyelitis eradication---India, January 2005--June 2006. MMWR 2006;55:772--6.
4. CDC. Progress toward poliomyelitis eradication---Pakistan and Afghanistan, January 2006--February 2007. MMWR 2007;56:340--3.
5. Grassly NC, Wenger J, Durrani S, et al. Protective efficacy of a monovalent oral type 1 poliovirus vaccine: a case-control study. Lancet 2007; 369:1356--62.
6. World Health Organization. Advisory committee on polio eradication---standing recommendations for responding to circulating polioviruses in polio-free areas. Wkly Epidemiol Rec 2005;80:330--1.
7. CDC. Resurgence of wild poliovirus type 1 transmission and consequences of importation---21 countries, 2002--2005. MMWR 2006;55: 145--50.

* Mass campaigns conducted during a brief period (days to weeks) in which 1 dose of OPV is administered to all children aged <5 years, regardless of vaccination history.

^† WHO/UNICEF estimates of OPV3 coverage from 2007 summary of WHO vaccine-preventable diseases monitoring system.

^§ Children with nonpolio acute flaccid paralysis who had never been vaccinated with OPV, according to their vaccination histories.

^¶ Two specimens are collected >24 hours apart, both within 14 days of paralysis onset, and shipped on ice or frozen ice packs to a WHO-accredited laboratory, arriving at the laboratory in good condition.

** Additional information available at http://www.who.int/immunization_monitoring/Supplement_polio_lab_manual.pdf.

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Date last reviewed: 7/11/2007