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Since the 1988 World Health Assembly resolution to eradicate polio, three WHO regions (Americas, Europe and Western Pacific) have been certified polio-free, and the number of polio-endemic countries has decreased from 125 in 1988 to 6 in 2003 (Afghanistan, Egypt, India, Niger, Nigeria and Pakistan). In 2003 and the first quarter of 2004, there were importations of wild poliovirus (WPV) to 8 previously polio-free countries in west and central Africa: 5 in the west African block (1) (Benin, Burkina Faso, Cote d'Ivoire, Ghana and Togo) and 3 in the central African block (Cameroon, Central Africa Republic (CAR) and Chad), resulting in 63 polio cases. (2) This report summarizes current epidemiological data and response activities to WPV importations and describes progress in implementing supplementary immunization activities (SIAs) and acute flaccid paralysis (AFP) surveillance in west and central Africa.
Routine and supplementary immunization
Reported routine coverage with 3 doses of oral poliovirus vaccine (OPV) was 73% for the 23 countries in the west and central African blocks, excluding Ghana (range: 13-93%) in 2002. All west and central block countries conducted SIAs annually between 1999 and 2002. In 2002, all countries except Algeria conducted national immunization days (NIDs), (3) resulting in the vaccination of 30.6 million children aged <5 years with at least 2 OPV doses.
Acute flaccid paralysis (AFP) surveillance
AFP surveillance quality is evaluated by two key indicators: annual reporting rate (target: non-polio AFP rate of [greater than or equal to] 1 case per 100 000 children aged <15 years) and completeness of specimen collection (target: two adequate stool specimens from [greater than or equal to] 80% of all persons with AFP). In 2002, these targets were met by all but 6 west and central block countries (Algeria, Cape Verde, Chad, Equatorial Guinea, Sao Tome and Principe, and Gambia). In 2003, the number of countries that did not reach the targets increased to 8 (Algeria, Cameroon, Cape Verde, Chad, Ghana, Liberia, Niger, Sao Tome and Principe).
Wild poliovirus (WPV) importation and spread
A total of 63 cases of polio resulted from importation of WPV type 1 into the polio-free countries of west and central Africa between January 2003 and March 2004 (Table 1 and Map 1). All the imported viruses can be traced back to common ancestral strains that were circulating in endemic reservoirs shared between northern Nigeria and southern Niger.
East of Nigeria, the first importation occurred in Chad in August 2003 from north-eastern Nigeria. This led to a local outbreak of polio cases resulting in 29 cases. The outbreak spread to the adjacent countries of Cameroon (2 cases) and CAR (1 case). The continued circulation following importation suggests that Chad is at high risk of re-establishment of endemic poliovirus transmission.
West of Nigeria, there have been three independent importations into Benin (5 cases) from different parts of Nigeria between late 2003 and early 2004. Genetic sequence data indicate that, following an importation in 2002, WPV spread from Burkina Faso to Ghana in early 2003. Closely related strains continued to circulate in 2003 in Ghana (8 cases) and in 2003 and 2004 in Burkina Faso (13 cases). The WPV isolated in Cote d'Ivoire (4 cases) and Togo (1 case) are genetically closely linked to the strains circulating in Burkina Faso and Ghana, indicating spread of poliovirus from Burkina Faso and possibly Ghana. These data suggest that independent circulation of WPV may have been re-established in Burkina Faso during 2003 and early 2004. (2)
Cross-border transmission of WPV types 1 and 3 continued between northern Nigeria and southern Niger during 2003 and early 2004.
Of the cases resulting from importation of WPV between January 2003 and March 2004, 76% (48/63) occurred during June-December 2003, coinciding with the peak transmission of indigenous WPV type 1 in Nigeria and Niger. Of these cases, 13% were aged <12 months, 20% were 12-23 months, 49% were 24-59 months, and 17% were >59 months. Of the 52 cases with known vaccination status, 16 (31%) had never been vaccinated with OPV, 26 (50%) had received 1-2 OPV doses, and 10 (19%) had received >3 OPV doses.