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Wild Poliovirus Importations --- West and Central Africa, January 2003--March 2004

Since the 1988 World Health Assembly resolution to eradicate poliomyelitis (1), three World Health Organization (WHO) regions (Americas, European, and Western Pacific) have been certified polio-free, and the number of countries with endemic polio has decreased from 125 in 1988 to six in 2003 (Afghanistan, Egypt, India, Niger, Nigeria, and Pakistan). During January 2003--March 2004, importations of wild poliovirus (WPV) occurred in eight countries that were previously polio-free: five in the West African block* (Benin, Burkina Faso, Côte d'Ivoire, Ghana, and Togo) and three in the Central African block (Cameroon, Central African Republic, and Chad), resulting in 63 polio cases (2,3). This report summarizes the 1) investigation and response to these WPV importations and 2) progress toward polio eradication in West and Central Africa.

Routine and Supplementary Immunization

In 2002, reported routine coverage with 3 doses of oral poliovirus vaccine (OPV) varied from 13% to 93% for countries in West and Central Africa, excluding Ghana. All West and Central African countries conducted supplementary immunization activities (SIAs) annually during 1999--2002. In 2002, all except Algeria conducted National Immunization Days (NIDs)§, vaccinating approximately 30.6 million children aged <5 years with >2 doses of OPV.

Acute Flaccid Paralysis (AFP) Surveillance

AFP surveillance quality is evaluated by two key indicators: 1) annual reporting rate (target: nonpolio AFP rate of more than one case per 100,000 children aged <15 years) and 2) completeness of specimen collection (target: two adequate stool specimens from >80% of all persons with AFP). In 2002, these targets were met by all but six West and Central African countries (Algeria, Cape Verde, Chad, Equatorial Guinea, Gambia, and Sao Tome and Principe). In 2003, the number of countries not meeting the targets increased to eight (Algeria, Cameroon, Cape Verde, Chad, Ghana, Liberia, Niger, and Sao Tome and Principe).

WPV Importation and Spread

During January 2003--March 2004, a total of 63 cases of polio resulted from importation of WPV into the previously polio-free countries of West and Central Africa (Table). All imported viruses were type 1 and could be traced to common ancestral strains that circulate in endemic reservoirs shared by northern Nigeria and southern Niger (Figure). During this same period, Nigeria and Niger have reported 497 cases of infection with WPV type 1 or type 3, with cross-border transmission of both serotypes between the two countries. Of the 63 polio cases resulting from importation of WPV, 48 (76%) occurred during June--December 2003, coinciding with the peak transmission of indigenous WPV type 1 in Nigeria and Niger.

East of Nigeria, the first importation occurred in Chad in August 2003 from northeastern Nigeria, leading to an outbreak of 29 cases. The outbreak spread to the adjacent countries of Cameroon (two cases) and the Central African Republic (one case) during October--December. The continued circulation of virus after importation suggests that Chad is at high risk for reestablishment of endemic poliovirus transmission.

West of Nigeria, three independent importations into Benin (five cases) occurred from different parts of Nigeria from late 2003 to early 2004. In addition, genetic sequencing data indicated that after a 2002 importation into Burkina Faso, in early 2003, WPV spread to Ghana. In 2003, closely related strains continued to circulate in Ghana (eight cases) and, during 2003--2004, in Burkina Faso (13 cases). The WPV strains isolated in Côte d'Ivoire (four cases) and Togo (one case) were linked genetically 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 might have been reestablished in Burkina Faso during 2003 and early 2004.

Among the 63 patients with WPV, 13% were aged <12 months, 21% were aged 12--23 months, 49% were aged 24--59 months, and 17% were aged >59 months. Of the 52 patients with known vaccination status, 16 (31%) had never received OPV, 26 (50%) had received 1--2 OPV doses, and 10 (19%) had received >3 OPV doses.

Response to WPV Importation

Investigations were initiated within 2 days of identifying the index patients in four of the eight countries (median: 4 days; range: 1--22 days). Clinical and epidemiologic information was verified, stool specimens were collected from immediate contacts, and the search for unreported AFP cases was intensified. Two of the eight index patients in the eight countries had traveled recently to a country with endemic polio, whereas the remaining six patients had no relevant travel history or immediate contact with persons who traveled to a country where polio is endemic. All index patients lived near commercial centers with substantial foreign trade with countries where polio is endemic.

All eight countries implemented SIAs in response to detection of imported WPV. The median duration from onset of paralysis to the start of SIAs was 12.5 weeks (range: 6--17 weeks). The magnitude of the response varied; four countries conducted NIDs, two countries conducted subnational immunization days** (SNIDs), and two countries conducted both SNIDs and NIDs. These campaigns provided approximately 21.7 million children aged <5 years with >2 doses of OPV.

Reported vaccination coverage exceeded 90% for all SIAs conducted; reported coverage at district level ranged from 48% to >100%. To determine the proportion of previously unvaccinated children, caregivers were asked whether their children were receiving OPV for the first time during the campaign. The Central African Republic (4%) and Côte d'Ivoire (8%) were the only countries reporting that >4% of children received their first dose of OPV during the most recent campaigns.

Four of the eight countries (Benin, Burkina Faso, Chad, and Ghana) detected WPV after at least two rounds of SIAs. The most recent WPV patient in Burkina Faso had onset in January 2004; two rounds of NIDs were then conducted in February and March 2004. The four WPV cases in Chad in 2004 were detected in provinces that had conducted at least two rounds of SNIDs in November and December 2003 and in January 2004; two rounds of NIDs were conducted in March and May 2004 after onset of these cases. The most recent WPV patient in Ghana had onset in September 2003 after two SNIDs in June and July 2003; since the most recent case, four rounds of NIDs were conducted (in October and December 2003 and February and March 2004).

Reported by: World Health Organization (WHO) Inter-Country Program Office, Abidjan, Côte d'Ivoire. WHO Inter-Country Program Office, Yaounde, Cameroon. Vaccine-Preventable Disease Unit, WHO Regional Office for Africa, Harare, Zimbabwe. Vaccines and Biologicals Dept; National, Regional, and Specialized Polio Reference Laboratories, Global Polio Laboratory Network; WHO, Geneva, Switzerland. Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases; Global Immunization Div, National Immunization Program, CDC.

Editorial Note:

During 1999--2000, West and Central African countries began intensifying and synchronizing NIDs, leading to a decrease in the number of countries with endemic WPV from 13 in 1999 to one in 2001 (4). During January 2003--March 2004, eight previously polio-free countries reported WPV importations from endemic poliovirus reservoirs shared by northern Nigeria and southern Niger, which were largely a result of suspension of immunization campaigns in certain northern states of Nigeria in August 2003 (3). Many of these countries had continued transmission after importation because of low routine vaccination coverage, increased intervals between SIAs, and possibly declining quality of SIAs. The importations and spread highlight the increased vulnerability of countries with low routine vaccination coverage that are no longer conducting SIAs.

Preparedness for response to WPV importation should be strengthened in West and Central African countries, which will continue to be at risk until WPV transmission in Nigeria and Niger is interrupted. According to WHO recommendations, >80% of all outbreaks should be investigated within 48 hours of their notification. However, only four (50%) of the outbreaks during 2003--2004 were investigated within the recommended period. In addition, WHO recommends that outbreak response vaccination occur within 4 weeks after confirmation of WPV; this was achieved in only one (13%) of the eight countries.

Four (50%) of the eight countries had continued transmission of WPV after completing two rounds of SIAs, indicating suboptimal quality of campaigns despite reported high coverage. The main challenges faced in implementing high-quality SIAs included 1) delayed provision of funds to support detailed planning aimed at vaccinating every eligible child, 2) gaps in supervision by national and subnational authorities, 3) lack of consistently effective social mobilization, and 4) inadequate commitment to conducting successful campaigns. The quality of SIAs has been improved in certain countries through enhanced political commitment and strengthened monitoring. In addition, experience was gained from the response to the importations during 2003--2004. This will lead to improved planning and more rapid implementation of high-quality SIAs during 2004--2005, an essential step to achieving eradication.

Ongoing transmission in Nigeria and Niger has set back the goal to interrupt poliovirus transmission in Africa by the end of 2004 (3). To restore gains made in polio eradication in West and Central Africa, WPV transmission must be interrupted in Nigeria and Niger. Until that time, neighboring countries must create a population immunity barrier by implementing high routine vaccination coverage and high-quality SIAs. In 2002, these steps proved successful in preventing importation of WPV into Bangladesh and Nepal during resurgence of polio in India. Surveillance standards also must be maintained to ensure rapid detection of any WPV importation, allowing for timely response and containment.


  1. World Health Assembly. Global eradication of poliomyelitis by the year 2000: resolution of the 41st World Health Assembly. Geneva, Switzerland: World Health Organization, 1988 (WHA resolution no. 41.28).
  2. Okwo-Bele JM, Lobanov A, Biellik RJ, et al. Overview of poliomyelitis in the Africa Region and current regional plan of action. J Infect Dis 1997;175(suppl 1):S10--S15.
  3. CDC. Progress toward global poliomyelitis eradication---Nigeria, January 2003--March 2004. MMWR 2004;53:343--6.
  4. World Health Organization. Global polio eradication initiative progress 2001. Geneva, Switzerland: World Health Organization, 2002.

* The African regional office of WHO has divided its member states into four blocks (East, Central, South, and West) plus Angola, Democratic Republic of the Congo, Ethiopia, and Nigeria. The Central African countries are as follows: Cameroon, Central African Republic, Chad, Congo, Equatorial Guinea, Gabon, and Sao Tome and Principe. The West African countries are as follows: Algeria, Benin, Burkina Faso, Cape Verde, Côte d'Ivoire, Gambia, Ghana, Guinea, Guinea-Bissau, Liberia, Mali, Mauritania, Niger, Senegal, Sierra Leone, and Togo.

Ghana reported 120% coverage.

§ Nationwide mass campaigns during a short period (usually a few days) in which 2 doses of OPV are administered to all children (usually aged <5 years), regardless of previous vaccination history, with an interval of 4--6 weeks between doses.

As of May 4, 2004.

** Campaigns similar to NIDs but confined to certain parts of the country.  


Table 1
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Figure 1
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