Progress Toward Poliomyelitis Eradication --- Nigeria, 2005--2006

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Only four countries (Afghanistan, India, Nigeria, and Pakistan) have never experienced interruption of poliovirus transmission (1--3). Nigeria had the largest number of cases in 2006, accounting for 1,129 (56%) of the 2,002 cases reported globally. However, major innovations to the national polio-eradication program in Nigeria were initiated in 2006. These innovations, if sustained, should advance the Global Polio Eradication Initiative. Nigeria (2006 population: 140 million) experienced a resurgence in wild poliovirus (WPV) transmission during 2003--2004 after a loss of public confidence* in oral polio vaccine (OPV) and suspension of supplementary immunization activities (SIAs) in certain northern states (4). Subsequently, WPV spread within Nigeria and into 19 polio-free countries (1,5,6). Even after national SIAs recommenced, limited acceptance and ongoing operational problems resulted in low vaccination coverage and continued poliovirus transmission. The number of confirmed polio cases in Nigeria attributed to both WPV type 1 (WPV1) and type 3 (WPV3) increased from 782 in 2004 to 830 in 2005 and to 1,129 in 2006 (as of March 23, 2007). To increase the effectiveness of polio-eradication measures and community acceptance of vaccination, in 2006, health authorities in Nigeria introduced monovalent type 1 OPV (mOPV1) vaccine^§ and changed the way SIAs were implemented. This report summarizes these new approaches and overall progress toward polio eradication in Nigeria during 2005--2006.

Immunization Activities

In 2005, national reported routine vaccination coverage for 3 doses of OPV among infants was 31%. Substantial variation was observed in coverage by state (range: 10%--57%), with lower coverage reported from northern states. In the same year, Nigeria adopted a new Reaching Every Ward strategic approach^¶ to improve routine vaccination coverage. This strategy focused on enhancing health-worker training and supervision and improving outreach at the local ward level.

Four national and two subnational SIA campaigns with trivalent OPV (tOPV) were conducted in 2005; one national campaign was conducted in February 2006. Despite these campaigns, the number of confirmed polio cases in the second half of 2005 and early 2006 indicated that a substantial proportion of children had not been immunized and remained susceptible to poliovirus infection, especially polio infection attributed to WPV1. The National Programme on Immunization began using mOPV1, which is more effective than tOPV against WPV1, during the March 2006 SIAs in 32 of the 37 states (including all northern states). In May 2006, the National Programme on Immunization introduced a modified strategy of SIA implementation, called immunization plus days (IPDs), during which OPV and other interventions were delivered using a combination of house-to-house vaccine delivery and fixed-post vaccination. Four IPD rounds were conducted during May--November 2006 in polio-affected northern states. The May, June, and September IPDs used mOPV1, whereas tOPV was used in the November round to provide protection against WPV3.

IPDs offered OPV and other vaccines (e.g., measles vaccine and diphtheria-tetanus-pertussis vaccine for eligible children and tetanus toxoid vaccine for pregnant women) at fixed vaccination posts, in addition to house-to-house delivery of OPV (and vitamin A twice in the year). In the targeted northern states, local government areas (LGAs) offered other health interventions when children were brought to vaccination posts. These interventions included distribution of soap, acetaminophen, oral rehydration salts, anthelminthics, and insecticide-treated bed nets. Additional modifications implemented as part of the IPDs were 1) holding community discussions to educate caregivers and address concerns before each round; 2) enhancing detailed SIA planning through involvement of local community leaders; 3) using qualified local health workers on the vaccination teams; and 4) enhancing field supervision by local, state, and federal authorities and through partner agencies.**

Acute Flaccid Paralysis (AFP) Surveillance

The Global Polio Eradication Initiative relies on an acute flaccid paralysis (AFP) surveillance system to identify cases of poliomyelitis. Through this system, AFP cases in all children aged <15 years and suspected polio in persons of any age are reported and investigated as possible poliomyelitis. AFP surveillance quality is monitored according to World Health Organization (WHO) operational targets. In 2005, Nigeria achieved a national nonpolio AFP detection rate of 7.6 cases per 100,000 population aged <15 years, compared with the WHO target of two cases, increasing to 7.9 per 100,000 children in 2006. In 2005, all 37 states and 85% of the 774 LGAs achieved nonpolio AFP rates of more than two cases per 100,000; in 2006, 90% of LGAs achieved this rate. In 2005, adequate stool specimens were collected for 85% of AFP cases nationally; this percentage increased to 90% in 2006. In 2005, 68% of states and 62% of LGAs reached the target of >80% AFP cases with adequate stool specimens; in 2006, 86% of states and 73% of LGAs reached this target. The proportion of LGAs that reached the target levels for both surveillance indicators increased from 52% in 2005 to 64% in 2006.

Vaccination histories of children aged 6--59 months with nonpolio AFP were used to estimate OPV coverage of the overall target population. In the 10 states with high polio incidence,^§§ the proportion of nonpolio AFP cases in children who had never received any OPV decreased from 45% in the first quarter of 2005 to 31% in the first quarter of 2006 (Table). After the introduction and continuation of IPDs, the average proportion of nonpolio AFP cases in children who had never received any OPV in these states decreased to 18% in the fourth quarter of 2006.

WPV Incidence

Of the 1,959 WPV cases reported during 2005--2006, a total of 830 (42%) occurred in children aged <2 years (1,867 [95%] were aged <5 years); 1,483 (76%) of cases were in children who had received <3 doses of OPV. During the late 1990s and early 2000s, WPV transmission in Nigeria peaked in July and August during the rainy season and reached its lowest during the dry season (7). In late 2005, the monthly case incidence of WPV1 was atypically high for the dry season. This was followed early in 2006 by a substantial increase in the number of cases compared with the same period in earlier years. The peak in WPV1 circulation in 2006 occurred in March, with a rapid decrease in cases commencing in June (Figure 1). The decrease in WPV1 incidence was pronounced in the three states with the highest incidence of poliomyelitis (Jigawa, Kano, and Katsina) (Figure 2).^¶¶ During July--December 2006, WPV1 incidence for these three states was 63% lower than the same period in 2005 (64 cases in 2006 versus 174 cases in 2005). WPV3 circulation did not decrease substantially during 2006.

Of the 830 WPV polio cases with onset in 2005 (580 WPV1 and 250 WPV3), a total of 224 (27%) were reported from Kano state (117 WPV1 and 107 WPV3), and 544 (65%) were reported from nine other high-incidence states (409 WPV1 and 135 WPV3). Of the 1,129 polio cases with onset in 2006 (851 WPV1 and 278 WPV3), 355 (31%) were from Kano (303 WPV1 and 52 WPV3) and 718 (64%) were from the other nine high-incidence states (504 WPV1 and 214 WPV3). Despite the increase in case numbers in 2006, the area of transmission decreased, from 21 affected states (57% of the 37 states in Nigeria) in 2005 to 18 states (49%) in 2006. No state in southern Nigeria has detected WPV since August 2005.

In 2005, a total of 27 WPV1 and 18 WPV3 genetic virus clusters were detected circulating in Nigeria.*** In 2006, fewer WPV1 and WPV3 genetic clusters were observed; pending completion of genetic analyses and further observation in 2007, the extent of this decrease is unclear. WPV1 and WPV3 found in Cameroon, Chad, and Niger during 2005--2006 were closely related to viruses found in nearby Nigerian states.

Reported by: National Programme on Immunization, Federal Ministry of Health; Country Office of the World Health Organization, Abuja; Poliovirus Laboratory, Univ of Ibadan, Ibadan; Poliovirus Laboratory, Univ of Maidugari Teaching Hospital, Maidugari, Nigeria. African Regional Polio Reference Laboratory, National Institute for Communicable Diseases, Johannesburg, South Africa. Vaccine Preventable Diseases, World Health Organization Regional Office for Africa, Brazzaville, Congo. Immunization, Vaccines, and Biologicals Dept, World Health Organization, Geneva, Switzerland. Div of Viral Diseases and Global Immunization Div, National Center for Immunization and Respiratory Diseases, CDC.

Editorial Note:

Although SIAs were resumed in all areas in Nigeria in mid-2004, they did not sufficiently curtail the resurgence of WPV transmission that began in 2003--2004. The number of cases in northern states increased substantially in late 2005 and early 2006, particularly WPV1 cases. In December 2005, the Nigerian government mandated the National Programme on Immunization to accelerate polio eradication and enhance routine vaccination. The introduction of mOPV1 in March 2006 and improved community acceptance in response to the introduction of IPDs in May 2006 have been associated with a decrease in WPV1 monthly incidence; in 2006, only 22% of 851 WPV1 cases occurred in the second half of the year. An increase in population immunity against poliovirus infection also is indicated by the decrease in the proportion of nonpolio AFP cases in children who have received zero doses since the introduction of IPDs in May 2006.

In 2006, polio-eradication measures were concentrated in the 10 states with the most intense transmission of WPV. Because approximately 50% of the target population remains undervaccinated in high-incidence states, further improvements in immunization levels are needed to interrupt poliovirus transmission. The continued involvement of traditional and religious community leaders will be essential to increase both SIA and routine vaccination coverage.

Although the sensitivity of AFP surveillance and the number of reported cases increased in 2006 compared with 2005, WPV was found in fewer states. Genetic sequence analysis suggests that several genetic virus clusters have been eliminated in 2006; however, by the end of the year, numerous independent chains of transmission persisted.

Compared with tOPV, mOPV1 is more effective, dose for dose, in immunizing children against WPV1; a single dose of mOPV1 has been estimated as the equivalent of 3 doses of tOPV in terms of seroconversion against WPV1 (8). Thus, in 2006, the Nigerian Polio Program decided to target preferentially WPV1 circulation using mOPV1 for most SIAs. Because of its greater transmissibility, WPV1 poses a greater threat of wide geographic spread compared with WPV3, as was observed during the 2003--2004 polio outbreak in Nigeria and neighboring countries. Use of tOPV in IPDs in northern states in November 2006 and early 2007 should curtail WPV3 transmission.

For 2007, priority has been given to further improving IPDs planning and supervision in the highest-risk LGAs in the high-incidence states. Communication and health-education activities will continue to be modified and strengthened according to findings from program monitoring and evaluation. These measures are expected to increase the impact of SIAs during the WPV low-transmission season and of SIAs scheduled for the remainder of the year. Nigeria achieved some key milestones in 2006 toward improving child survival. The Nigerian government and its immunization partners are committed to interrupting WPV transmission in Nigeria and to building sustainable means of enhancing child health. Continuation and expansion of IPDs and use of mOPV1 is needed to interrupt WPV1 transmission in Nigeria; periodic use of tOPV will continue to reduce WPV3 circulation.

References

* Loss of public confidence resulted from rumors regarding OPV safety, including false allegations that the vaccine could cause sterility or acquired immunodeficiency syndrome in vaccine recipients. Rumors were promoted, especially in northern Nigeria, in response to political tensions at national and state levels. All allegations were refuted on scientific grounds.

Mass campaigns conducted during a short period (days to weeks) during which a dose of OPV is administered to all children aged <5 years, regardless of previous vaccination history. Campaigns can be conducted nationally or in portions of the country.

^§ mOPV1 contains polio vaccine against WPV1 only and does not provide protection against other WPV types. mOPV1 provides greater immunity to a specific WPV type than does the same number of doses of trivalent OPV. mOPV3 is not yet available in Nigeria.

^¶ An adaptation of the World Health Organization Regional Office for Africa and global Reaching Every District initiative.

** National Programme on Immunization of the Nigeria Ministry of Health, Association of Local Governments of Nigeria, Nigerian state governments, World Health Organization, Rotary International, CDC, United Nations Children's Fund (UNICEF), European Union, International Federation of Red Cross/Red Crescent, World Bank, the Global Alliance for Vaccine and Immunization, the Vaccine Fund, and bilateral development agencies of Canada, Norway, Japan, the United Kingdom, and the United States (U.S. Agency for International Development [USAID]).

The current WHO operational targets for countries at high risk for polio transmission are a nonpolio AFP rate of at least two cases per 100,000 population aged <15 years at each subnational level and adequate stool specimen collection for >80% of AFP cases (i.e., two specimens collected >24 hours apart, both within 14 days of paralysis onset, and shipped on ice or frozen ice packs to a WHO-accredited laboratory and arriving at the laboratory in good condition).

^§§ Bauchi, Borno, Jigawa, Kaduna, Kano, Katsina, Kebbi, Sokoto, Yobe, and Zamfara.

^¶¶ As of March 23, 2007, with laboratory investigations for January AFP cases 99% complete, 27 cases of confirmed WPV have been reported provisionally in the country for January 2007 (seven WPV1 and 20 WPV3), compared with 24 in 2005 (nine WPV1 and 15 WPV3) and 89 in 2006 (80 WPV1 and nine WPV3).

*** All WPVs are sequenced across the interval encoding the major capsid protein (VP1) (approximately 900 nucleotides), and results are analyzed to determine the likely origin (by state and LGA) of the virus. Isolates within a cluster share >95% VP1 nucleotide sequence identity.

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