Progress Toward Poliomyelitis Eradication --- Nigeria, January 2009--June 2010

Progress Toward Poliomyelitis Eradication --- Nigeria, January 2009--June 2010
Weekly
July 9, 2010 / 59(26);802-807


Nigeria has maintained a high incidence of wild poliovirus (WPV) cases attributed to persistently high proportions of under- and unimmunized children, and, for many years, the country has served as a reservoir for substantial international spread (1). In 2008, Nigeria reported 798 polio cases, the highest number of any country in the world (2). This report provides an update on poliovirus epidemiology in Nigeria during the past 18 months, January 2009--June 2010, and describes activities planned to interrupt transmission. Reported WPV cases in Nigeria decreased to 388 during 2009 (24% of global cases), and WPV incidence in Nigeria reached an all-time low during January--June 2010, with only three reported cases. Cases of circulating type 2 vaccine-derived poliovirus (cVDPV2), which first occurred in Nigeria in 2005 (3), also declined, from 148 during the 12 months of 2009, to eight during the 6-month period, January--June 2010. One indicator of the effectiveness of immunization activities is the proportion of children with nonpolio acute flaccid paralysis (AFP) who never have received oral poliovirus vaccine (OPV). In seven high-incidence northern states of Nigeria, this proportion declined from 17.6% in 2008 to 10.7% in 2009. During 2009--2010, increased engagement of traditional, religious, and political leaders has improved community acceptance of vaccination and implementation of high-quality supplementary immunization activities (SIAs). Enhanced surveillance for polioviruses, further strengthened implementation of SIAs, and immediate immunization responses to newly identified WPV and cVDPV2 cases will be pivotal in interrupting WPV and cVDPV2 transmission in Nigeria.

Immunization Activities

Routine immunization against polio in Nigeria consists of trivalent OPV (tOPV, types 1, 2, and 3) at birth and at ages 6, 10, and 14 weeks. Immunization coverage is measured using both administrative data (estimated doses administered per targeted child population, determined by official census numbers) and coverage surveys. In 2009, using administrative data, national routine immunization coverage of children by age 12 months with three tOPV doses was 63% (range by state: 35%--90%) (4). Using coverage surveys, the estimated national coverage with three tOPV doses at 12--23 months was 39%, but lower in the northeast (28.6%) and northwest (24.3%) areas of Nigeria, including the seven high-incidence northern states (5).*

In addition to routine immunization, Nigeria conducts SIAs† for polio eradication using monovalent OPV type 1 (mOPV1), monovalent OPV type 3 (mOPV3), bivalent OPV types 1 and 3 (bOPV), or tOPV. Monovalent vaccines are more effective than tOPV in providing protection against the corresponding WPV serotype; bOPV is nearly equivalent to mOPV and superior to tOPV in producing seroconversion to WPV1 and WPV3 (6). Three national SIAs were conducted in 2009, using mOPV3, mOPV1, and tOPV. Five subnational SIAs were conducted in 2009, each using mOPV1, mOPV3, tOPV, or both mOPV1 and mOPV3. During January--June 2010, two national SIAs were conducted, one with bOPV and one with tOPV; bOPV, mOPV1, and mOPV3 were used in three subnational SIAs (Figure 1).

Vaccination histories of children with nonpolio AFP are used to estimate OPV coverage among the population of children aged 6--59 months. The proportion of children with nonpolio AFP reported to have never received an OPV dose (zero-dose children) from the seven high-incidence northern states declined from 17.6% in 2008 to 10.7% in 2009 (range: 0%--17.0%), with the highest proportions occurring in Zamfara and Kano states (Table). In contrast, the proportion of reported zero-dose children was 2.2% in 13 other northern states and 1.8% in 17 southern states in 2009. The proportion of children with nonpolio AFP reported to have received ≥4 OPV doses was 37.4% in the seven high-incidence northern states and 60.8% for the entire country.

AFP Surveillance

AFP surveillance is monitored using World Health Organization (WHO) targets for case detection and adequate stool specimen collection.§ The national annualized nonpolio AFP detection rate among children aged <15 years was 8.2 per 100,000 during January--March 2009 and 9.0 per 100,000 during January--March 2010. Nonpolio AFP detection rates meeting the WHO target were achieved in all 37 Nigerian states during January--December 2009 and in all but one state (Plateau) during January--March 2010.

The WHO adequate stool specimen target was reached in all 37 states and in 683 (88%) of 776 local government areas (LGAs) during January--December 2009, and in 36 states and 557 (72%) LGAs during January--March 2010. The proportion of LGAs meeting both surveillance indicators (nonpolio AFP detection rate meeting the target and adequate stool specimen collection rate) rose from 78% in 2008 to 86% in 2009.

WPV and cVDPV Incidence

Reported WPV type 1 (WPV1) cases declined from 67 during January--June 2009 to seven during July--December 2009, and to one case during January--June 2010 (provisional data, as of July 5, 2010) (Figure 2). Of the 75 WPV1 cases reported during the entire 18-month period, January 2009--June 2010, seven (9%) occurred in the seven high-incidence northern states, 33 (44%) in other northern states, and 35 (47%) in southern states. The number of LGAs with WPV1 cases declined from 49 during January--June 2009 to one during January--June 2010 (Figure 2). Reported WPV type 3 (WPV3) cases declined from 290 during January--June 2009 to 24 during July--December 2009, and to two during January--June 2010. Only three cases of WPV have been reported during the first 6 months of 2010. Among 316 WPV3 cases reported from January 2009--June 2010, 240 (76%) occurred in the high-incidence northern states, 75 (24%) in other northern states, and one (<1%) in southern states. The number of LGAs with WPV3 cases declined from 147 in January--June 2009 to two during January--June, 2010 (Figure 2). Of 391 WPV cases reported with onset during January 2009--June 2010, 270 (69%) occurred in children aged <3 years, 266 (68%) were in children reported to have received <4 OPV doses, and 66 (17%) were in zero-dose children. The number of cVDPV2 cases declined from 137 during January--June 2009 to 11 during July--December 2009, and to eight during January--June 2010.

All WPV isolates undergo partial genomic sequencing to determine genetic relatedness. Each 1% difference between two isolates correlates with approximately 1 year of undetected circulation between the specific chains of transmission. Differences greater than 1.5% indicate potential quality issues for surveillance. Three of the seven WPV1 isolates from July--December 2009 cases and the one WPV1 isolate from 2010 exhibited >1.5% divergence from the closest predecessor. Similarly, nine of the 24 (38%) WPV3 isolates from July--December 2009 and both 2010 WPV3 exhibited ≥1.5% divergence.

Reported by
National Primary Health Care Development Agency and Federal Ministry of Health; Country Office of the World Health Organization, Abuja; Poliovirus Laboratory, Univ of Ibadan, Ibadan; Poliovirus Laboratory, Univ of Maiduguri Teaching Hospital, Maiduguri, 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. Polio Eradication Dept, World Health Organization, Geneva, Switzerland. Div of Viral Diseases and Global Immunization Div, National Center for Immunization and Respiratory Diseases, CDC

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Progress Toward Poliomyelitis Eradication --- Nigeria, January 2009--June 2010