Outbreak of type 2 vaccine-derived poliovirus in Nigeria: emergence and widespread circulation in an underimmunized population

Abstract

Wild poliovirus has remained endemic in northern Nigeria because of low coverage achieved in the routine immunization program and in supplementary immunization activities (SIAs). An outbreak of infection involving 315 cases of type 2 circulating vaccine-derived poliovirus (cVDPV2; >1% divergent from Sabin 2) occurred during July 2005-June 2010, a period when 23 of 34 SIAs used monovalent or bivalent oral poliovirus vaccine (OPV) lacking Sabin 2. In addition, 21 "pre-VDPV2" (0.5%-1.0% divergent) cases occurred during this period. Both cVDPV and pre-VDPV cases were clinically indistinguishable from cases due to wild poliovirus. The monthly incidence of cases increased sharply in early 2009, as more children aged without trivalent OPV SIAs. Cumulative state incidence of pre-VDPV2/cVDPV2 was correlated with low childhood immunization against poliovirus type 2 assessed by various means. Strengthened routine immunization programs in countries with suboptimal coverage and balanced use of OPV formulations in SIAs are necessary to minimize risks of VDPV emergence and circulation.

Figure 1.

Map of Nigeria showing state geopolitical zones and population density by LandScan [36].

Figure 2.

Acute flaccid paralysis (AFP) cases associated with circulating vaccine-derived poliovirus type 2 (cVDPV2), pre-VDPV2, and co-infections with wild poliovirus type 1 (WPV1) or type 3 (WPV3), by month of onset and timing of supplementary immunization activities (SIAs) for northern states by oral poliovirus vaccine (OPV) formulation, Nigeria, 1 January 2005 through 30 June 2010. bOPV, bivalent oral poliovirus vaccine types 1 and 3; mOPV1, monovalent oral poliovirus vaccine type 1; mOPV3, monovalent oral poliovirus vaccine type 3; tOPV, trivalent oral poliovirus vaccine.

Figure 3.

Map of Nigeria showing state by proportion of children with diphtheria-tetanus-pertussis dose 3 (DPT3) coverage at 12–23 months of age (a proxy for oral poliovirus vaccine dose 3 or more [OPV3+] coverage) estimated from the 2008 Demographic Health Survey (2007 birth cohort) overlaid with cases of circulating vaccine-derived poliovirus type 2 (cVDPV2), pre-VDPV2, or coinfections with wild poliovirus type 1 (WPV1) or type 3 (WPV3), 2005–2010 (A–F). 1 dot equals 1 case. Cases were mapped randomly within local government area borders. FCT, Federal Capital Territory.

Figure 4.

Annual combined circulating vaccine-derived poliovirus type 2 (cVDPV2) and pre-VDPV2 case incidence rates, by year of birth and year of onset, Nigeria, 1 July 2005 through 30 June 2010. Table insert, Cumulative number of supplementary immunization activities (SIAs) with trivalent oral poliovirus vaccine (tOPV) in the affected states with cVDPV2 and pre-VDPV2 cases by year of birth since 2000 and year of onset, Nigeria, 2005–2010.

Figure 5.

Cumulative incidence of combined circulating vaccine-derived poliovirus type 2 (cVDPV2) and pre-VDPV2, by state, compared with state oral poliovirus vaccine (OPV) immunization coverage using LOESS polynomial regression line (r, Spearman rank correlation coefficients), Nigeria, 1 July 2005 through 30 June 2010. Estimated OPV3+ (≥3 doses of OPV) coverage (A) or absence of OPV vaccination (OPV0) (B) from caregiver recall histories for children aged 6–59 months with nonpolio acute flaccid paralysis (NP-AFP) in 2005. Also shown are the estimated diphtheria-tetanus-pertussis dose 3 (DTP3) coverage at 12–23 months of age (C) or absence of vaccination (ie, 100% – %DTP dose 1 [DTP1], [1–DTP1]) (D) from the 2006 national immunization coverage survey (NICS, 2005 birth cohort), as well as the estimated DPT3 coverage (E) or absence of vaccination (1–DTP1) (F) from the 2008 Demographic and Health survey (DHS, 2007 birth cohort). All correlations differed significantly from 0 (P < .001).