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Distribution of nucleotide substitutions in the VP1 regions of Sabin 2-related polioviruses isolated from patients with AFP from 2005 to 2011 in the northern (n = 612) (A) and southern (n = 83) (B) states of Nigeria. Each isolate is from an individual patient. The northern and southern states are identified in Fig. 1. Type 2 vaccine-derived polioviruses (VDPV2s) (black bars) are more divergent, having ≥6 nt substitutions in the VP1 region (48).
