Fig. 2.

An SEIR-type model for poliovirus, incorporating vaccination and environmental surveillance. The model represents infection by two strains of poliovirus (OPV1 as part of bOPV, with subscript $o$, and WPV1, with subscript $w$). Individuals infected with OPV1 acquire it through vaccination (superscript $v$) or transmission (superscript $t$). Model equations are given in Eqs. 3 and 4, and parameters are in Table 1.