where s(t) is a scalar, which describes the extent to which behaviours resulting in disease transmission are maintained. A reduction in s(t) indicates that disease transmission has decreased for some reason other than the depletion of susceptibles. For example, when the population is behaving normally (i.e. no social distancing), s(t)Z1, and when potentially infectious contacts are reduced by half, s(t)Z0.5. We consider that the population closely approached normal behaviour during periods B and F, and possibly during period D, i.e. s(B)Zs(D)Zs(F)Z1 (table 1) . Our aim is to use this model to estimate s(t) by estimating R(t) and s(t). More specifically, we seek to estimate R A ðtÞ Z R 0 sðtÞ Z RðtÞ sðtÞ ; ð4:2Þ