Fig. 1

Assignment of cases to group 1 and group 2. Example of cases in groups 1 (left) and 2 (right). The vertical solid red lines indicate a case’s date of onset of disease; the vertical dashed red lines indicate the period of probable infection (PPI), being 8 – 33 days before the date of onset of disease [23]. The grey polygon indicates the cumulative dose prior to the PPI (C_PRE); the red polygon indicates the cumulative dose during the PPI (C_PPI). The probability of infection has increased to 1.0 at the end of the PPI. For cases in group 1 the surface of the red polygon is greater than that of the grey polygon (C_PPI > C_PRE); and vice versa for cases in group 2 (C_PPI ≤ C_PRE)