We also fitted the data by assuming the per contact transmission probability of the infectors in i th group are averaged, i.e., b ij ≡ b j (see the last row in bold in Table  2 and Fig. 4 ). We will compare these results to ascertain the difference in age groups on per contact transmission probabilities to and from particular age groups. To give more insight to the data used and the goodness of fit, we also provide the daily observed and predicated mortality corresponding to the case b ij ≡ b j in [Additional file 1: Figs. S1-S2].