effective reproduction number with vaccination only, R V , does not necessarily decrease as the effective vaccination rate ν i π i increases. In other words, vaccination is not always beneficial in reducing incidence and the design of an effective vaccination program in multigroup model is highly nontrivial, unlike in simple epidemic models where there is a simple formula for the critical vaccination coverage level necessary for eradication (p c ) [20] . In fact, it has been shown mathematically by Hadeler and Castillo-Chavez [32] , using a model with a core group, that partially effective vaccination program may actually increase the total number of cases. Explicit quantification of an optimal vaccination policy ( [33] ) in a multi-group population scenario remains to be a challenge for mathematical modelers and is beyond the scope of this work. Moreover, the model fit was carried out with vaccination (of the elderly and small children of age 2 or less) as the only intervention since vaccination was the only intervention that was implemented during the flu seasonal for which the fitted P&I data was collected. Further simulations with quarantine and home withdrawal also can be easily carried out.