The mechanism of resistance to antibiotics (the anti bacterial) drugs and to antivirals drugs are quite different. For instance a bacteria may acquire an enzyme capable of degrading the antibiotic. The viruses have a limited genome and I am not aware of them acquiring enzymes to degrade the antiviral drugs. Though, viruses mutate a lot. The antiviral drugs are designed to bind to some key molecule of the virus. However, it may so happen that a mutation in that protein molecule of the virus which happens, such that - its function remain intact, but the structure changes just a tat bit and decreases efficiency of antiviral drug to bind to it. In such a case this resistant strain will be selected for. Though anti viral drugs are not prescribed as much as antibiotics and this problem is not as big. But it might be of concern to patient being treated for chronic viral infections - HIV, Hepatitis, HSV, HPV etc. Strasfeld and Chou in this review describe some of the antiviral drug resistance mechanisms. You can follow up the references for further studies and let me know if you have any further questions on this. I will answer about the flu vaccines in next comment.