Were such a thing to happen in a manner that was not limited by its own lethality (Ebola), the human race would have been long extinct, human intervention by antiviral or not.
However, viruses do not progress/mutate in that exact manner.
It's a pill. It's a treatment, not a mandated vaccine for prevention.
Tamiflu Pediatric Adverse Events: Questions and Answers
12 Japanese children out of 24 million total who were in Japan is equal to 1 dead, not necessarily from the Tamiflu but possibly from influenza itself, out of 2,000,000 courses of treatment.
Annual seasonal influenza mortality is 36,000 dead in a total population of aproximately 300,000,000 folks in this country is equal to 1 dead out of 8,333 potentially exposed.
It appears the odds are with Tamiflu.