In I Am Legend (2007), Robert and Sam get ambushed and Sam ends up getting bitten by the zombie dogs. Since dogs are only immune to the airborne version, Sam got infected. Robert takes Sam back to his lab where he ends up killing her after she turns.
Why not give Sam the antidote? He saw clearly that the rat had shown signs of regression or that the virus was no longer dominant. There wasn't any reason (that I could see at least) to not give Sam the antidote. The human subject was still alive at that point as well.