There's another angle that could occur here.
If Ellie is the 'cure' somehow, and they tell us this near the beginning, that begs for much deeper plot twists and events. I don't want to sit here and list off examples insinuating that I have the ability to do such. However, lets just say that Ellie is the cure, then at a certain point she gets bitten but doesn't transform, and instead loses her immunity status.
Or, they could simply treat her status as a minor importance to all the other hell the pair have to go through.
Ellie is 14 years old, and it took that long for anyone to know she can be the cure? Seems a bit far out there for that. As someone else said, if she was the cure, she'd be way too important for just one guy to take her out. I also find it incredibly cliché and unless Naughty Dog did something incredibly different with it, for a plot point it would be incredibly dull. I'd rather not see any cure at all. Let the world be screwed and keep it screwed.
I'd like to believe this as well. One of the things that I liked about TLoU was that Neil said in an interview they weren't going into specifics about what happened. Meaning its not going to be one of those games where we get text log after text log from scientists detailing what exactly went down, where it went down, why it went down, who saw it go down ect. And honestly for a horror element, that's the way to go I think. It's nice to be left in the dark.