The final Season 1 episode of The Last of Us is remarkably faithful to the conclusion of the game, with one minor difference. In the game, Marlene explains that the mutation in Ellie's brain could be the key to a vaccine. On the show, she pointedly says "cure," suggesting that Ellie's DNA could be even more powerful than it is in the game.

Instead of just being a preventative measure that will keep others from getting infected, Ellie's DNA could also work as a potential treatment for those who have been infected but haven't turned yet. It's ultimately a minor distinction. Either way, Ellie's DNA and her death could be the key to freeing the world from cordyceps and ultimately returning things to the way they were before.

However, Joel isn't super into the idea of sacrificing his new surrogate daughter in pursuit of saving the world. He takes Ellie from the hospital unconscious, and lies to her when she wakes up. He said that the Fireflies had stopped looking for a cure and there were actually at least a dozen people who were immune like her. In reality, she was the only one immune, and that immunity could have been the key to changing everything.