In the beginning of the summer, voice actor Nolan North (Uncharted's Nathan Drake) casually confirmed the development of The Last of Us 2, but then Joel voice actor Troy Baker said he had no knowledge of such a project.
Now, after several tips, rumors and clues, it should sound just natural that Naughty Dog is working on a sequel to the highly acclaimed The Last of Us (and perhaps something completely new), as the final touches are currently being added to Uncharted 4: A Thief's End. And it looks like developers from the studio feel the same way, as some of them commented the following during yesterday's livestream:
"(...) and he was the one in charge of face animation for the 3 Uncharted games and the first The Last of Us game... did I just say the first The Last of Us game?"
The quote, via Gamepur, doesn't specifically mention The Last of Us 2, but of course its intentional wording implies that there's a second game and that Naughty Dog want fans to start getting hyped.
However, no significant info should be expected until Uncharted 4: A Thief's End launches in March... or perhaps they're planning some sort of teaser at the PlayStation Experience in December?