Developer Naughty Dog said that it began work on the game alongside Uncharted 3: Drake's Deception, meaning that the studio was working on two projects for the first time in its history, according to Eurogamer.
"We were very serious about keeping it a secret, limiting the people that were exposed to it even internally in Sony," said co-president Evan Wells.
The project was at risk of being leaked several times, with creative director and writer Neil Druckmann leaving his iPad containing a trailer for it on a plane.
However, it was an Easter Egg left in Uncharted 3: Drake's Deception of a newspaper teasing a fungus virus inside the game's first level that "ratted" The Last of Us out to players.
Wells added that Uncharted 3: Drake's Deception saw a tough crunch period after it was given a final release date a year from launch, something he said the studio will never do again.
The Last of Us, which was announced at the Spike TV VGAs, may not be released until 2013.
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