It seems like director Marc Forster's adaptation of Max Brooks' oral history of the zombie war is diverging from the text even more than we thought—apparently it's being configured to allow for a couple of head-chopping sequels.
A piece on Pitt's evolving career in the L.A. Times had this little nugget buried in it, talking about what World War Z could mean for both Pitt and the studio releasing it:
For Pitt, the big sci-fi thriller also represents his strongest bid to have a big film franchise of his own, which might be viewed as the missing piece of his career jigsaw puzzle. Forster and Paramount Pictures each view World War Z as a trilogy that would have the grounded, gun-metal realism of, say, [Matt] Damon's Jason Bourne series tethered to the unsettling end-times vibe of AMC's The Walking Dead.
It makes sense: Paramount is spending more on World War Z—which now follows Pitt's United Nations fact finder who races, Contagion-style, to stop the spread of the zombie pandemic—than anyone's ever spent on a zombie movie before. They're going to want to milk it for as much as they can.