Will Saoirse Ronan Usher in 'The Host' Trilogy? 

I think they cnose young actors so they will still be age appropriate if and whenthe final movie is made. I!m taking that as a good sign there will be sequels.

That's a theory, I always thought that they gave the part to Saoirse because of the credibility of having an Oscar nominated actress gives to the project and then the young cast was just necessary.