The IESB has a great interview with the visual effects supervisor Jim Berney who has previously worked on The Polar Express, The Lion, the Witch, and the Wardrobe, and both Matrix sequels. In the interview, he discusses the challenges of creating an abandoned Times Square, the infected creatures, and the evacuation of New York City.
While the creatures were originally supposed to be shot practically, they decided to shoot them CG to get the proper effect they wanted. He says,
But even as those days came closer and closer to shooting, we didn’t know what we wanted the creatures to be but we knew it was a little bit more than what you can get out of make-up and costumes. We wanted to be able to see the underlying human structure. We wanted to see the muscle and the bone and the tendons and things like that. We wanted to see the light transmit through and the pulsing veins and things like that. So it was around the end of December, when we came to our Christmas hiatus, that we made the decision to go just completely CG.