Some of the adults are immune to the signal.
- Such as the cops fighting the parents during the school scene and the hospital staff fighting Kendall’s sister during the hospital scene.
- The immunity seems to come from 1) Simply not having any children of their own 2) Not being subjected to the signal yet. In all cases it was triggered by first seeing static from computer displays, television sets or just the display of a car infotainment system 3) Combination of both, as the parents only seemed to be interested in killing only their own children, not just any children.
People can build up immunity to the signal.
The signal is being sent by a malevolent alien force
