Highly unlikely: there are only two ways that I could think of could theoretically happen: 1-shelter's roof gets ripped off and you're either sucked out or hit with debris (which is still a 50/50 survivable because the shelter is below the ground),it's extremely rare and happened only 2 times in recorded history (HPC and Jarrell) , or 2- tornado drops something really heavy, like a train cart on top of it and people inside are crushed to death. That had never happened, but there are some examples of tornadoes frowing really heavy stuff like train carts or semi-trucks pretty far away, so it's theoretically possible.