An airbag for a vehicle, such as a side airbag, may be mounted to or proximate to a seat assembly on which a passenger may sit. The side airbag may include a chamber and an inflator configured to cause the chamber to expand toward the passenger to slow an acceleration of the passenger responsive to an imminent impact of the vehicle with another object or surface. Upon deployment of the chamber, the side airbag may cause a portion of the seat assembly (a seat tub, seat pan, and/or cushion coupled thereto) to deform toward the passenger seated thereon while remaining occluded from the passenger by the assembly. In some examples, the chamber may be stored in an unfolded position to allow for a more rapid deployment.