An underrun protection device for a vehicle comprising: a hybrid beam formed of a preform member and a thermoplastic formed onto the preform member, and configured to attach to a vehicle attachment element, the hybrid beam comprising: the preform member, which defines a channel and is configured to couple with the vehicle and extend along a substantial portion of a side of the vehicle parallel with a centerline of the vehicle, with an opening of the channel oriented away from the centerline, wherein the channel is defined by an inside surface at least partially facing away from the centerline, and an outside surface at least partially facing the centerline; and a thermoplastic beam formed of the thermoplastic and shaped to conform to the channel of the preform member, the thermoplastic beam at least partially abutting the inside surface of the channel, the thermoplastic beam defining a pattern of interstices defined by ribs (64, 66, 68, 69), with the ribs (64, 66, 68, 69) extending perpendicular to the inside surface of the channel; wherein the hybrid beam is configured to couple with the vehicle at least partially below an underbody of the vehicle, to at least partially occlude a passage to a space under the underbody from outside of the vehicle. a hybrid beam formed of a preform member and a thermoplastic formed onto the preform member, and configured to attach to a vehicle attachment element, the hybrid beam comprising: the preform member, which defines a channel and is configured to couple with the vehicle and extend along a substantial portion of a side of the vehicle parallel with a centerline of the vehicle, with an opening of the channel oriented away from the centerline, wherein the channel is defined by an inside surface at least partially facing away from the centerline, and an outside surface at least partially facing the centerline; and a thermoplastic beam formed of the thermoplastic and shaped to conform to the channel of the preform member, the thermoplastic beam at least partially abutting the inside surface of the channel, the thermoplastic beam defining a pattern of interstices defined by ribs (64, 66, 68, 69), with the ribs (64, 66, 68, 69) extending perpendicular to the inside surface of the channel; wherein the hybrid beam is configured to couple with the vehicle at least partially below an underbody of the vehicle, to at least partially occlude a passage to a space under the underbody from outside of the vehicle. the preform member, which defines a channel and is configured to couple with the vehicle and extend along a substantial portion of a side of the vehicle parallel with a centerline of the vehicle, with an opening of the channel oriented away from the centerline, wherein the channel is defined by an inside surface at least partially facing away from the centerline, and an outside surface at least partially facing the centerline; and a thermoplastic beam formed of the thermoplastic and shaped to conform to the channel of the preform member, the thermoplastic beam at least partially abutting the inside surface of the channel, the thermoplastic beam defining a pattern of interstices defined by ribs (64, 66, 68, 69), with the ribs (64, 66, 68, 69) extending perpendicular to the inside surface of the channel; wherein the hybrid beam is configured to couple with the vehicle at least partially below an underbody of the vehicle, to at least partially occlude a passage to a space under the underbody from outside of the vehicle.