An assembly for a vehicle (10, Fig.1) comprising a seat 12 and a collar 14 supported by the seat. The collar has a first end 16 and a second end 18, with a respective first 20 and second fastener 22. The first and second fasteners are releasably connectable with each other. An airbag 24 is disposed in the collar and inflatable to an inflated position (see fig.7A). The inflated position may define a head cavity (88) and face opening (90) with hood (86). An inflator 96 may be disposed on the seat, in fluid communication with the airbag via a resiliently flexible coiled tube 100. Alternatively, a fill tube connected to an intermediate portion 62 of the collar may support the collar on the seat. The collar may alternatively be supported by a back 36 of the seat. The first and second fastener may comprise a co-operating buckle 68 and tongue 70 (Fig.8A), or a zip 74 (Fig.8B). The assembly may comprise a retractor mounted on the seat, attached to the collar by a flexible webbing. The assembly may comprise a seat bottom 38 having a rotational mechanism 60, allowing seat rotation within the vehicle such as an autonomous vehicle.