A vehicle includes a bulkhead. The vehicle includes an airbag supported by the bulkhead and inflatable to an inflated position. The airbag in the inflated position includes a main chamber and a pair of side chambers that extend transversely from the main chamber. The vehicle includes a seat having a seatback and a seat bottom that extends from the seatback in a direction away from the bulkhead. The seatback is between the side chambers in the inflated position.