Abstract: A vascular access system comprising a sheath, a slitted sleeve, and a sealing portion of the slitted sleeve. The slitted sleeve comprises a tubular sleeve body having a longitudinal slit along its length, wherein the slitted sleeve is adapted to receive a first portion of a medical device therein. The longitudinal slit closes, but does not seal, after the first portion of the medical device is inserted therein. The sealing portion of the slitted sleeve is configured to fill a gap and form a seal between an inner surface of the sheath and a portion of the first portion of the medical device disposed in the sheath when the sheath is inserted in the blood vessel and the slitted sleeve is inserted into the sheath lumen. A vascular access device accessory and a method for inserting it into a sheath are also provided.