A needle assembly (10; 210) for a prefilled injection syringe is described. A needle holder (12; 212) has a needle (14; 214) secured to it and is adapted to be secured to a syringe barrel of the injection syringe. A needle sheath (16; 216) has a distal end forming a releasable sterile seal with the needle holder (12; 212), and surrounds and shields the needle (14; 214). A needle seal (18; 218) surmounts the needle tip and is arranged at least in part in the proximal end of the needle sheath (16; 216) and closed off by a closure element (20; 220). A closure connection between the closure element (20; 220) and the needle sheath (16; 216) is configured as a non-releasable and sterile barrier (54; 254) produced by means of bonding or welding.