This invention is directed to covers for holding and protecting needles of medical devices prior to use. It includes covers for holding and protecting needles of medical devices in which the needles are, in turn, contained within a retractable protective sleeve. In such case, the needle and its protective sleeve are held within the cover by a wedging action of a base portion with the cover. The cover contains projections which grip the base so that the needle and its protective sleeve can be turned with the cover to twist them on and off of the medical device safely. Longitudinal vanes in the cover can be included to hold a spring and locking collar over the protective sleeve in a retracted state prior to use. A cap with a flat end and square shoulders is aligned with a flat side of the cover and the entirety closed with a frangible security strip to indicate any possible tampering prior to use. A soft rubber seal inside the cover's closed end seals the needles of pre-filled devices against leakage. A pierceable capsule carried by the outside of the closed end of the cover contains a sealing material that can be used to seal the needle against further use.