The present invention is directed toward fluid transfer devices including a vial adapter having a top wall and a cannula with a cannula tip, and an elastic O-ring like sealing element sealingly encircling the cannula and initially disposed towards the cannula tip and spaced apart from the top wall, the sealing element being brought into initial contact with the vial stopper subsequent to the cannula tip contacting the vial stopper at a puncture site and thereafter being slidingly urged towards the top wall and continuously sealing the puncture site during snap fit mounting the vial adapter on the vial.
