No, you don’t throw the grenade and then vampires come out, as cool as that would be. The Transfusion Grenade bursts and drains health which then transfers to you in the form of healing orbs. That’s the weirdest of the bunch; other grenades like the Barrage Grenade and the Concussive Grenade work more or less the way you would expect these tools to function. So get ready to have some fun lobbing grenades around when the game patches them in on August 8th.
Source: Official Site