The gaseous form of the serum isn't exactly the same as with Captain America because it has granted him one superhuman attribute: an accelerated healing factor capable of rapidly regenerating damaged tissue much faster and more extensively than an ordinary human is capable of.
The gaseous form of the serum isn't exactly the same as with Captain America because it did not require the recipient to be exposed to Vita-Rays to stabilize it. Underworld has one more superhuman attribute: an accelerated healing factor capable of rapidly regenerating damaged tissue much faster and more extensively than an ordinary human is capable of.