"Vaccine that is effective against the virus infection.
The vaccine is an anti-"t" reagent, which makes a first and only appearance in Resident Evil 3: Nemesis. After Mikhail sacrificed himself in an explosion with "the pursuer", it returned and fought Jill. She won the battle, but was infected by a tentacle of the creature before her victory. Carlos took Jill to the Clock Tower's chapel and left for the Raccoon General Hospital to search for a cure.
This item can be obtained by combining the Vaccine Base and Vaccine Medium together. When Carlos has already created it or has the two items required for its creation and heads back to the Hospital Lobby, a bomb will appear in the nearby pillar. Nikolai has set up the bomb, and shortly after Carlos runs away from the hospital, Nemesis will appear in the clock tower.
After returning to the chapel, to cure Jill, Carlos can just talk to her, or use the Vaccine on her. After a short cutscene, the player takes control of Jill again.