booster shot

from The American Heritage® Dictionary of the English Language, 4th Edition

• n. An additional dose of an immunizing agent, such as a vaccine or toxoid, given at a time after the initial dose to sustain the immune response elicited by the previous dose of the same agent. Also called booster dose.

from Wiktionary, Creative Commons Attribution/Share-Alike License

• n. A dose of an antigen designed to strengthen immunity after an earlier immunizing dose.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• n. an additional dose that makes sure the first dose was effective