booster shot

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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.

• noun an additional dose that makes sure the first dose was effective