[ boo-ster ]
/ ˈbu stər /
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Origin of booster

An Americanism dating back to 1885–90; boost + -er1


boost·er·ish, adjective
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2022


What is a booster shot?

A booster shot is an additional dose of a vaccine given to a person in order to maintain or renew the effect of a previous dose. This is sometimes called a booster dose (especially since vaccines aren’t always given via shots) or simply a booster.

Vaccines strengthen the body’s immunity against disease. They work by introducing a killed, weakened, or otherwise modified version of a specific disease (such as a virus or bacterium) so that the body’s immune system will produce antibodies that will destroy those specific pathogens if they ever enter the body again.

A booster shot basically refreshes the immune system’s memory of a disease in order to strengthen immunity—it “boosts” the existing defenses.

Booster shots are commonly given to both children and adults. Common vaccines that typically involve one or multiple booster shots include those against tetanus, hepatitis A, human papillomavirus (HPV), and measles, mumps, and rubella (MMR). The timing of the booster shot varies depending on the vaccine—some are given a few months after the initial dose, while others are given years later. Medical organizations such as the Centers for Disease Control and Prevention (CDC) provide immunization schedules that recommend the frequency of booster shots.


Booster shots were a common topic of discussion in 2021 in the context of vaccination against the COVID-19 virus, especially after the emergence and spread of the more contagious Delta variant. Notably, the CEO of Pfizer, one of the companies that produced COVID-19 vaccines, said that it was likely that people would need COVID-19 booster shots to protect against mutations of the virus. Early data appeared to show that an additional dose significantly strengthened immunity. In July 2021, the CDC and Food and Drug Administration (FDA) issued a joint statement noting that the agencies were reviewing whether COVID-19 booster shots would be necessary, while also stating that they were not required at the time.

How to use booster in a sentence

British Dictionary definitions for booster

/ (ˈbuːstə) /

a person or thing that supports, assists, or increases power or effectiveness
Also called: launch vehicle the first stage of a multistage rocket
radio television
  1. a radio-frequency amplifier connected between an aerial and a receiver to amplify weak incoming signals
  2. a radio-frequency amplifier that amplifies incoming signals, retransmitting them at higher power
another name for supercharger
short for booster dose
slang, mainly US a shoplifter
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Medical definitions for booster

[ bōōstər ]

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.booster dose booster shot
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

Scientific definitions for booster

[ bōōstər ]

An additional dose of an immunizing agent, such as a vaccine or toxoid, given at a time period of weeks to years after the initial dose to sustain the immune response elicited by the first dose. Tetanus, diphtheria, and measles vaccines are commonly given in booster doses.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.