A follow-up immunization. Some vaccinations, most notably the Tetanus/Diphtheria shots, need to be updated every so often to maintain effectiveness. Each one after the first is called a booster shot, because they boost the effectiveness of the first one. This term is usually not used to describe one of a one-time series of shots - the third shot of the hepatitis B series, for instance, wouldn't be called a booster.

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