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Booster shots are additional doses of a vaccine needed periodically to "boost" the immune system.  

Booster shots are additional doses of a vaccine needed periodically to "boost" the immune system.  

For example, the tetanus and diphtheria (Td) vaccine require booster shots which are recommended for adults every ten years.

For example, the tetanus and diphtheria (Td) vaccine require booster shots which are recommended for adults every ten years.<ref>http://www.cdc.gov/vaccines/about/terms/glossary.htm#b</ref>