A COVID-19 booster shot is an additional dose of the COVID-19 vaccine that is given after the protection provided by the original shot(s) has begun to decrease over time. This would mean a third dose of vaccine from Pfizer-BioNTech or Moderna, or a second dose of Johnson & Johnson, which are the three approved vaccines in the U.S. 

The COVID-19 vaccines are very effective in lowering the risk of severe disease, hospitalization, and death, and they even work against the Omicron and Delta variant. The CDC has found that this protection may wear off over time, which is why COVID-19 boosters are recommended for some people to keep the vaccines effective for longer. Find out if you are eligible for a COVID-19 booster.

Booster shots are used in health care after someone's immunity from an initial vaccination naturally starts to decrease. Boosters are designed to help people maintain their level of immunity for longer. Many adults get a booster for vaccines they received as children, such as measles, whooping cough or meningitis. For a disease like tetanus, booster shots are recommended every 10 years as the original immunity wanes,