A booster shot is a dose of vaccine given after a person has had the original vaccine (sometimes called a primary dose or, if more than one dose, primary series). Immunity from the original vaccine can fade over time, and a booster shot can help the immune system “boost” the protection it provides.
Booster shots are given for many vaccines that kids and adults get. These include:
Experts recommend that everyone age 6 months or older get a flu vaccine every year. They also recommend that everyone age 6 months or older get an updated COVID-19 vaccine for the 2023–24 season. The number of additional doses kids need this season depends on their age, health status, and how many COVID-19 vaccines they've already had. It also depends on whether they get the vaccine made by Pfizer-BioNTech or the one made by Moderna. Some children may need as many as 3 shots this season, spread out over a few months, while others may only need 1 shot.
Even though these repeated flu and COVID-19 vaccines work by “boosting” the immune system response, experts don't call them booster shots. That's because they differ from the previous vaccines. They've been updated in order to fight the viruses as they change over time. They don’t just boost previous immunity — they provide new immunity. So instead of “booster shots,” health care experts call them the annual flu vaccine and updated COVID-19 vaccine. But COVID-19 vaccines given after the primary series were called booster shots, so you might still hear people refer to them that way.
Reviewed by: Elana Pearl Ben-Joseph, MD
Date Reviewed: Oct 1, 2023
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