What is the difference between an additional dose and a booster shot?

People with medical conditions that make their immune systems weaker may not have developed a strong antibody response to their two doses of the Pfizer or Moderna COVID-19 vaccines. For these people, a third dose at least 28 days after their second shot may help prompt the immune system to produce protective antibodies. Learn more about the additional dose of COVID-19 vaccine.

Booster shots are given to people who are likely to have developed a strong immune response with prior vaccination, in order to help increase, or boost, the immunity they already have. This is because there is some evidence from clinical trials that antibodies produced after the first two shots decline over time. Most of us are familiar with booster doses for other routine vaccinations, such as measles-mumps-rubella and tetanus.