What’s the difference between an additional dose and a booster dose?

An additional dose is recommended for people with weakened immune systems who need another dose in the vaccine series to increase their protection against COVID-19. The CDC recommends that immunocompromised people receive an additional dose of either the Pfizer or Moderna vaccine at least 28 days after their second dose, because they do not build enough immunity after the first two doses of vaccine and may be more vulnerable to serious and prolonged illness from COVID-19. Please follow up with your healthcare provider for additional information on whether you need additional doses.

A booster dose is another dose of the vaccine given to someone who has been fully vaccinated and gained immunity from the initial vaccination series, but would benefit from another dose due to waning immunity. Waning immunity occurs when the protection against the disease decreases over time.