What is the difference between a third dose and a booster shot?

Third doses are being provided to some immunocompromised people who may not have mounted a robust immune response despite already receiving two doses of either the Pfizer or Moderna COVID-19 vaccines. A third dose may help prompt the immune system to produce protective antibodies.

A booster shot is meant to supplement the full two-dose series provided to non-immunocompromised individuals to help increase, or boost, the immunity they already have from previous vaccination. Most of us are familiar with booster doses for other routine vaccinations, such as measles-mumps-rubella and tetanus.