A booster dose is given after a complete vaccine series to provide additional protection against a vaccine’s effectiveness has decreased over time, while an additional dose is given to people with compromised immune systems to improve their response to the initial vaccine series. 

People with compromised immune systems may have a reduced ability to respond to vaccines, and having a weakened immune system can increase the risk of becoming severely ill from COVID-19. The CDC recommends that immunocompromised people who received the Pfizer or Moderna vaccine get an additional dose at least 28 days after their second shot. Data show that an additional dose of the Pfizer or Moderna vaccines helps to increase protection for this group. 

Patients who are immunocompromised should consult with their health care provider to discuss additional precautions and any questions they have about protecting themselves from COVID-19.

Updated February 28, 2022