Currently, individuals with moderately to severely compromised immune systems are recommended for an additional dose of COVID-19 vaccine if they previously received a two-dose primary series of either the Pfizer BioNTech or Moderna COVID-19 vaccines. This includes people currently receiving treatment for cancer, people taking medications that weaken the immune systems, organ or stem cell transplant recipients, individuals with advanced or untreated HIV infection, and other conditions. A full list of conditions can be found on CDC’s website. If you are unsure if you are recommended for an additional dose, talk with your health care provider. The third dose of Pfizer BioNTech or Moderna COVID-19 vaccine can be given anytime 28 days after receiving the second dose.
An additional dose is being recommended after emerging data suggests some people with moderately to severely compromised immunes systems do not always build the same level of immunity as individuals who do not have a compromised immune system. An additional dose will help ensure people with compromised immune systems get as much protection as possible from COVID-19 vaccination.
More information can be found here: COVID-19 Vaccines for Moderately to Severely Immunocompromised People | CDC