As a growing number of Americans become eligible for a third dose of COVID-19 vaccines, CDC continues to expand its booster guidelines.

The agency has amended its criteria for moderately to severely immunocompromised people to allow a fourth dose of COVID-19 vaccine six months after receiving the third dose.

The criteria pertain to immunocompromised individuals who are 18 years of age or older who have received three doses of an mRNA vaccine.

CDC will allow COVID-19 vaccines from Pfizer-BioNTech (NYSE:PFE/NSDQ:BNTX), Moderna (NSDQ:MRNA) or Janssen (NYSE:JNJ) for the fourth dose. For Moderna recipients, the guidance recommends a 100-µg dose of mRNA for the third dose and a 50-µg dose for the fourth.

The guidelines, however, are more overt in their recommendations for moderately or severely immunocompromised individuals who have only received two doses of an mRNA vaccine. Assuming at least 28 days have elapsed since the second dose, CDC recommends that such individuals should receive an additional mRNA dose immediately.

CDC also included guidelines for moderately to severely immunocompromised recipients of a single dose of the Janssen COVID-19 vaccine. Assuming such individuals are 18 or older, they should receive a booster of any COVID-19 vaccine at least eight weeks after receiving their first shot. If the Moderna vaccine is used, CDC recommends a 50-µg dose for the booster.