Updated Nov. 11, 2021
In August, the CDC recommended a third dose of the two-dose vaccine series (i.e. Pfizer-BioNTech and Moderna Covid-19 vaccines) for individuals with moderately to severely compromised immune systems. Individuals who are immunocompromised often do not develop a protective level of immunity from just two doses, so a third dose is necessary to achieve protection.
In October, the CDC recommended Covid-19 vaccine boosters for a broader group of people based upon age, underlying medical conditions, and occupation. A booster dose of vaccine is for individuals with a normal immune system who developed protective levels of immunity from the first two doses of vaccine, but whose protection has waned with time. The recommendation of a booster dose now applies to all three vaccines approved or authorized for use in the U.S.