On January 7, CDC updated its vaccine guidance by discussing a fourth COVID-19 vaccine for certain people. For eligible individuals with a moderately or severely compromised immune system, CDC guidance recommends a third vaccine as an “additional primary shot” – to be followed by a booster as a fourth shot. As the U.S. Supreme Court considers reinstating the CMS vaccine mandate for healthcare workers, opinions may change regarding what it takes to be “fully vaccinated” as CDC recommendations evolve.