People with certain health conditions that make them moderately or severely immunocompromised may get a fourth mRNA Covid-19 shot, according to updated guidelines from the CDC. The CDC authorized a third dose for certain immunocompromised people 18 and older in August. It said a third dose, rather than a booster -- the CDC makes a distinction between the two -- was necessary because the immunocompromised may not have had a complete immune response from the first two doses.