A day after the F.D.A. authorized third doses of the Pfizer-BioNTech and Moderna vaccines for some immunocompromised people, an independent expert panel advising the C.D.C. recommended the move.,A day after the F.D.A. authorized third doses of the Pfizer-BioNTech and Moderna vaccines for some immunocompromised people, an independent expert panel advising the C.D.C. recommended the move.