On Mar. 29, 2022, the U.S. Food and Drug Administration (FDA) approved a second booster dose (a fourth shot) of the Pfizer/BioNTech messenger RNA (mRNA) biologic BNT162b2 ("Comirnaty") and Moderna/NIAID mRNA-1273 (“Spikevax”) for people 50 years of age and older, as well as younger individuals...