Solid organ transplant recipients (SOTRs) are less likely to develop antibodies and have lower median antibody levels after severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccination versus the general population.1-3 Some SOTRs with low or negative antibody levels after 2 mRNA vaccines can develop higher levels after a third dose (D3), but long-term antibody durability is unknown.4 Currently, the Centers for Disease Control and Prevention allows for a fourth vaccine dose 6 mo after an “additional primary vaccine,” but there is a paucity of information to inform this policy.5 We describe 6-mo SARS-CoV-2 antibody kinetics and durability in 32 SOTRs who received D3 (April 1, 2021–June 26, 2021) and were followed through December 9, 2021.