Solid organ transplant recipients (SOTRs) have suboptimal antibody responses to 2-dose severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccination, with some improvement after receiving a third dose.1,2 Further boosting with a fourth dose (D4) has been suggested in small cohorts of SOTRs, but the evidence remains limited and restricted to kidney recipients with poor preceding seroresponse.3-5 To expand on these early observations, we updated our original case series of 18 participants to a more granular evaluation of 128 participants, diverse in organ type and pre-D4 vaccine response.