Background: Recipients of solid organ transplant (SOT) are a high-risk group for severe SARS-CoV-2 infection. The mortality rate of patients with SOT during the COVID-19 pandemic has been reported to be approximately 20% (1). The anti–SARS-CoV-2 vaccines represent a hope to protect this population against this life-threatening infection. Objective: To assess the humoral response to messenger RNA (mRNA)–based vaccination in recipients of SOT. Methods: All patients with heart, kidney, liver, or pancreas transplants from the Midi-Pyrénées region (southwest France) are followed in our department. When the vaccination campaign started (7 January 2021), these patients were invited via text message, e-mail, or transplant patients' associations to be vaccinated. Patients were asked to register via a dedicated telephone number or website. They were vaccinated consecutively according to their registration date. According to the recommendations of the Francophone Society of Transplantation, anti–SARS-CoV-2 spike protein antibodies were monitored before and after vaccination. We used the SARS-CoV-2 total antibodies enzyme-linked immunosorbent assay test (Beijing Wantai Biological Pharmacy Enterprise) (80% of patients) or another validated anti–SARS-CoV-2 spike protein assay. According to French law (loi Jardé), anonymous retrospective studies do not require institutional review board approval. Findings: By 16 April 2021, 950 patients of the 2666 from our cohort had received at least 1 dose of an mRNA vaccine (BNT162b2 vaccine [Pfizer-BioNTech], n = 942; mRNA-1273 vaccine [Moderna], n = 8) and had anti–SARS-CoV-2 antibodies monitored. Fifty patients had vaccination without monitoring of antibodies, 80 patients are planned to be vaccinated within the next month, and 257 patients declined the vaccine. We had no feedback from the remaining 1329 patients.