Inclusion Criteria:  Patients who underwent solid organ transplantation prior to revaccination against COVID-19. Patients who have received the full COVID-19 vaccination regimen with Spikevax (Moderna) vaccine, with a minimum of 8 weeks having elapsed from the second dose to the time of trial initiation. Age > 18 years. All participants must have previously agreed to participate in the study by signing the informed consent form.  Exclusion Criteria:  Having suffered SARS-CoV-2 infection at any time prior to inclusion in the study. Subjects without solid organ transplantation or with a different solid organ (e.g. pancreas transplantation) and without any type of immunosuppression (immunocompetent non-transplanted) from the general population. Age < 18 years Temperature of at least 38°C in the 24 hours prior to immunization or other clinically relevant acute symptomatology. Clinical manifestations compatible with COVID-19 infection at the time of evaccination. Known allergy or history of anaphylaxis or other serious adverse events to the administration of vaccines or their excipients. Any other condition that contraindicates vaccination against SARSCov2, including pregnancy. Having presented graft rejection in the 15 days prior to the start of the study. Any condition or situation that may interfere with the ability to maintain adherence to study procedures and visits.