The health authority indicates that the official vaccination schedule differs from that described in the protocol.

Patients undergoing solid organ transplantation randomly selected from those who have received two doses of Spikevax (Moderna) vaccine, provided that a minimum of 8 weeks have elapsed from the second dose to the time of the start of the trial. It is planned to include 386 patients.

The main objective of this trial is to evaluate the immunogenicity of reimmunization against SARS-CoV-2 in patients undergoing solid organ transplantation with heterologous revaccination, testing whether in patients who received two doses of Spikevax Moderna vaccine there is a significant increase at 28 days in their antibody titers against SARS-CoV-2 after receiving a dose of Janssen.

The secondary objectives of this trial are:

1. To evaluate the efficacy of reimmunization against SARS-CoV-2 in subjects undergoing solid organ transplantation, in terms of incidence of infection and severity of COVID-19 after revaccination in the transplanted patient.
2. To gain insight into the humoral and cellular immune response conferred by the sequential combination of both homologous and heterologous vaccination over one year, as well as its duration in subjects undergoing solid organ transplantation.
3. To evaluate the safety of vaccines against SARS-CoV-2 in subjects undergoing solid organ transplantation.
4. To determine the potential existence of differences in humoral (Anti S Ac) and cellular immune response depending on the immunosuppression regimen.
5. To evaluate the occurrence of side effects, including rejection and anti-HLA and DSA antibodies after revaccination.

Inclusion Criteria:

1. Patients who underwent solid organ transplantation prior to revaccination against COVID-19.
2. 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.
3. Age > 18 years.
4. All participants must have previously agreed to participate in the study by signing the informed consent form.

Exclusion Criteria:

1. Having suffered SARS-CoV-2 infection at any time prior to inclusion in the study.
2. 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.
3. Age < 18 years
4. Temperature of at least 38°C in the 24 hours prior to immunization or other clinically relevant acute symptomatology.
5. Clinical manifestations compatible with COVID-19 infection at the time of evaccination.
6. Known allergy or history of anaphylaxis or other serious adverse events to the administration of vaccines or their excipients.
7. Any other condition that contraindicates vaccination against SARSCov2, including pregnancy.
8. Having presented graft rejection in the 15 days prior to the start of the study.
9. Any condition or situation that may interfere with the ability to maintain adherence to study procedures and visits.