This study will be a randomized, double-blind, placebo-controlled trial of a third dose of Moderna vaccine versus placebo. Participants will be those that have received two doses of mRNA-1273 COVID vaccine (Moderna) at 0 and 1 months. Participants will be randomized 1:1 to receive either a third dose of the mRNA-1273 vaccine or saline placebo at 3 months post initial vaccination.

Solid organ transplant (SOT) recipients are at high risk of COVID-19 complications. The Moderna vaccine (mRNA-1273) has proven highly efficacious and safe in a phase III large, randomized controlled trial of 30,420 persons in the general population, and has been in use in Canada since December 2020. SOT patients show diminished response to mRNA vaccines in several studies with approximately 40-50% positive antibody after the second dose. In positive patients, antibody titers are lower than the general population and adverse events mirror the general population. The current study will recruit 120 SOT recipients who have received both scheduled doses of the Moderna vaccine at 0 and 1 months. The hypothesis is that a third dose of vaccine will significantly increase antibody titers. SOT participants will be recruited and randomized 1:1 to receive an additional dose of Moderna COVID-19 vaccine two months after the last dose vs saline placebo. The outcomes will measure anti-RBD antibody titer, T-cell immunity, local/systemic side effects, and rejection events.

Inclusion Criteria:

• Solid organ transplant recipient who has previously received Moderna vaccine at 0 and 1 months
• Able to provide informed consent

Exclusion Criteria:

• Anaphylaxis or allergic reaction to Moderna vaccine

Participant and vaccinator will be blinded to vaccine vs placebo