Additional Study Information: The purpose of this study is to evaluate the safety, immune response, and efficacy of the Moderna mRNA-1273 SARS-CoV-2 vaccine in solid organ transplant recipients and healthy controls. Adult kidney and liver transplant recipients and healthy control participants who are at least 18 years of age will be enrolled. The current study aims to evaluate the antibody responses following the second dose of mRNA-1273 vaccine among adult recipients of kidney or liver transplants, and is expected to provide initial clinical information whether this population of immunocompromised participants, has comparable immune responses following vaccination as adult participants with normal immune function. Your participation in this study will last for approximately 1 year. About 240 people will take part in this study at several sites across the United States. You will receive compensation through a reloadable debit card, which will be provided to you.