Self-Amplifying mRNA COVID-19 Vaccine (LUNAR-COV19) Versus Comirnaty Vaccine in Adult Hematopoietic Cell Transplant Patients

Inclusion Criteria: * Aged ≥ 18 years * Willing and able to provide written informed consent, or with a legal representative who can provide informed consent (where locally approved) * Have received an allogeneic HCT within the prior 365 days * Have no relapse or progression of underlying malignancy * Have platelets ≥ 30,000/mm\^3 * Not pregnant (confirmed with negative urine or serum pregnancy test, if applicable) * Willingness to take study vaccine and complete necessary study procedures * If of childbearing potential, must agree to use a highly effective method of birth control or abstain from heterosexual activity for the course of the study through at least 60 days after the last dose of the study vaccine Exclusion Criteria: * Current infection with SARS-CoV-2 or infection within the prior 28 day period * Positive for SARS-CoV-2 by nasal swab polymerase chain reaction (PCR) at screening * Currently receiving any approved, authorized, or investigational direct-acting antiviral drug against SARS-CoV-2 * Received any approved, authorized, or investigational monoclonal anti-SARS-CoV-2 antibody therapy within the prior 180 days before screening * Received a SARS-CoV-2 vaccine after HCT or within 28 days prior to HCT * Participation in any other concurrent clinical trial of an experimental treatment or prevention for SARS-CoV-2 * Receiving \> 1 mg/kg/day corticosteroids within the prior 7 days * Active infection that is not adequately controlled, as determined by the invest…