The National Institutes of Health has started a Phase 1/2 clinical trial in which adult volunteers who have been fully vaccinated against COVID-19 will receive booster doses of different COVID-19 vaccines to determine the safety and immunogenicity of mixed boosted regimens. The National Institute of Allergy and Infectious Diseases (NIAID), part of NIH, is leading and funding the study through the Infectious Diseases Clinical Research Consortium, a clinical trials network that encompasses the Institute’s long-standing Vaccine and Treatment Evaluation Units (VTEUs).