RecruitingPhase 2Biological interventionSafety and Tolerability of a Newcastle Disease Virus-Based Mucosal COVID-19 Vaccine in Previously Vaccinated AdultsA Double-blind, Randomized, Controlled, Phase 2a Study to Evaluate the Safety and Tolerability of a Newcastle Disease Virus-based Mucosal Vaccine (NDV-HXP-S-KP.2) Relative to an Approved Systemic mRNA Vaccine in Previously Vaccinated Adults