Project Summary/Abstract Clinical rebound following treatment with nirmatrelvir-ritonar has been reported. The incidence, virology, and clinical consequence of this syndrome are not clear. The investigators will conduct a study of nirmatrelvir-ritonavir rebound with the following objectives: Objective 1: Determine the incidence and risk factors for symptomatic and asymptomatic virologic rebound after nirmatrelvir-ritonavir therapy Objective 2: Determine the duration and magnitude of virologic rebound and contagiousness of symptomatic and asymptomatic rebound after nirmatrelvir-ritonavir therapy Objective 3: Estimate the incidence of treatment-emergent drug resistance after rebound Objective 4: Describe clinical outcomes with and without virologic rebound following nirmatrelvir-ritonavir therapy in a real-world setting By carrying out these objectives, the investigators will define the incidence of rebound after nirmatrelvir-ritonavir and characterize the associated syndrome using quantitative measurement of viral load, viral culture, and whole virus genome sequencing.