Interim Guidance on Duration of Isolation and Precautions for Adults with COVID-19 | CDC

Summary of Key Findings

1. Concentrations of SARS-CoV-2 RNA in upper respiratory specimens decline after onset of symptoms. 
2. The likelihood of recovering replication-competent virus also declines after onset of symptoms. For patients with mild to moderate COVID-19, replication-competent virus has not been recovered after 10 days following symptom onset. (1,8,31,36,42,61,66) Recovery of replication-competent virus between 10 and 20 days after symptom onset has been reported in some adults with severe COVID-19; some of these cases were immunocompromised. (56) However, in this series of patients, it was estimated that 88% and 95% of their specimens no longer yielded replication-competent virus after 10 and 15 days, respectively, following symptom onset. 
3. In a large contact tracing study, no contacts at high risk of exposure developed infection if their exposure to a case patient started 6 days or more after the case patient's infection onset. (12)
4. Recovered patients can continue to have SARS-CoV-2 RNA detected in their upper respiratory specimens for up to 12 weeks after symptom onset.  Efforts to isolate replication-competent virus from these patients is usually not successful. 
5. Reinfection can happen due to COVID variants......To date, reports of reinfection have been infrequent.  

https://www.cdc.gov/coronavirus/2019-ncov/hcp/duration-isolation.html