Temporal profile and determinants of viral shedding and of viral clearance confirmation on nasopharyngeal swabs from SARS-CoV-2-positive subjects: A population-based prospective cohort study in Reggio Emilia, Italy

Objectives-To determine the timing of viral clearance (first negative RT-PCR on nasopharyngeal swab) and the probability of viral clearance confirmation (two consecutive negative swabs) in COVID-19 patients and to identify related determinants. Design-Population-based prospective cohort study on archive data. Setting-Preventive services and hospital care in the Reggio Emilia Province, northern Italy. Participants-All 1162 subjects testing positive to RT-PCR on nasopharyngeal swabs and diagnosed with COVID-19 in the Reggio Emilia Province with at least 30 days of follow-up by April 22, 2020. Main outcome measures-Median times from diagnosis and from symptom onset to viral clearance with interquartile range (IQR) assessed using the Kaplan-Meier estimator, stratified by included characteristics. The probability of viral clearance confirmation, stratified by time from diagnosis, and putative determinants assessed using a multivariate logistic regression model. Results-Viral clearance was achieved by60.6% (704/1162) of patients, with a median time of 30 days from diagnosis (IQR 23-40) and 36 days from symptom onset (IQR 28-45). Of those negative and retested, 78.7% (436/554) had viral clearance confirmation, suggesting one in five false negative tests. The time from symptom onset to viral clearance slightly increased with age, from 35 (IQR 26-44) days under age 50 to 38 (IQR 28-44) in over age80, and with disease severity, from 33 (IQR 25-41) days in non-hospitalised subjects to 38 (IQR 30-47) days in hospitalisedpatients. The probability of confirmed viral clearance reached 86.8% after 34 days from symptom onset and increased with time, even when adjusting for age and sex (OR 1.16 95%CI 1.06-1.26 per day from diagnosis). Conclusions-Postponing follow-up testing of clinically recovered COVID-19 patients could increase the efficiency and performance of testing protocols. Understanding viral shedding duration also has implications for containment measures of paucisymptomatic subjects.