NEW YORK — Recovered COVID-19 patients should not be retested for SARS-CoV-2 infection until more than four weeks have passed since their symptoms first appeared in order to prevent further transmission of the virus, according to a population-based study conducted in one of Italy's hotspots.
The research, which appeared in BMJ Open on Wednesday, indicates that it takes an average of 30 days from COVID-19 diagnosis and 36 days from the onset of symptoms for the virus to clear from a patient. It also points to a higher rate of false negatives with RT-PCR-based nasal swab tests performed early in a patient's recovery.
Based on these findings, postponing follow-up testing of clinically recovered COVID-19 patients could increase the efficiency and performance of testing protocols, the study's authors write.
Alongside contact tracing and social distancing, the isolation of SARS-CoV-2-infected individuals has been a key aspect of controlling the ongoing pandemic. However, little is known about the timing of viral clearance in patients after their symptoms subside, complicating efforts to minimize unnecessary isolation.
To better understand the issue, scientists from the Istituto di Ricovero e Cura a Carattere Scientifico tracked 4,538 COVID-19 patients from Italy's Reggio Emilia province who were diagnosed between February 26 and April 22. Diagnosis was performed using Seegene's Allplex 2019-nCoV, which is designed to detect the RdRp, E, and N genes of the virus.
Of the 4,480 patients who could be included in the researchers' preliminary analysis, 1,259 achieved viral clearance as indicated by at least one negative swab test following the initial positive test result, with a median time to viral clearance of 31 days. Among this group, 428 individuals died.
The investigators then performed follow-up testing on 1,162 people who had been diagnosed at least 30 days prior and for whom the date of symptom onset was available. Patients were tested an average of three additional times, with the mean times of retesting about 15 days after the first positive test, 14 days after a second positive test, and 9 days after a third positive test.
Viral clearance was detected in 704 — nearly 61 percent — of the 1,162 patients and confirmed in roughly 79 percent of those who underwent a second test, suggesting that there was about one false negative for every five negative test results. Median time to viral clearance in this cohort was 30 days from the first positive swab and 36 days from symptom onset, with the time increasing based on age and disease severity.
The rate of false negatives declined, however, when the first negative test was performed more than 34 days after the first positive test, the study's authors found.
Based on the findings, the study's authors concluded that testing at 14 days from diagnosis, which is recommended by many regional surveillance protocols, will likely result in most cases still being positive. Therefore, this timeline should be pushed back to more than four weeks, they wrote.
The investigators also recommend that COVID-19 patients should either isolate for more than 30 days after their symptoms begin or until at least one follow-up test is performed to confirm viral clearance to avoid generating secondary cases.