New study finds COVID-19 patients remain infectious for only this number of days

Although complete viral clearance typically occurs roughly two weeks after COVID-19 transmission, a new joint research paper published by Singapore’s National Centre for Infectious Diseases and the Academy of Medicine, Singapore posits that patients are no longer contagious 11 days after infection.

“COVID-19 is caused by the SARS-CoV-2 virus, a beta-coronavirus which emerged in Wuhan, China in December 2019,” the authors write. “Based on the accumulated data since the start of the COVID-19 pandemic, the infectious period of SARS-CoV-2 in symptomatic individuals may begin around two days before the onset of symptoms, and persists for about seven to 10 days after the onset of symptoms. Active viral replication drops quickly after the first week, and a viable virus was not found after the second week of illness despite the persistence of PCR detection of RNA. These findings are supported by epidemiologic, microbiologic, and clinical data.”

Period of Infectivity to Inform Strategies for de-isolation for COVID-19 Patients

The paper was written following a meta-analysis of a previously conducted study of 73 COVID-19 patients. 

After the SARS-CoV-2 outbreak achieved pandemic status from the Word Health Organization swab tests became an important part of the surveillance process. Unfortunately, these tests react to active and dead viral material alike.

This coupled with the fact that some populations become more ill than others, remain ill for longer than others or don’t get sick at all made it all but impossible for experts to determine exactly how long positive cases are a danger to their communities. 

Early reports from mainland China motioned an asymptomatic infection rate of about 1%. However, ensuing literature and increased contact tracing indicate that a larger proportion of infected individuals might remain asymptomatic than what was previously assumed. 

It is now believed that asymptomatic infections range somewhere between 17.9% – 78%, depending on the population being studied. 

For symptomatic cases of COVID-19, just about  80% will remain mild and will not require the need for medical intervention, whereas about 15% will develop the more severe disease (mainly pneumonia) and about 5% may require critical care.

All of these cases have the potential to get well of course, though health status does not directly correspond with detectable traces of coronavirus. 

In actuality, a positive test does not equate to infectiousness or a viable virus. In fact, the new paper concludes that SARS-CoV-2 likely can not be isolated or cultured after day 11 of illness.

Separate data reviewing 766 confirmed COVID-19 patients, found that by day 15 of infection 30% of cases are PCR-negative according to nasopharyngeal swab tests. 

By day 21 this figure rises to 88% and by day 33 95% of all patients are negative according to NCID data. 

A small portion of cases may continue to shed viral debris after a month or longer but again viral RNA detection by PCR does not equate to infectiousness or viable virus.

We cannot rely on broad estimates to gauge infectious potential but this new research allows virologists to start with a principled base for further analysis. 

“These new findings allow for revised discharge criteria based on the data on the time course of infectiousness rather than the absence of RNA detection by PCR testing, taking into consideration both the clinical and public health perspectives, including the individual patient’s physical and mental well-being,” the authors continued. “In addition, given these findings, resources can focus on testing persons with acute respiratory symptoms and suspected COVID-19 in early presentation, allowing timelier public health intervention and containment.”

 CW Headley is a reporter for the Ladders and can be reached at cheadley@theladders.com.