Study shows how easily coronavirus can potentially spread

Medical workers at Kaiser Permanente French Campus test a patient for the novel coronavirus, COVID-19, at a drive-thru testing facility in San Francisco on March 12, 2020. Between 70 million to 150 million people in the United States could eventually be infected with the novel coronavirus, according to a projection shared with Congress, a lawmaker said March 12, 2020. less Medical workers at Kaiser Permanente French Campus test a patient for the novel coronavirus, COVID-19, at a drive-thru testing facility in San Francisco on March 12, 2020. Between 70 million to 150 million ... more Photo: Josh Edelson, AFP Via Getty Images Photo: Josh Edelson, AFP Via Getty Images Image 1 of / 107 Caption Close Study shows how easily coronavirus can potentially spread 1 / 107 Back to Gallery

According to a virologic study by researchers in Germany, COVID-19 can be spread before it causes symptoms and for as many as 12 days after recovery. The study of nine infected patients at a Munich hospital that was published Monday has not yet been peer-reviewed.

In comparison to findings from the 2003 outbreak of SARS, the novel coronavirus began producing high viral loads — a term used to describe the quantity of virus in a given volume — more quickly.

"In SARS, it took 7 to 10 days after onset until peak RNA concentrations (of up to 5x105 copies per swab) were reached," the researchers wrote. "In the present study, peak concentrations were reached before day 5, and were more than 1,000 times higher."

This makes the novel coronavirus far harder to contain. SARS was contained after about 8,000 cases — we've already seen the novel coronavirus exceed that by far worldwide.

The study also showed very high viral shedding — meaning people are very contagious — during the first week of symptoms. This could mean very high transmission of the virus before those infected show any clinical symptoms at all, or when they are only beginning to experience very mild symptoms and are still going about their daily routines, according to Michael Osterholm, director of the University of Minnesota’s Center for Infectious Diseases Research and Policy.

This may help explain how quickly the virus has spread around the world.

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However, the study also suggests that those with mild infections are likely not still infectious by about 10 days after symptoms begin, even if they can still test positive for the virus for days or weeks after symptoms subside. This could mean changes in how long hospitals are advised to wait before discharging COVID-19 patients.

"Based on the present findings, early discharge with ensuing home isolation could be chosen for patients who are beyond day 10 of symptoms with less than 100,000 viral RNA copies per ml of sputum," wrote the authors. "Both criteria predict that there is little residual risk of infectivity, based on cell culture."

For two of the patients in the study, viral shedding dropped after day 5. However, two patients who developed more serious illness continued to emit high levels of the virus from the throat until about day 10 or 11.

Madeline Wells is an SFGATE associate digital reporter. Email: madeline.wells@sfgate.com | Twitter: @madwells22