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Coronavirus patients most contagious in first week of symptoms, new research finds

University College London (UCL)/
University College London (UCL)/

Covid-19 is at its most infectious during the first week of symptoms, according to new research.

Scientists from one of Europe’s largest university hospitals conducted the study, which they said could help inform when a coronavirus patient is discharged from hospital.

Their research showed that the disease was most commonly spread via "droplets" emitted through sneezing or coughing, rather than through the contamination of materials.

The team, from Berlin’s Charite Universitatsmedizin centre, analysed data from coronavirus patients in Germany to check how contagious they were at different points of their infection.

They looked at nine adults from Munich with relatively mild symptoms and measured how much of the virus they shed over the course of their illness.

The researchers analysed throat, lung, stool, blood and urine samples collected from the patients during their clinical course.

They found the virus was most active – in terms of reproduction and “shedding” – in the upper respiratory tract during the first seven days of symptoms.

They also noticed that two of the participants, who showed some early signs of pneumonia, continued to shed high levels of the virus in their mucus until day 10 or 11.

According to the study, published in the journal Nature, the virus remained detectable in patients’ mucus even after symptoms stopped.

The virus was not present in blood or urine samples, and the authors didn’t find a replicating form of the virus in stool samples.

The scientists, led by Institute of Virology director Christian Drosten, called for further studies to further investigate the disease’s possible transmission route.

The researchers wrote in the paper: “Our initial results suggest that measures to contain viral spread should aim at droplet, rather than fomite-based (via clothes or materials), transmission.

“The prolonged viral shedding in sputum (mucus) is relevant not only for hospital infection control, but also for discharge management.

“In a situation characterised by limited capacity of hospital beds in infectious diseases wards, there is pressure for early discharge following treatment.

“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.

“Both criteria predict that there is little residual risk of infectivity, based on cell culture.”

The scientists said further research could help to assess whether increases in the viral load of a patient after the first week of symptoms could signal an aggravation of symptoms.