Viral shedding

Viral shedding

Viral shedding is the release of contagious virus into the environment after it has replicated inside the host.

Viral shedding is the release of contagious virus into the environment after it has replicated inside the host. The quantity of virus shed and the pattern in which the virus is shed from individuals over time are of interest to infectious disease researchers. Shed quantity can be a proxy for infectiousness and the shed pattern can indicate how long an infected individual is able to spread the infection to others. Viral shedding can be tested by PCR but this technique can detect inactive viral fragments that are not be able to grow and be infectious. Culturing virus from patient samples is a way to test if actual infectious virus is present.


Viral shedding for the SARS-CoV-2 coronavirus responsible for COVID-19 disease has been reported to be high early in the infection. A study of nine patients based in Germany found virus shedding dropped after day 5 in most patients. The two patients that continue to shed high levels of virus from the throat developed early signs of pneumonia. SARS coronavirus, responsible for an outbreak of respiratory disease in 2002-2003 showed peak shedding later when the virus had moved into the deep lungs. Since SARS-CoV-2 sheds from the upper airway early in infections, it is more difficult to contain than SARS. At peak shedding, individuals with SARS-CoV-2 infections are emitting more than 1000 times more virus than SARS coronavirus during its peak shedding.

A report led by researchers in China and the WHO found that while viable virus was found in stool it is not likely the driver for viral transmission. The German researchers were not able to grow virus from stool samples where they recovered viral fragments, nor were they able to be grown from blood or urine samples from COVID-19 patients.

A study of 94 COVID-19 patients conducted by the WHO and researchers in China found the highest viral load in throat swabs at the time of symptom onset and inferred that infectiousness peaked on or before the onset of symptoms.

Shedding of SARS-CoV-2 RNA for a median of 17 days and as long as 6 weeks was reported at a hospital in Changsha, China. The study used PCR which cannot distinguish between live virus and non-infectious viral debris. The study found that risk factors for prolonged duration of viral shedding were highest temperature at admission, time from symptom onset to admission and hospital length of stay.




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