Thread by @andrewleedr

Does prolonged viral shedding cause disease transmission of COVID19 later on?

There have been reports of prolonged viral shedding beyond 25 days and people with more severe illness appear to shed virus for longer.

thelancet.com/journals/lanin…

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This has led to concern that the advised #quarantine period for patients with #Covid19 of 7 days is insufficient, with fears that the patients may still be infectious.

However, this assumption of prolonged infectiousness may not be correct.

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I'm not a virologist but have been listening into researchers discussing this topic from around the world and here are a few interesting points that emerged:

Firstly, our PCR tech is very sensitive and allows us to detect viral genetic material (RNA) at very low levels.

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Viral RNA is what has been detected in clinical samples e.g. in stool. But just because a patient is shedding the virus weeks later does not necessarily mean they are infectious. There will be people who keep shedding virions, but are no longer infectious
How so?
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There’s a lower limit of viral load detection (~ CT 30) that may correlate with the transmissibility of the virus. This is supported by animal studies.

In addition to the viral load, context matters too e.g. the duration of exposure and closeness of contact.

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The stage of the disease matters too.

Early on, there may be little in the way of an immune response & antibodies, so virions will be infectious. Later on, virions are covered with neutralizing antibodies so they are harmless even though their RNA can still be detected.

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Why does this matter?

One of the big concerns at present is whether we can discharge infected patients safely back to care homes without risking infecting others.

Establishing evidence based criteria for safe discharge is key.

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There are systematic reviews underway which should hopefully present a further clarity and summary of the evidence soon.