So this week had the opportunity to talk with local care home managers (thanks @michellecarrin3 & @sarahfiori76 for the invite).

Unsurprisingly they had concerns about risk of introducing COVID19 infection into their care home & keen to protect their residents & staff.

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So here's my long winded (but hopefully simple) answer. Firstly, need to understand test used. The COVID19 test uses a technique called PCR (Polymerase Chain Reaction). Works by testing for bits of viral genetic material called RNA in samples taken from the nose & throat.
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PCR is quite a good test but depends on how well the samples are taken but also when in the disease it was taken.

A positive test means viral genetic material is present, but it will not tell you if the virus is viable ("live") or not viable ("dead").

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The PCR test is most accurate when done within 7 days of symptoms starting. A positive test result taken at that time indicates infection.

However, a positive test result from a sample taken later on in the infection does not necessarily mean a person is infectious.

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How long do infected persons shed virus?
From early Chinese studies, most patients shed virus for 13-24 days. But it has also been reported a few cases shed virus for 60+ days!

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Detecting viral genetic fragments is of no value. There are patients that keep shedding viral fragments, but are not infectious any more.

Studies suggest infected persons are infectious possibly 2 days before they start having symptoms, & first 7-10 days of symptoms.

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Study from Singapore reported patients not infectious after 11 days.
https://www.ams.edu.sg/view-pdf.aspx?file=media%5c5556_fi_331.pdf&ofile=Period+of+Infectivity+Position+Statement+(final)+23-5-20+(logos).pdf
Viral RNA detection by PCR does not equate to infectiousness or viable virus...Viable virus was not found after the 2nd week of illness despite persistence of PCR detection of RNA.
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So, if a person has no symptoms for at least 2 days & their illness started more than 10 days ago, they are unlikely to be infectious.
Also, in recovering patients, any virus that is shed is probably covered with neutralizing antibodies which stops them being infectious.
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So is re-testing needed before admitting a person to a care home? No because there is a high chance you will detect viral genetic fragments. A positive result only tells you viral genetic fragments still being shed. It doesn't tell you if the person is still infectious.
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So how can we tell if a person is infectious?

Not easy.

Commonest symptoms are fever & cough, but only 83% have a cough and 67% have a fever! (Chan et al., 2020)

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Also, proportion of infected people with no symptoms varies between 31-70.0%!
(Japan Ministry of Health, Labor and Welfare, 2020; Dorigatti et al., 2020)

So there are folk out there who may be unknowingly infected who could infect others...

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That’s why continued hygiene measures, PPE & physical distancing etc... are important.

We need to have a low threshold to suspect infection & ISOLATE EARLY.

Applies to hospitals, clinics, care homes, schools, & many other settings.

Final reflection this virus is going to be around awhile. We've got to learn to live with it & find ways to reduce risk. Zero risk not possible, ie impossible to stop all further infections, but with quick public health action we can try to stamp out outbreaks & limit the damage.