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Working From Home - Impacts of Coronavirus
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bd139:
Exactly.

The gutter press likes to omit those facts when scaremongering readers for clicks or sales however.
cdev:
 It is possible to test positive again after testing negative.

Its rare but it has happened with COVID-19, and they still dont know why it seems more common in some places than others.
 I am pretty certain they don't know why.

Some viruses never go away. You immune system usually keeps them in check, but if you get an illness that requires you take immunosuppressant drugs or there are environmental causes too, then you become more suceptible to those viruses or if you already have them they may reactivate.



--- Quote from: bd139 on May 18, 2020, 12:19:45 pm ---It is possible to get tested positive again if you are subjected to enough virus particles to overwhelm the immune system or the immune system is in the process of fighting it off. Also possible to be tested positive again if it's a PCR test and is contaminated or you have bits of dead non-viable virus fragments all over you.

--- End quote ---
Zero999:

--- Quote from: cdev on May 18, 2020, 04:32:02 pm --- It is possible to test positive again after testing negative.

Its rare but it has happened with COVID-19, and they still dont know why it seems more common in some places than others.
 I am pretty certain they don't know why.

Some viruses never go away. You immune system usually keeps them in check, but if you get an illness that requires you take immunosuppressant drugs or there are environmental causes too, then you become more suceptible to those viruses or if you already have them they may reactivate.



--- Quote from: bd139 on May 18, 2020, 12:19:45 pm ---It is possible to get tested positive again if you are subjected to enough virus particles to overwhelm the immune system or the immune system is in the process of fighting it off. Also possible to be tested positive again if it's a PCR test and is contaminated or you have bits of dead non-viable virus fragments all over you.

--- End quote ---

--- End quote ---
Just because someone tests positive, it doesn't mean they're infectious. As he said, the test is sensitive enough to pick up old, inactive virus particles tucked away in the tissues.

Tests can also come back false negative, but as long as that's rare enough, it will have minimal impact on mitigating the spread.

I think we as engineers like to rely on certainties, but in medicine they are rarely any. In a large scale system such as a pandemic, something only needs to work most of the time, in order for it to be effective.
paulca:
Thought Emporium has an interesting video on the PCR variant tests.  Basically it's a live stream where he actually creates a PCR test for Speckled tobacoo virus to find out if his plants have it.  It's very obvious it would pick up broken shards of viral RNA/DNA by how the tests work.  It only takes 1 or 2 particles containing the primer sequence to exist in the sample and the enzyme(?) will copy that sequence millions of times any amplify it.  So the test only tells you the particular sequence was present, usually only a small section, enough to uniquely identify the virus.  It cannot tell you if the piece came from a complete RNA/DNA viral component or from fragment.  Nor can it tell you how much of that RNA/DNA was in the sample.
cdev:
PCR tests can tell how much is there by a process of progressive dilution. They describe it in terms of "titers" (as in titration). If SARS-CoV-2 RNA titers are described as high that means that the presence of the virus still registered positive through many successive dilutions (of the substance under test). However, if a test is in short supply, it stands to reason that instead of doing multiple tests to determine stength of a virus, they might simply go for a positive or negative, because determining the titer is too resource intensive for an emergency situation.


--- Quote from: paulca on May 19, 2020, 07:25:22 am ---Thought Emporium has an interesting video on the PCR variant tests.  Basically it's a live stream where he actually creates a PCR test for Speckled tobacoo virus to find out if his plants have it.  It's very obvious it would pick up broken shards of viral RNA/DNA by how the tests work.  It only takes 1 or 2 particles containing the primer sequence to exist in the sample and the enzyme(?) will copy that sequence millions of times any amplify it.  So the test only tells you the particular sequence was present, usually only a small section, enough to uniquely identify the virus.  It cannot tell you if the piece came from a complete RNA/DNA viral component or from fragment.  Nor can it tell you how much of that RNA/DNA was in the sample.

--- End quote ---
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