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I am giving a talk on "SARS-CoV-2 testing and Immunity" this week. Thought it would make a good #Tweetorial.
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First testing.
You can categorize testing as follows:
Pathogen based (test for presence of virus)
Host based (test for patient's response)
You can categorize testing as follows:
Pathogen based (test for presence of virus)Host based (test for patient's response)
There have been a LOT of Emergency Use Authorizations granted by the FDA. For a variety of tests.
Here are the types of tests that detect virus.
Molecular/Genetic (RT-PCR)
Antigen
Viral culture
Molecular/Genetic (RT-PCR)AntigenViral culture
Host-based test look for antibodies against virus that are produced due to your adaptive immune response.
Either IgG or IgM.
Either IgG or IgM.
Here is a graphic that explains the difference between a RT-PCR and Antibody test.
RT-PCR tests are the most commonly available test out there currently. Typically requires a nasopharyngeal sample.
They target a variety of RNA genes.
envelope (env)
nucleocapsid (N)
spike (S)
RNA-dependent RNA polymerase (RdRp)
ORF1 genes.
They target a variety of RNA genes.
envelope (env)nucleocapsid (N)spike (S)RNA-dependent RNA polymerase (RdRp)ORF1 genes.
RT-PCR detects genetic material. Fragments. That's it.
How long can you you have a positive RT-PCR test?
This study reported as long as 8 days after resolution of symptoms. https://www.nature.com/articles/s41591-020-0869-5
How long can you you have a positive RT-PCR test?
This study reported as long as 8 days after resolution of symptoms. https://www.nature.com/articles/s41591-020-0869-5
However, you know from my recent tweet that there are many people out there who have seen positive RT-PCRs for much longer periods. Beyond 50 days even. https://twitter.com/chungk1031/status/1258843412129705984?s=20
Does this mean you are infectious that entire time?
No.
What test can determine if we have active (whole) virus?
Nothing commercially available. You have to do this in a lab via cell culture.
https://wwwnc.cdc.gov/eid/article/26/6/20-0516_article
No.
What test can determine if we have active (whole) virus?
Nothing commercially available. You have to do this in a lab via cell culture.
https://wwwnc.cdc.gov/eid/article/26/6/20-0516_article
There is a report that demonstrates active virus via cell culture cannot be isolated from the respiratory system after about 8 days. https://www.nature.com/articles/s41586-020-2196-x
How about Antigen testing. Can that determine 'infectivity'?
Maybe.
Well...one company was granted an EUA last week. We are told many will soon follow.
But problem is that it may not be sensitive enough. Lots of false negatives according to this. https://www.fda.gov/news-events/press-announcements/coronavirus-covid-19-update-fda-authorizes-first-antigen-test-help-rapid-detection-virus-causes
Maybe.
Well...one company was granted an EUA last week. We are told many will soon follow.
But problem is that it may not be sensitive enough. Lots of false negatives according to this. https://www.fda.gov/news-events/press-announcements/coronavirus-covid-19-update-fda-authorizes-first-antigen-test-help-rapid-detection-virus-causes
How about antibodies?
Here's a pre-print with good info on IgM and IgG.
https://www.medrxiv.org/content/10.1101/2020.03.24.20042382v1
Here's a pre-print with good info on IgM and IgG.
https://www.medrxiv.org/content/10.1101/2020.03.24.20042382v1
As you can see. Not everyone mounts the same antibody response. Some strong. Most weak. Some not much at all.
Who's protected? And for how long?
Unknown.
Who's protected? And for how long?
Unknown.
That's why the WHO issued this statement.
"There is currently no evidence that people who have recovered from COVID-19 and have antibodies are protected from a second infection." https://www.who.int/news-room/commentaries/detail/immunity-passports-in-the-context-of-covid-19
"There is currently no evidence that people who have recovered from COVID-19 and have antibodies are protected from a second infection." https://www.who.int/news-room/commentaries/detail/immunity-passports-in-the-context-of-covid-19
This paper circulated last week on #MedTwitter.
I found it a quite useful summary.
https://jamanetwork.com/journals/jama/fullarticle/2765837
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I found it a quite useful summary.
https://jamanetwork.com/journals/jama/fullarticle/2765837
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Check out this amazing artwork on sensitivity, specificity, PPV, and NPV.
https://twitter.com/stefantigges/status/1260168301436719104?s=21 https://twitter.com/StefanTigges/status/1260168301436719104
https://twitter.com/stefantigges/status/1260168301436719104?s=21 https://twitter.com/StefanTigges/status/1260168301436719104
