Thread by @MonicaGandhi9

WHY DID THE CDC RECOMMEND NOT TO TEST ASYMPTOMATIC PEOPLE AFTER VACCINATION (nor to quarantine after exposure if no symptoms) A thread. I start with this piece by my colleague @AmeshAA at Johns Hopkins: how testing algorithms change after vaccination
thehill.com/opinion/health…

Okay, why did we test asymptomatic people at all with COVID-19? Because there is asymptomatic transmission with this virus (see @NEJM paper we wrote early in pandemic below) unlike so many viruses where "syndromic surveillance" (testing if have symptoms)
nejm.org/doi/full/10.10…

works to curb spread. In fact, this is true of the original SARS in 2002-03: because you were infectious when you had symptoms, people could be isolated if they felt unwell & pandemic was quickly curtailed after 8098 cases total (although it caused severe disease like this one)

This is also true of influenza which is why I think best way to curtail flu in future winters is to ask people who are sick to stay home (as I said in this @scientificamerican piece. You can wear mask but again, flu is not spread asymptomatically.
scientificamerican.com/article/some-p…

Then, with more careful contact tracing studies, we realized your viral load matters. This study from Spain showed that increasing viral load values in nasopharyngeal swabs of COVID-19 patients were associated with greater risk of transmission
thelancet.com/journals/lanin…

And that asymptomatic patients had much lower viral loads (4-fold) than symptomatic patients which is why they were less likely to transmit. Sadly, we seldom incorporate the "CT" value when telling people the results of their PCR test (given as yes/no) sciencemag.org/news/2020/09/o…

which is of course a huge mistake. "CT" stands for cycle threshold and means the number of times a PCR machine has to cycle before giving you a positive result. A high number of cycles (>30 or so) means the viral load is extremely low & not infectious by other studies.

Okay, so let's now turn our attention to the post-vaccination setting where we - unfortunately- are still doing a lot of swabbing of asymptomatic people after vaccination (often in nursing homes, for example or Yankees). 4 studies show us important finding that viral loads low

in the nose if you are asymptomatic after vaccination. These studies were well done because they actually bothered to measure viral loads via CT values to determine viral loads low. Those studies here:
nature.com/articles/s4159…
medrxiv.org/content/10.110…
academic.oup.com/cid/advance-ar…

I can go through all of them if you want but I found this one the most amazing: The UK performed 1,610,562 PCR tests (with CT values) on 373,402 people as vax rolling out. Vaccines reduced asymptomatic infection (as many studies show) but also took CT >30
medrxiv.org/content/10.110…

Meaning, even if you were asymptomatic & had some virus in your nose, the CT was >30, very low viral loads, can't transmit. And I don't call this a "breakthrough", I call this vaccine success because if you are exposed, all that IgA, IgG and T cells found in your nasal mucosa

from the vaccines rush in to fight the virus as it tries to replicate & stops it from replicating much so the viral load is very low: you have no symptoms and you can't transmit. To remember that these vaccines produce IgA (which is an antibody that protects your mucosal

surfaces like the inner lining of your nose, please see this thread. Also reminds us that IgG (which we know is generated by vaccines) goes happily from bloodstream into nasal mucosa and acts there too:

https://twitter.com/monicagandhi9/status/1375707615351595008?lang=en

And what, of course, of our beloved T cells generated by the vaccines so aptly as summarized in previous threads? They get into the nasal mucosa as well, extremely readily, and there to fight the virus if it ever dares come near a vaccinated person.
pubmed.ncbi.nlm.nih.gov/9490657/

So, if you a) have no symptoms after vaccination; b) fight the virus with your nasal immunity if you see it so that viral load remains low & can't transmit, then testing asymptomatic individuals after vax is not only not indicted, but drain on public health resources

In fact- eek- I just paid CA taxes Monday & wondered about public health resources in CA with our high levels of vaccination & low cases and low test positivity rate (0.8% or 8/1000 so we are doing a lot of testing). So CDC right to follow symptomatic breakthroughs

And of course, relevant to this thread are all the many many studies that show us even asymptomatic infection itself (having in your nose) massively reduced by vaccines: hividgm.ucsf.edu/covid-19-vacci…

Someone asked me how they can download this table with links: we posted here. I believe asymptomatic testing after vaccination still being done in some hospitals/nursing homes; athletes; schools: above thread gives justification to only do if symptomatic
hividgm.ucsf.edu/covid-19-vacci…

If we continue to do asymptomatic testing after vaccination (not recommended by CDC), looks like we can get into situations like the ones below with athletes not being able to play (or patients not getting procedures or teachers staying out of school etc.)
politifact.com/article/2021/j…

Simple screening algorithm for monitoring if breakthrough happening or not after vaccination- if virus still circulating after vax, you can get it in nose, fight it with your IgA/IgG/T cells and bring it down to low levels- that is vax success not failure so look at CT carefully

6/29/21: So, what is happening in UK with delta? Because UK testing asymptomatic after vax, finding "cases" (or successful vax recipients with dead/low level delta virus in nose), hospitalizations not high. Vax "decoupling" link between cases & admissions
reuters.com/world/uk/uk-co…

Article to add to this from @NEJM yesterday showing 81% reduction in infection (even asymptomatic as routine swabs weekly) after 1 dose, 91% after 2 doses and importantly, CT values of PCR tests show lower viral load 40% in vax'd, making harder to transmit
nejm.org/doi/10.1056/NE…