*** Natural immunity against COVID-19? ***

SARS-CoV-2 is one of several coronaviruses. Coronaviruses sometimes cause the common cold, buts sometimes produce very serious infections such as SARS and MERS.
https://www.thestar.com.my/news/regional/2020/01/22/defending-against-coronavirus
#COVID19
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Several recent papers show that people can have their T cells respond to SARS-CoV-2 without having prior exposure. In other words, some of us might be naturally “immune.”
#COVID19
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The reason I put quotation marks around immune is that it is a cross-resistant immunity that occurs without prior exposure to SARS-CoV-2.
#COVID19
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At the risk of being repetitive, the concept is that a given individual could be immune by virtue of the T cell response they show, and not necessarily have detectable protective antibodies.
#COVID19
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It is also possible for people to develop immunity that is only detectable via a T cell response and without antibodies as we will see below.
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* How common is this natural “immunity?”

We do not know for sure but the estimates suggest a significant fraction of the population Three studies show T cell reactivity against SARS-CoV-2 in the range of 20%, 50%, and 80%.
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One of the first of such studies suggested that 50% of samples collected viably from 2015 to 2018 (pre-SARS-CoV-2) had this reactivity.
https://www.cell.com/action/showPdf?pii=S0092-8674%2820%2930610-3&fbclid=IwAR2FZeBrpOxPd3AStAavzW0qDyGfprEkZSe8s9nSQFa4-y-oqTSNlfomZJ8
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T cell responses were seen in a large number of unexposed individuals. Using blood collected from 2015-2018 and found cross-reactive responses by those T cells.
Figure 2A
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Another study shows reactive T cells in 2 out of 10 healthy controls not previously exposed to SARS-CoV-2, which is indicative of cross-reactivity due to past infection with ‘common cold’ coronaviruses (albeit mild).
H/T @Oncotastic
https://immunology.sciencemag.org/content/5/48/eabd2071
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In another study, this cross-reactivity to "common cold" viruses was seen in 81% of those tested and not exposed to SARS-CoV-2
H/T @BallouxFrancois
https://www.researchsquare.com/article/rs-35331/v1
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What about infection leading to T cell responses but no antibodies in those exposed to SARS-CoV-2? More work is needed, but it might be possible. Of course, you always have to wonder how good are we with regards to antibody tests?
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In one study the authors investigated seven families, including 9 index patients and eight exposed contacts. Ten unexposed healthy donors were enrolled as controls.
H/T @RockRat2
https://www.medrxiv.org/content/10.1101/2020.06.21.20132449v1
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Intrafamilial Exposure to SARS-CoV-2 Induces Cellular Immune Response without Seroconversion

Background. In the background of the current COVID-19 pandemic, serological tests are being used to assess past infection and immunity against SARS-CoV-2. This knowledge is paramount to determine the...

https://www.medrxiv.org/content/10.1101/2020.06.21.20132449v1
COVID-19 patients developed anti-SARS-CoV-2 antibodies and had a T cell response. Some of the exposed people developed mild symptoms but remained without antibodies. 6/8 had a T cell response, suggesting a past SARS-CoV-2 infection.
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The authors conclude that exposure to SARS-CoV-2 can induce virus-specific T cell responses without the formation of antibodies (the so-called seroconversion). 
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These studies are small but seem to consistently show nice T cell responses. Some of the samples used were collected way before SARS-CoV-2 was among us.
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The findings of these studies might mean that we can reach a state of “functional herd immunity” with a fraction of the population being infected that is smaller than usually predicted.
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One recent mathematical model suggests that this fraction is lower than the 60% usually invoked.
H/T @Amir_Ahmed_Toor
https://science.sciencemag.org/content/early/2020/06/22/science.abc6810.full
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A mathematical model reveals the influence of population heterogeneity on herd immunity to SARS-C...

Despite various levels of preventive measures, in 2020 many countries have suffered severely from the coronavirus 2019 (COVID-19) pandemic caused by the severe acute respiratory syndrome coronavirus...

https://science.sciencemag.org/content/early/2020/06/22/science.abc6810.full
One study in Spain showed a prevalence of antibodies of 11.4%, and Spain at large of 5.2%. Despite this low prevalence of antibodies the mortality and incidence data keeps going down.
https://www.mscbs.gob.es/ciudadanos/ene-covid/docs/ESTUDIO_ENE-COVID19_SEGUNDA_RONDA_INFORME_PRELIMINAR.pdf
https://ourworldindata.org/coronavirus 
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Keep in mind “herd immunity” is a description of a state of the population, and NOT A POLICY recommendation per se. Pursuing herd immunity is an action, not the state.
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Several others have pointed out this before, and the empirical data keeps pointing in this direction. It is increasingly likely that not all humans are susceptible to SARS-COV-2
H/T @noid043
https://twitter.com/noid043/status/1277354454795378688?s=20
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In a recent thread, I explore the possibility of whether the data we are seeing supports the notion that: not all humans are susceptible to SARS-CoV-2 that causes COVID-19
https://twitter.com/Rfonsi1/status/1276213480127897602?s=20
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As always these data are only the basis of formulating hypotheses based on conjecture. I suer hope to be right (others have pointed this out too). We do not know if a second wave will come but need to remain vigilant.
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These data also suggest that pursuing the vaccine is important. There will always be people at risk even if we are to be lucky and have no further large waves. Especially with a change of locations.
#COVID19
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As always I have to place the caveat that we should not let our guard down. This is not a call to relax recommendations. Use masks and stay physically distant. Retain hope!
#COVID19
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Here is a continuous readable fashion ( http://Tumbler.com ) https://liberteriandoc.tumblr.com/post/622276660963819520/natural-immunity-against-covid-19
Natural immunity against COVID-19?

SARS-CoV-2 is one of several coronaviruses. Coronaviruses sometimes cause the common cold, buts sometimes produce very serious infections such as SARS and MERS....

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