Thread by @AtomsksSanakan

1/M

For months, COVID-19 contrarians spread misinformation about the immune system and COVID-19.
So I'll combine some rebuttal points I've made elsewhere, with a focus on T cells.

2/M

An important concept here is 'herd immunity'.

In simplified terms:

The 'herd immunity threshold' is the number of people who need to be immune to infection, in order for 'infections per unit time' to stop increasing, at baseline.

medium.com/@silentn2040/t…

3/M

So for the graph below:
y-axis = # of new infections per day
x-axis = time

This is under 'baseline' conditions; i.e. people don't change their behavior in response to infection, + no further public health interventions (ex: lockdown).

At x = 10, the threshold was reached.

4/M

So why would herd immunity matter to you?

Well, once the herd immunity threshold (HIT) is reached, you and everyone else can go back to *living as you did before the viral pandemic, without worrying about infections per day increasing.*

medium.com/@silentn2040/t…

5/M

So herd immunity sounds nice.

Unfortunately, many politically-motivated non-experts misrepresented it, since they view it as a way to dodge policies they dislike, such as lockdowns.

6/M

Many non-expert contrarians assume that if infections per day (or hospitilzations per day, or COVID-19 deaths per day, or...) decrease, then that automatically means HIT was reached.

The contrarians typically apply this to Sweden or New York City.

judithcurry.com/2020/05/10/why…

7/M

This fails b/c HIT is about baseline conditions, without public health interventions or behavior changes (in technical terms: it's about R0).

In other words: factors other than herd immunity can decrease infections per day.



ncbi.nlm.nih.gov/pmc/articles/P…

8/M

Another contrarian myth is that immune cells known as T cells greatly limit the number of infections, allowing us to reach herd immunity with less people infected.

Anyone who grasps basic immunology can debunk that, as other immunologists have:

9/M

An analogy on this might help those unfamiliar with immunology:

A key entering a lock is somewhat analogous to a receptor binding to the receptor's target.

T cells have their T cell receptor (TCR), and immune cells known as B cells have a BCR.

immunology.org/public-informa…

10/M

B cells also make antibodies; these small proteins float freely and bind to viruses (like a key to a lock). That can prevent the virus from infecting your cells.

The BCR is basically an antibody the B cell keeps with it to bind stuff like viruses.

memorangapp.com/flashcards/110…

11/M

The BCR can recognize freely floating virus.

In contrast, the TCR *cannot* recognize freely-floating virus. Instead, it needs your cells to take up the virus, and then present parts of the virus on the cell's surface on a molecule known as MHC.

faculty.ccbcmd.edu/courses/bio141…

12/M

To go back to our analogy:

If the virus is a key, then the BCR is a lock that fits that key, with no additional help needed.

And the TCR is a lock that only fits the key if the key has an adaptor (or keychain) attached.
MHC is the keychain.

13/M

T cells known as CD8+ T cells kill virus-infected cells that show virus proteins on MHC.

So these T cells don't greatly limit infection: the TCR's function here *depends* on cells getting infected. That's immunology 101.

westburg.eu/immunotherapy-…

14/M

CD4+ T cells, in contrast, don't kill certain types of cells that present virus to them.

But these CD4+ T cells limit infection by encouraging B cells to make antibodies. That flies in the face of what contrarians (who don't grasp immunology) say:

15/M

So if someone makes the following claims to u, they're misleading u on immunology:

- 'T cells will help us get to herd immunity early, way beyond what we see with antibodies!'

- 'Sweden (New York City, or some other place not at baseline) achieved herd immunity!'