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Age-targeted strategies are often interpreted in extreme ways: "cut off", "perfectly segregate", etc, some part of society, while everyone else "gets on with normal life".

Obviously in this formulation, skepticism is merited. Nothing is perfect, and "cutting" things off...1/9

Obviously in this formulation, skepticism is merited. Nothing is perfect, and "cutting" things off...1/9

sounds hard!

But this is an absolutist interpretation of ideas which should, like most, be interpreted quantitatively. The same misinterpretation could be given for all current strategies, by suggesting that masks, restrictions, etc., aim to "perfectly" stop all transmission. 2/

But this is an absolutist interpretation of ideas which should, like most, be interpreted quantitatively. The same misinterpretation could be given for all current strategies, by suggesting that masks, restrictions, etc., aim to "perfectly" stop all transmission. 2/

In fact, the goal of current strategies is to have a quantitative effect on transmission, reducing it for all groups essentially "as much as the market will bear".

In practice, this has involved implementing policies which begin harshly (lockdown), and gradually relax. 3/

In practice, this has involved implementing policies which begin harshly (lockdown), and gradually relax. 3/

A quick note on this thread by @joel_c_miller before bedtime.

Joel is summarizing a nice argument coming from random graphs to suggest that increasing transmission rates among low-risk groups cannot be good, unless accompanied by other decreases. 🧵 1/9

Joel is summarizing a nice argument coming from random graphs to suggest that increasing transmission rates among low-risk groups cannot be good, unless accompanied by other decreases. 🧵 1/9

This argument is valid if comparing two scenarios with constant transmissions. It is not valid if we expect (as I think we do!) that transmission patterns will eventually increase.

In particular, it is worth noting, that... 2/9

In particular, it is worth noting, that... 2/9

even in the simplest single-population models with time-varying transmission rates, epidemic sizes (and thus mortality) can be decreased by increases in transmission.[🤯]

Time dynamics make coupled systems complex, and intuitive reasoning about the effects of changes is tricky.

Time dynamics make coupled systems complex, and intuitive reasoning about the effects of changes is tricky.

In our paper w/ @ChikinaLab we

1) account for preferential mixing in age groups

2) do not require perfect isolation of at risk groups

3) do not assume that we can "turn off transmission" (indeed, we assume transmission gradually reverts to normal levels)1/

1) account for preferential mixing in age groups

2) do not require perfect isolation of at risk groups

3) do not assume that we can "turn off transmission" (indeed, we assume transmission gradually reverts to normal levels)1/

Joel is also concerned that aged-care facilities could support an epidemic (not localized outbreaks at individual centers) on their own without any involvement of younger individuals (staff/doctors/etc). 2/8

An earlier discussion on this ended abruptly:

An earlier discussion on this ended abruptly:

I think its fair to say this is an unusual viewpoint.

Threads like this which dismiss age-targeted strategies out of hand inevitably work with the most simplistic and absolutist version of an idea. I have not seen an argument that we cannot have a quantitative... 3/8

Threads like this which dismiss age-targeted strategies out of hand inevitably work with the most simplistic and absolutist version of an idea. I have not seen an argument that we cannot have a quantitative... 3/8

In light of the most credible reinfection case to date (👇) there has been discussion of what this means for herd immunity from infection or vaccines.

Quick thread with a simplified explanation of the basic concept of population immunity.🧵 1/8

Quick thread with a simplified explanation of the basic concept of population immunity.🧵 1/8

What does it take for an epidemic to die out, so that a people are no longer being infected in significant numbers?

When first confronting this question, it might seem like the answer is that nearly everyone has to be protected from infection, so that any infected person is...2/

When first confronting this question, it might seem like the answer is that nearly everyone has to be protected from infection, so that any infected person is...2/

unlikely to infect anybody else.

However, it is not required that each infected person infects 0 new people for an epidemic to be unsustainable, we need only that each infected person infect less than 1 person on average, so that new infections are outnumbered by recoveries. 3/8

However, it is not required that each infected person infects 0 new people for an epidemic to be unsustainable, we need only that each infected person infect less than 1 person on average, so that new infections are outnumbered by recoveries. 3/8

🧵I talk a lot about the mistake people make thinking it is always safer to err on the side of extreme action.

Here is a simple thought experiment to illustrate this point.

Imagine you are given a magic wand which, if you wave it, will reduce COVID transmission by... 1/11

Here is a simple thought experiment to illustrate this point.

Imagine you are given a magic wand which, if you wave it, will reduce COVID transmission by... 1/11

90% for 2 weeks. The magic wand has no side effects, and it can only be used today.

Should you wave it?

It doesn't sound like a hard question. The magic wand only confers benefits, it seems, and no costs. What could be the problem?

Surprisingly, in some situations... 2/11

Should you wave it?

It doesn't sound like a hard question. The magic wand only confers benefits, it seems, and no costs. What could be the problem?

Surprisingly, in some situations... 2/11

waving the magic wand can result in more COVID mortality and morbidity than not waving it.

The reason is that even in the simplest epidemic models, where all individuals are identical, the final size of an epidemic is not monotone with respect to a time-varying pattern... 3/11

The reason is that even in the simplest epidemic models, where all individuals are identical, the final size of an epidemic is not monotone with respect to a time-varying pattern... 3/11

Is there a seasonal component to the COVID-19 mortality rate?

A thread with some figures with @ChikinaLab on what seems to be a question of underestimated importance. 🧵 1/8

Slightly longer version here: math.cmu.edu/~wes/aus.html

A thread with some figures with @ChikinaLab on what seems to be a question of underestimated importance. 🧵 1/8

Slightly longer version here: math.cmu.edu/~wes/aus.html

Backstory:

Much of the coverage of summer second waves has noticed the reassuring feature that the mortality rate seems to be lower. Possible theories explaining this include different age-distributions of cases, or that the effect is actually an illusion from more testing. 2/

Much of the coverage of summer second waves has noticed the reassuring feature that the mortality rate seems to be lower. Possible theories explaining this include different age-distributions of cases, or that the effect is actually an illusion from more testing. 2/

Another explanation for the effect could be that there is a seasonal effect on mortality rates. There is some precedent for the plausibility of this (👇) and it's important to note that it is unclear what we should expect to be driving the effect... 3/