Thread by @ashishkjha

As Omicron cases explode

We need a strategy for isolating folks who test positive

We need to think about the purpose of isolation clearly

Because if we don't get it right,

It'll both be hugely disruptive and won't keep us safe

So let's discuss what we need to do

Thread

First principles:

Why ask people to isolate at all?

Well, that’s obvious

We don’t want them spreading

So what we care about is CONTAGIOUSNESS

We want folks to isolate when they're contagious

So when are people contagious?

Well, it varies. A lot

OK, so what to do?

2/n

How long folks are contagious depends on two things

1. When they test positive (i.e. early in the phase of infection vs. late)

2. Their immune system's ability to clear the virus

Take the average person who tests PCR+ today

How long will they be contagious?

3/n

The average person who tests PCR+ today?

Some will be contagious for 2 days

Others for 5 or (rarely) 10 days

But here’s the kicker

Some for zero days – because they tested + after no longer being contagious!

Confusing

But thankfully, we have tools to figure this out

4/n

So let’s look at what we know

Here’s a graphic from @FT by @jburnmurdoch that's adapted from @michaelmina_lab piece in @NEJM

Simply speaking, there are three periods of time you can be PCR+ (I label as A, B, and C)

Its an excellent graphic and critical to this whole story

5/n

There is very short pre-contagious period

Where folks are PCR + but rapid antigen negative

Labeled A

There is the contagious period: PCR + and Antigen +

Labeled B

There is post contagious period: PCR +, Antigen neg, which can last many, many days

Labeled C

Graph again

The graph shows “infectious” period of about 5 days

But it can be longer

That’s why CDC defaults to 10 days to be safe

But here's the thing

10 day isolation uses the mental model that either

People test + in Phase A/early Phase B

Or have symptoms beginning of phase B

7/n

But many people have little or no symptoms (especially vaccinated folks)

And many folks get tested late in phase B or even in phase C

Which is why studies show that many PCR+ have high CT values (low viral load)

These are people testing positive but no longer contagious!

8/n

So what to do?

We need a simple, workable solution that

Works across scenarios

Works for everyone, not just in healthcare

So here's why 5 days of isolation and negative antigen test is reasonable, even cautious

Antigen tests ("lateral flow") are contagiousness tests (graph)

So for average person

If you isolate for 5 days, you are likely no longer contagious

But for rare person who might still be?

That’s where antigen “contagiousness” test comes in

Negative antigen on top of 5 day isolation?

Pretty good evidence you are no longer contagious

But, what about false negative antigen test (i.e. person still contagious)?

That's pretty rare

But if you want to be extra careful (for nursing home workers), you could require 2 negative antigen tests

At that point, I’m not sure why you’d continue isolating someone

11/14

Two more things

1. If you test + on rapid antigen, you are contagious (vaccinated or not)

And isolating for 5 days and repeating antigen test makes sense

2. Data from Delta suggests vaccinated folks have shorter contagiousness period

Expect same with Omicron

12/15

Therefore, if you want to be even more cautious

Limit 5 days isolation and a neg antigen test to only vaccinated folks

NYC strategy: 5 days isolation and then high quality masking for another 5 days

That’s reasonable

I’d love a negative antigen test on top if possible

13/14

Bottom line?

Isolating COVID + people all about stopping transmission

Different people contagious for different periods of time

10 days isolation for everyone is unnecessary

Luckily, we don’t have to guess

We have antigen tests (I know, not enough…but they're coming)

14/15

5 days and 1-2 neg antigen tests should be enough to end isolation

Not just for healthcare workers

But for moms and dads getting back to kids

Hourly workers who don’t get paid when home

Really for anyone who doesn’t want to be isolating when they don’t need to be

End