Antigen test kits are ramping up and should become readily available before long. But what’s the protocol? What do we do with the information?
Dr. Ashish Jha posted an informative threat that gives some practical guidance:
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
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?
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?
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. 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.
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
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. 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!
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. 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.
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. 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
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.) 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.
Originally tweeted by Ashish K. Jha, MD, MPH (@ashishkjha) on December 26, 2021.
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