Thread by @kakape

One of the complexities in talking about #omicron is that we are constantly talking about the variant’s properties on two levels:
in a naive population and in an immune population (in reality many populations with different levels of immunity)
That matters.

Take transmissibility:
It’s been clear from watching #omicron that it spreads faster than delta.
But from the beginning the question has been: Is that because it can infect people delta can’t? Or is it inherently more transmissible?

If you look at the UK’s risk assessment of #omicron for instance, you can see that that question is still not settled:
"there is no clear epidemiological demonstration of transmissibility as distinct from other contributors to growth advantage”
assets.publishing.service.gov.uk/government/upl…

We now have some intriguing data from Denmark that goes directly to this question.

Preprint is here:
medrxiv.org/content/10.110…
And a great thread from an author is here:

https://twitter.com/LyngseF/status/1476223591138140167?s=20

The authors essentially looked at how much more likely people were to get infected from an #omicron infected household member vs. a #delta infected household member.
They found a secondary attack rate of 31% for #omicron vs. 21% with #delta.

But crucially, that difference was almost entirely due to vaccinated people.
In unvaccinated people the likelihood of infection barely differed whether they had an omicron case in their household or a delta case.
That suggests, omicron is inherently not much more transmissible.

Remember this is just one preprint and there are a lot of caveats. There are other pieces of evidence that seem to suggest omicron might be inherently more transmissible.
But I just wanted to point out that even this seemingly obvious trait of omicron is anything but.

Now why does this matter?
For one, if omicron really isn’t inherently more transmissible that means the same level of NPIs that worked against Delta should work against omicron.

(Personally, I find that difficult to square with delta cases falling, but omicron cases rising in Germany at the moment, but there are a lot of caveats to that that I don’t want to go into here. Just want to point out that we have conflicting data points…)

Another implication:
"although we showed that booster vaccines did offer some protection against household transmission, the reduced level of protection means that vaccination is less likely to be sufficient to curb transmission within a population"

In other words:
If omicron’s super power is mostly to evade immunity, then beefing up immunity is not the best way to stop it from spreading.

Again, all of this comes with a ton of uncertainty and that’s my point.
Untangling how different characteristics of the virus have changed and what role widespread immunity plays is hard.
But it does matter for what is going to happen in the next weeks.