Thread by @ritwik_priya

It may be a case of hoping more than anything else, but I'm going to put my thoughts on Omicron out there, just for the record. stylised, no deep analysis.
1. A lot of evidence seems to point towards immune evasion - reinfection rates, neutralisation titres, superspreading...1/

..events with % infected ranging in 50-70% despite being vaccinated etc.
2. But immune evasion for infection or even symptomatic disease does not mean immune evasion for severity. Every year our bodies get exposed to new versions of old respiratory viruses. We get sick but....2/

..don't have to try to fight the whole thing anew - it's something 'similar yet dissimilar'. We are not immunologically naive, even if there is some immune evasion.
3. Omicron may be the first emerging 'endemic' variant (idea received in to from @BallouxFrancois ) after the...3/

..'pandemic' variants we have seen.

4. It may thus be similar to many other seasonal viruses, which get through to 10-50% of entire populations through a season, over the course of just 3-4 months, despite something similar yet dissimilar having done the same last season....,4/

5. Thus, how badly it affects a population will almost entirely be driven by how immunologically naive a population is (+age structure, but that we know already). But such a population has to worry about any variant, not just Omicron.

5/

6. There is emerging evidence that in a population that has a high degree of prior immunity (SA) through infections, Omicron is behaving in the immune-evasion-but-not-severe manner of endemic seasonal viruses. (This has absolutely nothing to do with being inherently 'mild')

6/

7. So *if* protection against severity is largely maintained in individuals and population networks with high vaccination / prior infection rates, it will start coming down to what overall level of risk we live with.

7/

8. Because we have omicron today, we will have pi early next year, rho by maybe this time next year. It will go on.
In the UK, with 95%+ of 16+ population vaccinated/infected (and 69%+ of sub-16 as well), hosp/case ratio is running at < 2% and CFR around 0.3%.

8/

9/ this is at 40-50% ascertainment, so vs. infections we are looking at 0.8-0.9% and 0.15% respectively.
If reinfection/ breakthrough risk increases by 2-3x with severity risk 1 or <1.5x, the ratios will drop by 2-2.5x.
0.4% IHR and 0.05%-0.1% IFR.

9/

10. At those levels - much as I hate to say it - we are in seasonal flu territory. Can you imagine what case rate graphs would look like if we PCR tested synptomatics and contacts for flu? And measured transmissibility of various competing variants based on immune evasion?

10/

11. So - we will have to see for a few more weeks of course - but Omicron may be the entry point of Sars-Cov-2 into endemicity for high prior immunity populations. Life will have to restart as normal, though we could prepare a little better for *all* respiratory viruses.

11/

12. Low prior immunity populations - they'll need to protect themselves for a little more, much as they would for any other variant. Until they get the IHR and IFR down to 'we accept this much risk as routine' territory.

12/

13. I may of course be terribly wrong and Omicron may be a monster. I hope I'm not.

fin/