Thread by @trvrb

It seems that the common assumption has been that Omicron will displace Delta, just as Delta displaced Alpha, Beta, Gamma, etc... before it. This may well be the case, but it's by no means definite. 1/15

Depending on Omicron's mix of intrinsic transmissibility and immune escape (and what happens with continued evolution), we could see:
1. Displacement of Delta by Omicron
2. Long-term co-circulation
3. Omicron wave followed by resurgence of Delta and extinction of Omicron
2/15

Intuitively, the more immune escape Omicron has from Delta-specific immunity the more the two variants have distinct ecological niches and so are able to co-exist without stepping on each other's toes. 3/15

We can model this as a two-strain SIRS model with polarizing immunity (bedford.io/projects/mitii…). Here, β1 is transmission rate of strain 1, β2 is transmission rate of strain 2, γ is recovery rate, ω is rate of immune waning and σ is cross-immunity. 4/15

For these purposes, I assume that strain 1 (ie Delta) is at endemic equilibrium, where epidemic growth is balanced by population immunity, but where the strain continues to circulate due to waning immunity (ω = 1/365 per day). 5/15

The second strain (ie Omicron) is introduced and depending on its transmissibility β2 and its degree of immune escape 1-σ, it will spread more or less rapidly as described previously. 6/15

https://twitter.com/trvrb/status/1466076797670363140

Assuming Delta R0 is 6, with lower intrinsic transmissibility and higher immune escape (Omicron R0 = 4, immune escape = 66%), Omicron has initial Rt of 2 and undergoes a large wave. However, this wave has relatively little impact on Delta. 7/15

With moderate intrinsic transmissibility and immune escape (Omicron R0 = 6, immune escape = 50%), Omicron also has initial Rt of 2, a wave of similar magnitude, but more impact on Delta. 8/15

And with higher intrinsic transmissibility and lower immune escape (Omicron R0 = 8, immune escape = 33%), Omicron's wave really knocks down Delta. 9/15

In fact, we can plot out parameter combinations that result in scenario 1 (extinction of Delta, yellow), scenario 2 (co-circulation of Delta and Omicron, light gray) and scenario 3 (extinction of Omicron after epidemic wave, dark gray). 10/15

Co-circulation occurs when R0 is similar between Omicron and Delta and/or when there is little cross-immunity, and displacement occurs when there is strong cross-immunity and lopsided R0. See Restif and Grenfell for further discussion (royalsocietypublishing.org/doi/10.1098/rs…). 11/15

If we compare to seasonal flu, displacement is largely the rule with new strains replacing existing diversity. However, we do have an example of scenario 2 with B/Victoria and B/Yamagata diverging around 1980 and co-circulating since (figure from elifesciences.org/articles/44205). 12/15

Although this question of displacement vs co-circulation is not immediately critical to pandemic understanding or public health response, its resolution will have significant impact on eventual endemic state of SARS-CoV-2. 13/15

If Omicron and Delta co-circulate then annual vaccines would be best formulated as multiple valencies (just like flu vaccine has both B/Victoria and B/Yamagata) and we can expect generally higher levels of viral circulation than in the displacement scenario. 14/15

I'm very much hoping we'll see displacement, but we should have first hint at outcome in the next few weeks as we see to what degree the Omicron wave reduces Delta circulation in the US and Europe. 15/15