Modelling plausible scenarios for the Omicron SARS-CoV-2 variant from early-stage surveillance

We used a spatially explicit agent-based model of SARS-CoV-2 transmission combined
with spatially fine-grained COVID-19 observation data from Public Health Scotland to in vestigate the initial rise of the Omicron (BA.1) variant of concern. We evaluated plausible
scenarios for transmission rate advantage and vaccine immune escape relative to the Delta
variant based on the data that would have been available at that time. We also explored
possible outcomes of different levels of imposed non-pharmaceutical intervention. The initial
results of these scenarios were used to inform the Scottish Government in the early outbreak
stages of the Omicron variant.
Using the model with parameters fit over the Delta variant epidemic, some initial assumptions about Omicron transmission rate advantage and vaccine escape, and a simple growth rate fitting procedure, we were able to capture the initial outbreak dynamics for Omicron.
We found that the modelled dynamics hold up to retrospective scrutiny. The modelled imposition of extra non-pharmaceutical interventions planned by the Scottish Government at
the time would likely have little effect in light of the transmission rate advantage held by the
Omicron variant and the fact that the planned interventions would have occurred too late in
the outbreak’s trajectory. Finally, we found that any assumptions made about the projected
distribution of vaccines in the model population had little bearing on the outcome, in terms
of outbreak size and timing. Instead, it was the landscape of prior immunity that was most
important.