  In this paper we used an adapted version of an existing simulation model of
SARS-CoV-2 transmission in Scotland to investigate the rise of the Omicron
variant of concern, in order to evaluate plausible scenarios for transmission
advantage and vaccine immune escape relative to the Delta variant. 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.
  We use an explicitly spatial agent-based simulation model combined with
spatially fine-grained COVID-19 observation data from Public Health Scotland.
Using the model with parameters fit over the Delta variant epidemic, some
initial assumptions about Omicron transmission advantage and vaccine escape,
and a simple growth rate fitting procedure, we were able to capture the initial
outbreak dynamics for Omicron. We also find the modelled dynamics hold up to
retrospective scrutiny.
  We found that 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 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, rather that the
detailed landscape of immunity prior to the outbreak was of far greater
importance.
