Executive summary

Since July 2022, Covid-19 Modelling Aotearoa have been using a compartment-based ordinary differential equation (ODE) model to simulate the spread of Covid-19 in Aotearoa New Zealand.

This age-structured model includes waning of vaccine-derived and infection-derived immunity, immune evasion of new Omicron subvariants, age-dependent hospitalisation and death rates, and changes in transmission resulting from behavioural and policy changes. The model is calibrated to data on Covid-19 cases, hospitalisations and deaths using an approximate Bayesian computation (ABC) method.

A detailed description of the underlying model and fitting procedure as applied to the BA.5 wave in July 2022 can be found in Lustig et al. (2023).

The model has been periodically updated to reflect the changing transmission and immunity landscape in New Zealand. This document is a technical report summarising the different modelling assumptions and results relating to changes in transmission and immunity between August 2022 and February 2023.

It includes:

  • A description of the fitting method used to calibrate the model to epidemiological
  • A description of how the model was used to estimate the potential impact of
    behavioural and policy changes at key time points.
  • A description of the observed Omicron subvariants in New Zealand and how
    these were implemented in the model.
  • A comparison of the model’s outputs and data produced after each change in
    modelling assumptions.
  • A note on possible factors leading to the reduction in observed Covid-19 case
    fatality ratio since September 2022.