This document collates results and advice given to the NZ Ministry of Health in August 2022 as part of a rapid response to requests for modelling advice on the on consequences of changing policy settings for cases, contacts, and community context.
Using the Network Contagion Model (NCM) we model the estimated relative change in the effective reproduction number (Rt) for COVID-19 in Aotearoa, across a range of scenarios with different policy settings for:
• isolation requirements for confirmed cases
• testing and quarantine requirements for household contacts of confirmed cases, and
• community context: transmission reduction behaviours, including mask wearing, reducing in-person interactions, and improved ventilation.
Approximating the effects of changing each intervention as independent, we can estimate the percentage increase in Rt for each intervention with the following heuristic:
| Intervention | Policy change from baseline | Percentage point change in Rt (from baseline) |
| Removal of community transmission reduction behaviour | CPF Orange → CPF Off | +5 % |
| Reduced case isolation | 7 days isolation requirement → 7 days isolation guidance only | +11% |
| No contact quarantine | Quarantine required, day 3 and 7 testing → No quarantine required but daily testing | +2.5% |
| No contact quarantine | Quarantine required, day 3 and 7 testing → No quarantine required and only testing if symptomatic | +5% |
We model the shift from case isolation to guidance in two ways – one where ability/proclivity to isolate is the same across all people, and one where ability/proclivity to isolate is unevenly spread throughout the population. We raise equity concerns that shifting from requirements to guidance is likely to result in increased infection risk in communities that are less able to choose to follow guidance (for example those with lower-income jobs).
This report is the first of three related documents that use a similar method to consider the effect of these policy changes.
- A second report Estimating the effect of changes in case isolation on the effective reproduction number of COVID-19 in Aotearoa: September 2022 presents further modelling in response to requests from mid-August to early September specific to case isolation policy.
- An Addendum to these reports Addendum: Assumption of perfect case isolation within the home when estimating the effect of a change in case isolation policy presents findings of sensitivity testing of transmission within households during quarantine.