This pair of reports were provided to the NZ Ministry of Health and the COVID-19 Modelling Government Steering Group on May 3rd, 2023 in response to a request on April 21st, 2023 for advice on the impact of changing case isolation policies.

The six different isolation policy scenarios requested by the COVID-19 Modelling Government Steering Group were: (1) 7-day isolation (status quo); (2) no isolation mandate, 7-day guideline (low compliance); (3) no isolation mandate, 7-day guideline, high compliance; (4) 5-day isolation; (5) 5-day minimum isolation with test-to-release (one negative test result required) up to 7 days maximum isolation; (6)  5-day minimum isolation with test-to-release (one negative test result required) up to 10 days maximum isolation.

We were also asked for estimates on the impact of isolation for asymptomatic cases, and to give advice on changing the definition of ‘Day 0’ to the day of the first positive test result.

The first of these reports builds on previous work by CMA, using updated parameter estimates from NZ case data and recent scientific literature, to estimate the individual case level impact for a range of case isolation policies. These individual level impacts are then translated into estimates of the overall transmission rate changes under a range of uncertainties, including the proportion of infections that are being detected (testing rates) and the proportion of people following the isolation rules.

We find that changing from the current isolation settings to a policy with test-to-release (TTR) does not appreciably increase transmission and can offer a significant reduction in the burden of excess isolation, especially for asymptomatic cases or cases detected through periodic (e.g. workplace) testing. And that reducing case isolation from 7 to 5 days, with no TTR, results in an increase in transmission, but much less than what would result from removing case isolation entirely.

Although the impact of isolation settings (as at May 3 2023) is subject to a large number of unknowns and uncertainties, by considering a range of assumptions we estimate that the reduction in transmission is between 10% and 25% when compared to no isolation requirements.

This report also gives a detailed exposition of how the different model parameter estimates and assumptions have been developed, and includes critical assessment and discussion of the extensive uncertainties in both the disease dynamics and behavioural factors, and sensitivity analysis related to these uncertainties.

The second of these reports uses a mathematical model (CMA’s “ordinary differential equation” model) to investigate the potential effect of the six different COVID-19 isolation policy options on the cumulative number Covid-19 infections, hospitalisations and deaths over the 6 months following the policy change.

We note that when considering the scenarios with a shift from an isolation ‘mandate’ to isolation ‘guidance’, scenario (3) “high compliance” assumes that there are high levels of compliance with non-mandatory guidance for isolation, which is likely to require measures such as clear guidance from government agencies, strong public health messaging, provision of free tests, and providing financial support to isolate. If these conditions are not met, it is more likely that impacts on transmission from removing the isolation mandate will be higher, e.g. as in scenario (2) “low compliance”.

The model was run under three different assumptions about the effect of seasonality on transmission during the upcoming winter period, namely that there is no effect of seasonality, or that seasonality causes transmission rates (per infectious person) to increase gradually from 1 April to a peak on 1 July that is either 10% or 20% higher than on 1 April.

We find that removal of the isolation mandate, in the scenario where there is high compliance with non-mandatory guidelines, could lead to a 6-21% increase in hospitalizations in the subsequent 6 months and a 6-24% increase in deaths. In a low-compliance scenario, the model estimates an 11-30% increase in hospitalisations and a 12-35% increase in deaths. We note that over a longer period of time, the relative increases are likely to be smaller than this. We also emphasise uncertainty in these estimates due to uncertainty in the current proportion of infected people who get tested and follow the isolation mandate. If the current level of testing and compliance is low, the relative increase in infection rates following an end to the isolation mandate would be smaller.