COVID-19 Modelling Aotearoa used a mathematical model to investigate the potential impact of shifting from mandatory COVID-19 case isolation to case isolation guidance on the number of infections, hospital admissions, and deaths over the next 6 months.
Irrespective of legal requirements, staying at home when sick is an important public health message for reducing disease transmission.
In the model, ending mandatory case isolation caused a wave of infections in the subsequent 1-2 months. Under our best estimate, ending mandatory case isolation led to a 13-25% increase in the total number of COVID-19 hospital admissions and deaths in the subsequent 6 months. After 4-6 months, infections settled to a level that was only slightly higher than if mandatory isolation was maintained. However, relative increases in severe health outcomes in the longer-term tend to be slightly larger than the associated increase in infections. This is because the additional infections caused by the transmission increase tend to occur disproportionately in older age groups due to their lower rates of prior infection.
There is a lot of uncertainty about the exact impact of replacing mandatory isolation with guidance, because firstly the current behaviour of people with COVID-19 is not well known and secondly the change in behaviour in response to the policy change is difficult to predict. Not everyone follows the current requirements, while others will continue to follow isolation guidelines even if they are no longer mandatory.
To account for some of this uncertainty, we ran alternative model scenarios. In a scenario with a smaller transmission increase, which could occur if many people continue to test and isolate, the increase in hospital admissions and deaths could be as low as 6%. In a scenario with a larger transmission increase where more people leave their dwelling while infectious, the increase in hospital admissions and deaths could be over 35%.
An important note is that the results only relate to total national numbers and that this modelling assumes that the change in behaviour is the same across the whole population. It is likely that groups at higher risk of severe COVID-19, including Māori and Pacific people, and shielding individuals will be disproportionately affected by an increase in transmission.
This modelling assumes that the behavioural change in response to any policy update occurs immediately. If instead the behavioural change occurs more gradually, this would reduce the size of the initial peak and lead to a more gradual transition onto the same long-term trends. The model results suggest that, beyond the 4-6 months following a policy change, long-term outcomes are relatively insensitive to the timing of any decision to end mandatory isolation.
The modelling does not include considerations such as the effect of new variants of concern or seasonal effects. However, we believe that the relative changes due to ending isolation are robust under a range of underlying transmission environments.
Provided to NZ Ministry of Health: 22 March 2023; This was version updated: 27 Mar 2023 following internal peer review.