Suppression and Mitigation Strategies for Control of COVID-19 in New Zealand

Alex James; Shaun C Hendy; Michael J Plank; Nicholas Steyn

Mitigation strategies, which aim to allow the epidemic to go ahead at a controlled rate, keep demand on healthcare systems under capacity, and deliver herd immunity, are a tempting approach for the control of Covid-19. However, model results show that for these to be successful requires the ability to reduce transmission to a level where the effective reproduction number Rc is close to or below 1. It remains unknown whether this will be achievable in practice in New Zealand. There is no evidence that it has yet been achieved in comparable, western democracies, including those that have instigated major lockdowns such as Italy. The only regimes that have conclusively achieved this level of control are China and South Korea. In these countries, this has been achieved by extremely intensive measures, including mandatory and strictly enforced quarantine, huge amounts of resources devoted to contact tracing, electronic surveillance of citizens' movements, etc. In addition, successful mitigation requires periods of these intensive control measures to be continued for up to 2.5 years before the population acquires a sufficient level of herd immunity. This could be an underestimate as these models do not include population turnover via birth-death, which will become significant over this time frame and may act to reduce the build-up of herd immunity. Furthermore, correct timing of strong control measures is crucial to successfully keeping healthcare systems from being overloaded. Small uncertainties in case trajectories could lead to drastically overshooting hospital and ICU capacity. If hospitalisation and/or ICU admission rates are in reality higher than assumed here, e.g. closer to the CDC (2020) estimates (Table 1) , then mitigation strategies aimed at keeping ICU load under become even more difficult. Expanding New Zealand's ICU capacity would alleviate this somewhat, and would be a sensible precaution in any case.