Executive summary

  • Evaluating the effectiveness of New Zealand’s Covid-19 response, relative to counterfactual (alternative ‘what-if’) scenarios, is important for guiding future response strategies. We assess the importance of early implementation of interventions for controlling Covid-19.
  • We model counterfactual scenarios in which the timings of three policy interventions are varied: border restrictions requiring 14-day quarantine of all international arrivals, border closure except to returning residents and citizens, and Alert Level 4 restrictions. We compare these to a modelled factual scenario in which intervention timings are the same as occurred in reality.
  • Key measures describing the dynamics of a Covid-19 outbreak (notably peak load on the contact tracing system, the total number of reported Covid-19 cases and deaths, and the probability of elimination within a specified time frame), are used to compare outcomes between scenarios.
  • Key measures were more sensitive to the timing of Alert Level 4, than to timing of border restrictions and border closure. Of the counterfactual scenarios, an earlier start to Alert Level 4 would have resulted in the greatest reduction in numbers of cases and deaths.
  • Delaying the start of Alert Level 4 by 20 days could have led to over 11,500 cases and 200 deaths, and would have substantially reduced the probability of eliminating community transmission of Covid-19, requiring a longer period at Alert Level 4 to achieve control.