15 September 2020


Effective reproduction number and likelihood of cases outside Auckland


Executive Summary

  • The effective reproduction number 𝑅𝑒𝑓𝑓 measures the potential for COVID-19 to spread. If 𝑅𝑒𝑓𝑓 > 1, new daily cases are likely to increase over time, if 𝑅𝑒𝑓𝑓 < 1 new daily cases will decrease over time.
  • For the March-April outbreak, 𝑅𝑒𝑓𝑓 was between 1.2 and 2.2 before moving to Level 4 and between 0.35 and 0.55 during Level 4.
  • For the August-September outbreak, 𝑅𝑒𝑓𝑓 was between 2.3 and 2.7 before Auckland moved to Level 3 and between 0.5 and 0.8 during Level 3.
  • In both March and August, the Alert Level response was successful in reducing 𝑅𝑒𝑓𝑓 below 1 and hence containing the outbreak.
  • The bigger relative reduction in 𝑅𝑒𝑓𝑓 achieved by lockdown in August relative to lockdown in March may be explained by better performance of the testing and contact tracing system.
  • The higher value for pre-lockdown 𝑅𝑒𝑓𝑓 in August compared to March may be due to a combination of factors, including Level 1 conditions (no gathering size restrictions, etc.), different behaviour of cases associated with international travel in March/April, higher transmission rates in winter, and potentially higher rates of crowded housing in affected communities.
  • It is still too early to produce a reliable estimate for 𝑅𝑒𝑓𝑓 at Alert Level 2.5 in Auckland. The confidence interval for 𝑅𝑒𝑓𝑓 is very wide and it is possible that 𝑅𝑒𝑓𝑓 > 1 under current conditions.
  • The likelihood of undetected active cases outside the Auckland region is also uncertain and could be up to 40% for the North Island and up to 20% for the South Island. The possibility of spread to other regions will remain as long as there are active cases in the Auckland cluster combined with near normal rates of inter-regional travel.