Executive summary
- The effective reproduction number Reff measures the potential for Covid-19 to spread. If Reff is less than 1, new daily cases are likely to increase over time, if Reff is greater than 1, new daily cases will decrease over time.
- For the March-April 2020 outbreak, Reff was between 1.2 and 2.2 before moving to Alert Level 4 and between 0.35 and 0.55 during Alert Level 4.
- For the August-September 2020 outbreak, Reff was between 2.3 and 2.7 before Auckland moved to Alert Level 3 and between 0.5 and 0.8 during Alert Level 3.
- In both March and August 2020, 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 2020 relative to lockdown in March 2020 may be explained by better performance of the testing and contact tracing system.
- The higher value for pre-lockdown Reff in August 2020 compared to March 2020 may be due to a combination of factors, including Alert 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 is greater than 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.