Effective reproduction number for Covid-19 in Aotearoa New Zealand

Abstract

The effective reproduction number, R_eff, is the average number of secondary cases infected by a primary case, a key measure of the transmission potential for a disease. Compared to many countries, Aotearoa New Zealand has had relatively few Covid-19 cases, many of which were caused by infections acquired overseas. This makes it difficult to use standard methods to estimate R_eff. In this work, we use a stochastic model to simulate Covid-19 spread in Aotearoa New Zealand and report the values of R_eff from simulations that gave best fit to case data.

We estimate that Aotearoa New Zealand had an effective reproduction number R_eff = 1.8 for Covid-19 transmission prior to moving into Alert Level 4 on 25 2020 and that after moving into Alert Level 4 this was reduced to R_eff = 0.35. Our estimate R_eff = 1.8 for reproduction number before Alert Level 4, is relatively low compared to other countries. This could be due, in part, to measures put in place in early-to mid-March, including: the cancellation of mass gatherings, the isolation of international arrivals, and employees being encouraged to work from home.

Executive summary

• The effective reproduction number, R_eff, is the average number of secondary cases infected by a primary case, a key measure of the transmission potential for a disease.
• Compared to many countries, New Zealand has had relatively few Covid-19 cases, many of which were caused by infections acquired overseas. This makes it difficult to use standard methods to estimate R_eff.
• We use a stochastic model to simulate Covid-19 spread in New Zealand, and report the values of R_eff from simulations that gave best fit to case data.
• We estimate that New Zealand had an effective reproduction number R_eff = 1.8 for Covid-19 transmission prior to moving into Alert Level 4 on March 25 and that after moving into Alert level 4 this was reduced to R_eff = 0.35.
• Our estimate R_eff = 1.8 for reproduction number before Alert Level 4, is relatively low compared to other countries. This could be due, in part, to measures put in place in early- to mid-March, including: the cancellation of mass gatherings, the isolation of international arrivals, and employees being encouraged to work from home.