Effective reproduction number for COVID-19 in Aotearoa New Zealand

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 . In this work, 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 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.