Effect of Alert Level 4 on Reff: review of international COVID-19 cases

The effective reproduction number, R eff , is an important measure of transmission potential in the modelling of epidemics. It measures the average number of people that will be infected by a single contagious individual. A value of R eff > 1 suggests that an outbreak will occur, while R eff < 1 suggests the virus will die out. In response to the COVID-19 pandemic, countries worldwide are implementing a range of intervention measures, such as population-wide social distancing and case isolation, with the goal of reducing R eff to values below one, to slow or eliminate transmission. We analyse case data from 25 international locations to estimate their R eff values over time and to assess the effectiveness of interventions, equivalent to New Zealand’s Alert Levels 1-4, for reducing transmission. Our results show that strong interventions, equivalent to NZ’s Alert Level 3 or 4, have been successful at reducing R eff below the threshold for outbreak. In general, countries that implemented strong interventions earlier in their outbreak have managed to maintain case numbers at lower levels. These estimates provide indicative ranges of R eff for each Alert Level, to inform parameters in models of COVID-19 spread under different intervention scenarios in New Zealand and worldwide. Predictions from such models are important for informing policy and decisions on intervention timing and stringency during the pandemic. Executive Summary In response to the COVID-19 pandemic, countries around the world are implementing a range of intervention measures, such as population-wide social distancing and case isolation, with the goal of reducing the spread of the virus. R eff , the effective reproduction number, measures the average number of people that will be infected by a single contagious individual. A value of R eff > 1 suggests that an outbreak will occur, while R eff < 1 suggests the virus will die out. Comparing R eff in an early outbreak phase (no or low-level interventions implemented) with a later phase (moderate to high interventions) indicates how effective these measures are for reducing R eff . We estimate early-phase and late-phase R eff values for COVID-19 outbreaks in 25 countries (or provinces/states). Results suggest interventions equivalent to NZ’s Alert Level 3-4 can successfully reduce R eff below the threshold for outbreak.