Public Health Measures and the Reproduction Number of SARS-CoV-2

Coronavirus disease 2019 (COVID-19) is a respiratory infectious disease caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), first detected in early December 2019 in Wuhan, China. It has since spread throughout the world. One measure of viral spread is the R0, the expected number of secondary infectious cases produced by a primary infectious case. This calculation is used to determine the potential for epidemic spread in a susceptible population. The effective reproduction number, Rt, determines the potential for epidemic spread at a specific time t under the control measures in place (Figure 1). To evaluate the effectiveness of public health interventions, the Rt should be quantified in different settings, ideally at regular and frequent intervals (eg, weekly). In an article published in JAMA, Pan and colleagues1 evaluated the association of public health interventions with the epidemiological features of the COVID-19 outbreak in Wuhan by 5 periods, according to key events and interventions, including cordons sanitaire, traffic restriction, social distancing, home confinement, centralized quarantine, and universal symptom survey. In their study, Pan et al1 determined the Rt as an indicator to measure the transmission of SARS-CoV-2 before and after the interventions. In a figure in their article, the authors show the extraordinary change in the rate of transmission of SARS-CoV-2 associated with reducing social interaction (Figure 2). In early through mid-January 2020, the SARS-CoV-2 epidemic in Wuhan had an Rt of 3 to 4. In other words, each case spread to an average of 3 to 4 others. That is a striking number: compare it to the Rt of 1.4 to 1.7 for influenza, which is a disease that spreads widely around the world every year. Couple that with the fact that each new generation of SARS-CoV-2 cases occurs every 5 days, and it is clear to see how this epidemic was spreading out of control. On January 23, a series of major actions were taken by the Chinese government, including a city lockdown and home and centralized quarantines. Some of the measures put in place in Wuhan