The Effect of Large-Scale Anti-Contagion Policies on the Coronavirus (COVID-19) Pandemic

Solomon Hsiang; Daniel Allen; Sebastien Annan-Phan; Kendon Bell; Ian Bolliger; Trinetta Chong; Hannah Druckenmiller; Andrew Hultgren; Luna Yue Huang; Emma Krasovich; Peiley Lau; Jaecheol Lee; Esther Rolf; Jeanette Tseng; Tiffany Wu

We estimate that in the absence of policy, early infection rates of COVID-19 grow 45% per day on average, implying a doubling time of approximately two days. Country-specific estimates range from 25.23% per day (p< 0.05) in China to 65.04% per day (p< 0.001) in Iran, although an estimate only using data from Wuhan, the only Chinese city where a meaningful quantity of pre-policy data is available, is 55% per day (p< 0.001). Growth rates in South Korea, Italy, France, and the US are very near the 45% average value (Figure 2A ). These estimated values differ from the observed growth rates because the latter are confounded by the effects of policy. In the early stages of most epidemics, a large proportion of the population remains susceptible to the virus, and if the spread of the virus is left uninhibited by policy or behavioral change, exponential growth will continue until the fraction of the susceptible population declines meaningfully. 7, 29 This decline results from members of the population leaving the transmission cycle, due to either recovery or death. 29 At the time of writing, the minimum susceptible population fraction in any of the administrative units analyzed is 99.4% of the total population (Lodi, Italy: 1,445 infections in a population of 230,000). This suggests that all administrative units in all six countries would likely be in a regime of uninhibited exponential growth if policies were removed today.