Early Warning for a Potential Pandemic

From December 2019 to August 2020, more than 22 million confirmed cases of coronavirus disease 2019 (COVID-19) have been identified in more than 200 countries and regions around the world, causing severe damage to the health of people and the economy. The speed and intensity of COVID-19 transmission varies greatly among countries. However, a common measure to make this comparison is lacking. The existing indicators, such as basic reproduction number (R 0 ) and doubling time, cannot reflect the speed and intensity of an epidemic simultaneously. Additionally, these parameters are usually estimated by mathematical models such as Susceptible-Exposed-Infectious-Recovered (SEIR) model, which can only fit data well after the inflection point has been reached and is hence too late for an early warning of a pandemic. In light of these problems, we propose a novel indicator, the number of days needed for the cumulative incidence to reach 1 case per 100,000 population (days to 1/100,000), as a comprehensive indicator to reflect both the speed and intensity of COVID-19 in a particular country or region. Additionally, because “days to 1/100,000” is a pre-prediction variable which has already reflected the partial influence of several possible factors on the inflection point, it was confirmed to be a good early predictor for the number of days for the epidemic to reach the inflection point. This indicator could be generalised to other infectious diseases as an early warning of a potential pandemic. Funding Statement: This work was supported by the Natural Science Foundation of China (NSFC) (Grant No. NSFC 41531179 and NSFC 41421001) and the Ningxia Key Research & Development Plan (Special project for foreign cooperation) under the "Western Young Scholars" project of Chinese Academy of Sciences. Declaration of Interests: The authors declare that we have no conflicts of interest.