Government Responses Matter: Predicting Covid-19 cases in US under an empirical Bayesian time series framework

Ziyue Liu; Wensheng Guo

When facing an epidemic, people and government of a country may underestimate its seriousness in the beginning but will eventually step up their responses. Hence the case numbers tend to increase exponentially in the early stage, while the trends will gradually bend and plateau. Therefore, similarities in the case number trajectories can be observed in different countries, though the timing and severity can differ substantially due to different responses. Figure 1 displays the trajectories of total Covid-19 case numbers for China, S. Korea, Italy, France, Iran, Germany, Spain and USA using Johns Hopkins data. These countries have more days from time zero than US, where time zero is defined as first day with 100 or more (100+) cases as a heuristic but widely used choice 1 . The curve of South Korea increased rapidly early on but quickly bended and plateaued, for which S. Korea's swift and deterministic policy responses are credited 2 . China exhibits similar but later flattening pattern, which agrees with its missing early intervention window, but later extreme lockdown policy implementation 3 . On the other hand, the cases in Italy and France have grown exponentially until recent days, which have partially been attributed to their late and weak policy responses 4 . The US trajectory is almost linear on the logarithm scale. While the US government is catching up with policies such as work/study from home, social distancing and self-quarantine, the effect has not seen in the trajectory.