Four overlooked facts about COVID-19

Four Stylized Facts about COVID-19 | National Bureau of Economic Research Working Paper 27719 - Andrew Atkeson, Karen Kopecky, and Tao Zha:

August 2020 - "We document four facts about the COVID-19 pandemic worldwide relevant for those studying the impact of non-pharmaceutical interventions (NPIs) on COVID-19 transmission. 

• First: across all countries and U.S. states that we study, the growth rates of daily deaths from COVID-19 fell from a wide range of initially high levels to levels close to zero within 20-30 days after each region experienced 25 cumulative deaths. 
• Second: after this initial period, growth rates of daily deaths have hovered around zero or below everywhere in the world. 
• Third: the cross section standard deviation of growth rates of daily deaths across locations fell very rapidly in the first 10 days of the epidemic and has remained at a relatively low level since then. 
• Fourth: when interpreted through a range of epidemiological models, these first three facts about the growth rate of COVID deaths imply that both the effective reproduction numbers and transmission rates of COVID-19 fell from widely dispersed initial levels and the effective reproduction number has hovered around one after the first 30 days of the epidemic virtually everywhere in the world.... 

"In this paper, we document these facts regarding COVID deaths using both simple data smoothing procedures and a Bayesian estimation procedure that allows us to construct probability bands around our estimates of the growth of COVID deaths. We then use an SIR epidemiological model based on that in Kermack and McKendrick (1927) to interpret these data on the growth rate of COVID deaths ... and show that our conclusions about the worldwide decline in the transmission rate for COVID-19 are not much affected by the choice of epidemiological model....

"Several prominent studies, including Dehning et al. (2020), Hsiang et al. (2020), and Flaxman et al. (2020), have studied empirically the role of government-mandated non-pharmaceutical interventions (NPIs) in reducing the transmission of COVID-19, and many of these studies  ... may substantially overstate the role of government-mandated NPI’s in reducing disease transmission due to an omitted variable bias. Moreover, given the observation that disease transmission rates have remained low ... as NPI’s have been lifted, we are concerned that estimates of the effectiveness of NPI’s in reducing disease transmission from the earlier period may not be relevant for forecasting the impact of the relaxation of those NPI’s in the current period....

"What might this omitted variable or variables be?... [T]he literatures in both epidemiology and economics offer several candidates. COVID-19 is not the first epidemic for which transmission rates have fallen faster than would be predicted by simple epidemiological models....

"The first of these hypotheses is that humans spontaneously take action to avoid disease transmission once an epidemic breaks out. For COVID-19, a great deal of real-time mobility and economic data indicate that human social and economic interactions have fallen substantially across a large number of locations. Further research is required, however, to determine whether this decline in human interactions is sufficiently large and widespread to account for the apparent decline in transmission rates....

"The second ... hypothesis is that the network structure of human interactions naturally leads to a slowdown in disease transmission faster than would be predicted from a simple epidemiological model in which the population interacts uniformly with each other. Further research is required to determine whether a network structure of human interactions can indeed explain the global decline....

"Finally, we must consider the possibility that ... some unobserved natural factor may have driven the decline to date in the transmission of COVID-19. Clearly, the existence of such an unobserved factor would complicate empirical studies of the causal driving forces behind COVID-19 transmission....

"One of the central policy questions regarding the COVID-19 pandemic is the question of which non-pharmaceutical interventions [NPIs] governments might use to influence the transmission of the disease. Our ability to identify empirically which NPI’s have what impact on disease transmission depends on there being enough independent variation in both NPI’s and disease transmission across locations as well as our having robust procedures for controlling for other observed and unobserved factors that might be influencing disease transmission. The facts that we document in this paper cast doubt on this premise."

Read more: https://www.nber.org/papers/w27719.pdf