![]() ![]() Instead, we are fitting a curve to observed case data at the state and county levels, so that the estimated outbreak rate is independent of the population in the respective unit. This is a statistical, but not an epidemiological model, that is, we are neither trying to model infection transmission nor estimate epidemiological parameters, such as the pathogen's reproductive or attack rate. The doubling time T d can be calculated from the outbreak rate as T d = ln ( 2 ) b. Solutions to this differential equation have the form y = a e b t, where a is the initial value of cases y. Going forward, we simply refer to b as the outbreak rate. On average, the number of unallocated cases is small, but a few states contribute as many as 4866 (New Jersey), 1300 (both Rhode Island and Georgia), or 1216 (Washington State) unallocated cases, resulting in an average of 308 unallocated cases per state, again as of April 14, 2020.įollowing approaches by the Institute for Health Metrics and Evaluation at the University of Washington 4 and the COVID-19 Modeling Consortium at the University of Texas at Austin, 5 we statistically model the outbreak in the USA at state and county levels using the exponential growth equation d y d t = b y, where b is a positive constant called the relative growth rate it has units of inverse time. For our county-level analysis, we discard cases which USA Facts can only allocate at the state, but not at the county level because of a lack of information. Since January 22, 2020, the latter database has aggregated data from the Centers for Disease Control and Prevention and state- and local-level public health agencies, confirming them by referencing state and local agencies directly. We obtain COVID-19 outbreak data from the China Data Lab published at Harvard Dataverse (as of April 13, 2020) and USA Facts (as of April 14, 2020) 1, 3 we check for consistency between the two databases. In this article, we show the necessity of analyses on a lower level. We show, both through visualization and statistical analysis, that the outbreak varies significantly across counties and that an aggregate view at the state level, as it is most often reported in media, hides differences at a lower level. In the present study, we examine the relative growth rate of the COVID-19 outbreak and its variation on a state and county levels across the USA. 2 As these factors vary significantly across the USA, there is likely to be considerable intra-country variation in the outbreak as well. A community's susceptibility to any virus is determined by a variety of factors, including but not limited to biological determinants, demographic profiles, type of habitat, and socio-economic characteristics. At the federal level, international and domestic travel restrictions were imposed, and at a state level, closing down of businesses, stay at home orders, and social distancing mandates were enacted. 1 In the struggle to contain the pandemic's growth rate, the US government took unprecedented action. On January 20, 2020, the first case of the novel coronavirus disease 2019 (COVID-19) was reported on the US soil, with cases in the USA growing to over 579,197 as of April 13, 2020. ![]()
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