Initial Model for USA CoVID-19 Resurgence

Early CoVID-19 growth obeys: N{t*}=NI exp[+Ko t* ], with Ko=[(ln2)/(tdbl)], where tdbl is the pandemic growth doubling time. Given N{t*}, the daily number of new CoVID-19 cases is {rho}{t*} = dN{t*}/d{t*}. Implementing society-wide Social Distancing increases the tdbl doubling time, and a linear function of time for tdbl was used in our Initial Model: No[t] = 1 exp[+KA t / (1 + {gamma}o t) ] = eGo exp(-Zo[t] ) , to describe these changes, where the [t]-axis is time-shifted from the t*-axis, back to the pandemic start, and Go = [ KA / {gamma}o ]. While this No[t] successfully modeled the USA CoVID-19 progress from 3/2020 to 6/2020, this equation could not easily model some quickly decreasing {rho}[t] cases ("fast pandemic shutoff"), indicating that a second process was involved. This situation was most evident in the initial CoVID-19 data from China, South Korea, and Italy. Modifying Zo[t] to allow exponential cutoffs: Zo[t] {equiv} +[Go / (1+{gamma}o t)] [exp(-{delta}ot)] = Zo[t] exp(-{delta}ot) , NA[t] = eGo exp(-ZA[t]) , resulted in an Enhanced Initial Model (EIM) that significantly improved data fits for these cases. After 6/2020, many regions of the USA "opened up", loosening their Social Distancing requirements, which led to a sudden USA CoVID-19 Resurgence. Extrapolating the USA No[t] 3/2020-6/2020 results to 9/2020 as an Initial Model Baseline (IMB), and subtracting this IMB from the newer USA data gives a Resurgence Only function, which is analyzed here. This USA CoVID-19 Resurgence function differs significantly from the No[t] IMB functional form, but it was well-modeled by the NA[t] fast pandemic shutoff function. These results indicate that: (a) the gradual increase in tdbl doubling time from society-wide shut-downs is likely due to eliminating of a large number of population gathering points that could have enabled CoVID-19 spread; and (b) having a non-zero {delta}o fast pandemic shutoff is likely due to more people wearing masks more often [with 12 Figures].