Top of Page Box 1 Description of the Five COVID-19 Pandemic Planning Scenarios For each Pandemic Planning Scenario: Parameter value for viral transmissibility is the Basic Reproduction Number (R0) Parameter value for disease severity is the Infection Fatality Ratio (IFR) Parameter values for the pre-symptomatic and asymptomatic contribution to disease transmission are: Percentage of transmission occurring prior to symptom onset (from pre-symptomatic individuals) Percentage of infections that are asymptomatic Infectiousness of asymptomatic individuals relative to symptomatic individuals For Pandemic Scenarios 1-4: These scenarios are based on parameter values that represent the lower and upper bounds of disease severity and viral transmissibility (moderate to very high severity and transmissibility). The parameter values used in these scenarios are likely to change as we obtain additional data about the upper and lower bounds of disease severity and viral transmissibility of COVID-19. For Pandemic Scenario 5: This scenario represents a current best estimate about viral transmission and disease severity in the United States, with the same caveat: that the parameter values will change as more data become available. Scenario 1: Lower-bound values for virus transmissibility and disease severity Lower percentage of transmission prior to onset of symptoms Lower percentage of infections that never have symptoms and lower contribution of those cases to transmission Scenario 2: Lower-bound values for virus transmissibility and disease severity Higher percentage of transmission prior to onset of symptoms Higher percentage of infections that never have symptoms and higher contribution of those cases to transmission Scenario 3: Upper-bound values for virus transmissibility and disease severity Lower percentage of transmission prior to onset of symptoms Lower percentage of infections that never have symptoms and lower contribution of those cases to transmission Scenario 4: Upper-bound values for virus transmissibility and disease severity Higher percentage of transmission prior to onset of symptoms Higher percentage of infections that never have symptoms and higher contribution of those cases to transmission Scenario 5: Parameter values for disease severity, viral transmissibility, and pre-symptomatic and asymptomatic disease transmission that represent the best estimate, based on the latest surveillance data and scientific knowledge.  Parameter values are based on data received by CDC through August 8, 2020. Top of Page Table 1. Parameter Values that vary among the five COVID-19 Pandemic Planning Scenarios. The scenarios are intended to advance public health preparedness and planning.  They are not predictions or estimates of the expected impact of COVID-19.  The parameter values in each scenario will be updated and augmented over time, as we learn more about the epidemiology of COVID-19.  Additional parameter values might be added in the future (e.g., population density, household transmission, and/or race and ethnicity). Parameter values Table 1 Parameter	Scenario 1	Scenario 2	Scenario 3	Scenario 4	Scenario 5: Current Best Estimate R0*	2.0	4.0	2.5 Infection Fatality Ratio†	0-19 years: 0.00002 20-49 years: 0.00007 50-69 years: 0.0025 70+ years: 0.028	0-19 years: 0.0001 20-49 years: 0.0003 50-69 years: 0.010 70+ years: 0.093	0-19 years: 0.00003 20-49 years: 0.0002 50-69 years: 0.005 70+ years: 0.054 Percent of infections that are asymptomatic§	10%	70%	10%	70%	40% Infectiousness of asymptomatic individuals relative to symptomatic¶	25%	100%	25%	100%	75% Percentage of transmission occurring prior to symptom onset**	30%	70%	30%	70%	50% *The best estimate representative of the point estimates of R0 from the following sources: Chinazzi M, Davis JT, Ajelli M, et al. The effect of travel restrictions on the spread of the 2019 novel coronavirus (COVID-19) outbreak. Science. 2020;368(6489):395-400; Imai N., Cori, A., Dorigatti, I., Baguelin, M., Donnelly, C. A., Riley, S., Ferguson, N.M. (2020). Report 3: Transmissibility of 2019-nCoV. Online report Li Q, Guan X, Wu P, et al. Early Transmission Dynamics in Wuhan, China, of Novel Coronavirus-Infected Pneumonia. N Engl J Med. 2020;382(13):1199-1207 Munayco CV, Tariq A, Rothenberg R, et al. Early transmission dynamics of COVID-19 in a southern hemisphere setting: Lima-Peru: February 29th-March 30th, 2020 [published online ahead of print, 2020 May 12]. Infect Dis Model. 2020; 5:338-345 Salje H, Tran Kiem C, Lefrancq N, et al. Estimating the burden of SARS-CoV-2 in France [published online ahead of print, 2020 May 13] [published correction appears in Science. 2020 Jun 26;368(6498):]. Science. 2020;eabc3517. The range of estimates for Scenarios 1-4 represent the upper and lower bound of the widest confidence interval estimates reported in: Li Q, Guan X, Wu P, et al. Early Transmission Dynamics in Wuhan, China, of Novel Coronavirus-Infected Pneumonia. N Engl J Med. 2020;382(13):1199-1207. Substantial uncertainty remains around the R0 estimate. Notably, Sanche S, Lin YT, Xu C, Romero-Severson E, Hengartner N, Ke R. High Contagiousness and Rapid Spread of Severe Acute Respiratory Syndrome Coronavirus 2. Emerg Infect Dis. 2020;26(7):1470-1477 (https://dx.doi.org/10.3201/eid2607.200282external icon) estimated a median R0 value of 5.7 in Wuhan, China. In an analysis of 8 Europe countries and the US, the same group estimated R0 of between 4.0 and 7.1 in the pre-print manuscript: Ke R., Sanche S., Romero-Severson, & E., Hengartner, N. (2020). Fast spread of COVID-19 in Europe and the US suggests the necessity of early, strong and comprehensive interventions. medRxiv. † These estimates are based on age-specific estimates of infection fatality ratios from Hauser, A., Counotte, M.J., Margossian, C.C., Konstantinoudis, G., Low, N., Althaus, C.L. and Riou, J., 2020. Estimation of SARS-CoV-2 mortality during the early stages of an epidemic: a modeling study in Hubei, China, and six regions in Europe. PLoS medicine, 17(7), p.e1003189. Hauser et al. produced estimates of IFR for 10-year age bands from 0 to 80+ year old for 6 regions in Europe. Estimates exclude infection fatality ratios from Hubei, China, because we assumed infection and case ascertainment from the 6 European regions are more likely to reflect ascertainment in the U.S. To obtain the best estimate values, the point estimates of IFR by age were averaged to broader age groups for each of the 6 European regions using weights based on the age distribution of reported cases from COVID-19 Case Surveillance Public Use Data (https://data.cdc.gov/Case-Surveillance/COVID-19-Case-Surveillance-Public-Use-Data/vbim-akqf). The estimates for persons ≥70 years old presented here do not include persons  ≥80 years old as IFR estimates from Hauser et al., assumed that 100% of infections among persons ≥80 years old were reported. The consolidated age estimates were then averaged across the 6 European regions. The lower bound estimate is the lowest, non-zero point estimate across the six regions, while the upp  50b  er bound is the highest point estimate across the six regions.