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||Scenario 1||Scenario 2||Scenario 3||Scenario 4||Scenario 5:
Current Best Estimate
|Infection Fatality Ratio, Overall†||0.005||0.005||0.008||0.008||0.0065|
|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**||35%||70%||35%||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; and 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 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 Europe 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.
† Meyerowitz-Katz, G., & Merone, L. (2020). A systematic review and meta-analysis of published research data on COVID-19 infection-fatality rates. medRxiv.
§ The percent of cases that are asymptomatic, i.e. never experience symptoms, remains uncertain. Longitudinal testing of individuals over time is required to accurately indicate the absence of symptoms for the full period of infectiousness. Current peer-reviewed and preprint studies vary widely in follow-up times for re-testing, or do not include re-testing of cases. Additionally, studies vary in the definition of a symptomatic case, which makes it difficult to make direct comparisons between estimates. Furthermore, the percent of cases that are asymptomatic may vary by age and studies also vary in the age groups included. Given these limitations, the range of estimates for Scenarios 1-4 is wide. The lower bound estimate approximates the lower 95% confidence interval bound estimated from: Byambasuren, O., Cardona, M., Bell, K., Clark, J., McLaws, M. L., & Glasziou, P. (2020). Estimating the extent of true asymptomatic COVID-19 and its potential for community transmission: systematic review and meta-analysis. Available at SSRN 3586675. The upper bound estimate approximates the upper 95% confidence interval bound estimated from: Poletti, P., Tirani, M., Cereda, D., Trentini, F., Guzzetta, G., Sabatino, G., Marziano, V., Castrofino, A., Grosso, F., Del Castillo, G. and Piccarreta, R. (2020). Probability of symptoms and critical disease after SARS-CoV-2 infection. arXiv preprint arXiv:2006.08471. The best estimate is the midpoint of this range and aligns with estimates from: Oran DP, Topol EJ. Prevalence of Asymptomatic SARS-CoV-2 Infection: A Narrative Review [published online ahead of print, 2020 Jun 3]. Ann Intern Med. 2020; M20-3012.
¶ The relative infectiousness of asymptomatic cases to symptomatic cases remains highly uncertain as asymptomatic cases are difficult to identify and transmission is difficult to observe and quantify. The estimates for relative infectiousness are assumptions based on studies of viral shedding dynamics. The upper bound of this estimate reflects studies that have shown similar durations of viral shedding between symptomatic and asymptomatic cases: Zou L, Ruan F, Huang M, et al. SARS-CoV-2 Viral Load in Upper Respiratory Specimens of Infected Patients. N Engl J Med. 2020;382(12):1177-1179; and Zhou R, Li F, Chen F, et al. Viral dynamics in asymptomatic patients with COVID-19 [published online ahead of print, 2020 May 11]. Int J Infect Dis. 2020; 96:288-290. The lower bound of this estimate reflects data indicating that viral loads are higher in severe cases relative to mild cases (Liu Y, Yan LM, Wan L, et al. Viral dynamics in mild and severe cases of COVID-19. Lancet Infect Dis. 2020;20(6):656-657) and data showing that viral loads and shedding durations are higher among symptomatic cases relative to asymptomatic cases (Noh JY, Yoon JG, Seong H, et al. Asymptomatic infection and atypical manifestations of COVID-19: Comparison of viral shedding duration [published online ahead of print, 2020 May 21]. J Infect. 2020; S0163-4453(20)30310-8). The current best estimate is an assumption.
** The lower bound of this parameter is approximated from the lower 95% confidence interval bound from: Casey, M., Griffin, J., McAloon, C.G., Byrne, A.W., Madden, J.M., McEvoy, D., Collins, A.B., Hunt, K., Barber, A., Butler, F. and Lane, E.A. (2020). Estimating pre-symptomatic transmission of COVID-19: a secondary analysis using published data. medRxiv. The upper bound of this parameter is approximated from the upper 95% confidence interval bound from: He, X., Lau, E.H., Wu, P., Deng, X., Wang, J., Hao, X., Lau, Y.C., Wong, J.Y., Guan, Y., Tan, X. and Mo, X. (2020). Temporal dynamics in viral shedding and transmissibility of COVID-19. Nature medicine, 26(5), pp.672-675.The best estimate is the geometric mean of the point estimates from these two studies.
Table 2. Parameter Values Common to the Five COVID-19 Pandemic Planning Scenarios. The parameter values are likely to change as we obtain additional data about disease severity and viral transmissibility of COVID-19.
Parameter values are based on data received by CDC through June 29, 2020, including COVID-19 Case Surveillance Public Use Data (https://data.cdc.gov/Case-Surveillance/COVID-19-Case-Surveillance-Public-Use-Data/vbim-akqf); data from the Hospitalization Surveillance Network (COVID-NET) (through June 27); and data from Data Collation and Integration for Public Health Event Response (DCIPHER).
Assumption, ASPR and CDC
|No pre-existing immunity before the pandemic began in 2019. It is assumed that all members of the U.S. population were susceptible to infection prior to the pandemic.|
|Time from exposure to symptom onset*||~6 days (mean)|
|Time between symptom onset in an individual and symptom onset of a second person infected by that individual†||~6 days (mean)|
|Parameter Values Related to Healthcare Usage|
|Number of days from symptom onset to seeking outpatient care, percentage of patients§||
≤2 days: 35%
3–7 days: 47%
≥8 days: 18%
|Median number of days from symptom onset to hospitalization (interquartile range)¶||
18-49 years: 6 (3, 9) days
50-64 years: 6 (2, 9) days
≥65 years: 3 (0, 7) days
|Median number of days of hospitalization among those not admitted to ICU (interquartile range)**||
18-49 years: 3 (2, 5) days
50-64 years: 4 (2, 7) days
≥65 years: 6 (3, 10) days
|Median number of days of hospitalization among those admitted to ICU (interquartile range)**, §§||
18-49 years: 10 (6, 17) days
50-64 years: 14 (8, 22) days
≥65 years: 12 (6, 19) days
|Percent admitted to ICU among those hospitalized**||
18-49 years: 23.6%
50-64 years: 36.2%
≥65 years: 35.1%
|Percent on mechanical ventilation among those hospitalized. Includes both non-ICU and ICU admissions**||
18-49 years: 11.7%
50-64 years: 21.8%
≥65 years: 21.3%
|Percent that die among those hospitalized. Includes both non-ICU and ICU admissions**||
18-49 years: 2.0%
50-64 years: 9.8%
≥65 years: 28.1%
|Median number of days of mechanical ventilation (interquartile range)¶||
Overall: 6 (2, 12) days
|Median number of days from symptom onset to death (interquartile range)¶||
18-49 years: 15 (9, 23) days
50-64 years: 15 (9, 25) days
≥65 years: 12 (7, 19) days
|Median number of days from death to reporting (interquartile range)††||
18-49 years: 7 (3, 18) days
50-64 years: 7 (2, 19) days
≥65 years: 6 (2, 18) days
*McAloon, C.G., Collins, A., Hunt, K., Barber, A., Byrne, A., Butler, F., Casey, M., Griffin, J.M., Lane, E., McEvoy, D. and Wall, P. (2020). The incubation period of COVID-19: A rapid systematic review and meta-analysis of observational research. medRxiv.
† He, X., Lau, E.H., Wu, P., Deng, X., Wang, J., Hao, X., Lau, Y.C., Wong, J.Y., Guan, Y., Tan, X. and Mo, X. (2020). Temporal dynamics in viral shedding and transmissibility of COVID-19. Nature medicine, 26(5), pp.672-675.
§ Biggerstaff, M., Jhung, M. A., Reed, C., Fry, A. M., Balluz, L., & Finelli, L. (2014). Influenza-like illness, the time to seek healthcare, and influenza antiviral receipt during the 2010–2011 influenza season—United States. The Journal of infectious diseases, 210(4), 535-544.
¶ Estimates only include symptom onset dates between March 1, 2020 – May 15, 2020 to ensure cases have had sufficient time to observe the outcome (hospital discharge or death). Data for 17 year olds and under are suppressed due to small sample sizes.
§§ Cumulative length of stay for persons admitted to the ICU, inclusive of both ICU and non-ICU days.
†† Estimates only include death dates between March 1, 2020 – May 15, 2020 to ensure sufficient time for reporting. Data for 17 year olds and under are suppressed due to small sample sizes.