The Centers for Disease Control and Prevention (CDC) and the Office of the Assistant Secretary for Preparedness and Response (ASPR) have developed five COVID-19 Pandemic Planning Scenarios that are designed to help inform decisions by modelers and public health officials who utilize mathematical modeling.
The planning scenarios are being used by mathematical modelers throughout the federal government. Models developed using the data provided in the planning scenarios can help evaluate the potential effects of different community mitigation strategies (e.g., social distancing). The planning scenarios may also be useful to hospital administrators in assessing resource needs and can be used in conjunction with the COVID-19 Surge Tool.
Each scenario is based on a set of numerical values for biological and epidemiological characteristics of COVID-19. These values—called parameter values—can be used to estimate the possible effects of COVID-19 in U.S. states and localities. The parameter values in each scenario will be updated and augmented over time, as we learn more about the epidemiology of COVID-19.
New data on COVID-19 is available daily; information about its biological and epidemiological characteristics remain limited, and uncertainty remains around nearly all parameter values.
The parameters in the scenarios:
· Are estimates intended to support public health preparedness and planning.
· Are not predictions of the expected effects of COVID-19.
· Do not reflect the impact of any behavioral changes, social distancing, or other interventions.
The five COVID-19 Pandemic Planning Scenarios represent a range of possible parameters for COVID-19 in the United States. All parameter values are based on current COVID-19 surveillance data and scientific knowledge.
Scenarios 1 through 4 are based on parameter values that represent the lower and upper bounds of disease severity and viral transmissibility (moderate to very high). 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 the transmissibility of SARS-CoV-2, the virus that causes COVID-19. Scenario 5 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.
The Parameter Values Definitions
· Parameter values that vary across the five COVID-19 Pandemic Planning Scenarios include measures of viral transmissibility, disease severity, and pre-symptomatic and asymptomatic disease transmission. Where sufficient data are available, age-stratified estimates are provided.
Viral Transmissibility
· Basic reproduction number (R0): The average number of people that one person with COVID-19 is likely to infect in a population without any immunity (from previous infection) or any interventions. R0 is an estimate of how transmissible a pathogen is in a population.
Disease Severity
· Symptomatic Case Fatality Ratio: The number of symptomatic individuals who die of the disease among all individuals experiencing symptoms from the infection. This parameter is not necessarily equivalent to the number of reported deaths per reported cases, because many cases and deaths are never confirmed to be COVID-19, and there is a lag in time between when people are infected and when they die. This parameter reflects the existing standard of care and may be affected by the introduction of new therapeutics.
· Symptomatic Case Hospitalization Ratio: The number of symptomatic individuals who are hospitalized from the disease among all individuals experiencing symptoms from the infection. This parameter is not necessarily equivalent to the number of reported hospitalizations per reported cases, because many cases and deaths are never confirmed to be COVID-19, and there is a lag in time between when people are infected and when they die. This parameter reflects the existing standard of care and may be affected by the introduction of new therapeutics.
Pre-symptomatic and Asymptomatic Contribution to Disease Transmission
· A pre-symptomatic case of COVID-19 is an individual infected with SARS-CoV-2 who has not exhibited symptoms at the time of testing, but who later exhibits symptoms during the course of the infection. An asymptomatic case is an individual infected with SARS-CoV-2 who does not exhibit symptoms during the course of infection. Parameter values that measure the pre-symptomatic and asymptomatic contribution to disease transmission include:
· Percentage of infections that are asymptomatic: The percentage of persons who are infected with SARS-CoV-2 but never show symptoms of disease. Asymptomatic cases are challenging to identify because individuals do not know they are infected unless they are tested, typically as a part of a scientific study.
· Infectiousness of asymptomatic individuals relative to symptomatic individuals: The contribution to transmission of SARS-CoV-2 from asymptomatic individuals compared to the contribution to transmission of SARS-CoV-2 from symptomatic individuals. A parameter value of 50% means that an asymptomatic individual is half as infectious as a symptomatic individual, while a parameter value of 100% means that an asymptomatic individual is just as likely to transmit infection as a symptomatic individual. This parameter is especially challenging to estimate because studies that repeatedly test asymptomatic individuals over time are limited.
Parameter values that do not vary across the five Pandemic Planning Scenarios include:
· Percentage of transmission occurring prior to symptom onset: The percentage of new cases of COVID-19 due to transmission from a person with COVID-19 who transmits infection to others before exhibiting symptoms.
· Mean number of days from symptom onset to seeking outpatient care
· Mean number of days from symptom onset to hospitalization
· Mean number of days of hospitalization
· Percentage of patients admitted to the ICU among those hospitalized
· Percentage of patients on mechanical ventilation among admitted to the ICU
· Mean number of days on mechanical ventilation
· Median number of days from symptom onset to death
These healthcare-related parameters are included to assist assessment of resource needs as the pandemic progresses.
For each Pandemic Planning Scenario:
· Parameters values for viral transmissibility include the Basic Reproduction Number (R0)
· Parameters values for disease severity include Symptomatic Case Fatality Ratio and Symptomatic Case Hospitalization Ratio
· Parameter values for the pre-symptomatic and asymptomatic contribution to disease transmission include:
· 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). 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 prior to 4/29/2020.
