How CFA Conducts Risk Assessments

Overview

Our assessments are qualitative. This means subject-matter experts make judgments based on available evidence, including epidemiological and clinical data, modeling analyses, and data on past outbreaks. We also consider information from international partners and media reporting.

For each population examined, we estimate overall risk by combining the likelihood of infection and the impact of the disease. The likelihood of infection refers to the probability that individuals in the population acquire infection, which depends on the likelihood of exposure, infectiousness of the pathogen, and susceptibility of the population (see definitions). The impact of infection considers factors such as the severity of disease, level of population immunity, availability and effectiveness of treatments, and necessary public health response resources (see definitions). We also assign confidence levels to each of our assessments, based on the quality and amount of evidence that underpins the assessment, as well as how well different lines of evidence corroborate one another (see definitions).

Our risk assessment methods are adapted from those developed by the World Health Organization and the European Centre for Disease Prevention and Control.

Next Steps

We are collaborating across CDC and with external partners to standardize and improve these methods, which will help facilitate comparisons across disease areas. We are also working with international partners to align on methods, address common challenges, and ensure we can more easily compare risk assessments across countries.

Definitions

Likelihood of infection definitions

• Extremely low: An extremely small number of people are likely to be exposed, the pathogen is not very infectious, or the population is highly immune. We expect an extremely low prevalence of infection in the population, far less than 1% of the population.

• Very low: A very small number of people are likely to be exposed, the pathogen is not very infectious, or the population is highly immune. We expect a very low prevalence of infection in the population.

• Low: There are limited opportunities for exposure for most of the population, but exposure may be high in some areas or subgroups. The pathogen has at least moderate infectiousness or significant gaps in population immunity. We expect a low prevalence of infection in the population, potentially with pockets of higher prevalence.

• Moderate: Many people are likely to be exposed, the pathogen has moderate to high infectiousness, or the population has low levels of immunity. We expect a moderate prevalence of infection in the population.

• High: Most people are likely to be exposed, the pathogen has high infectiousness, or the population has very low immunity. We expect a high prevalence of infection in the population, with most of the population affected.

• Very high: The vast majority of the population is likely to be exposed, the pathogen has very high infectiousness, or the population has extremely low immunity. We expect a very high prevalence of infection in the population, with the vast majority of the population affected.

Impact of infection definitions

• Very low: The pathogen is very unlikely to cause severe disease for this population, there is a very high proportion of population immunity protecting against severe disease, and/or effective treatments are widely available. The disease is very unlikely to cause disruption to normal activities or require additional resources for public health measures.

• Low: The pathogen is unlikely to cause severe disease for this population, there is a very high proportion of population immunity protecting against severe disease, and/or effective treatments are widely available. The disease is unlikely to cause disruption to normal activities or require additional resources for public health measures.

• Moderate: The pathogen causes severe disease for a substantial proportion of this population or pockets within this population, there is limited population immunity protecting people from severe disease, and/or effective treatments are not widely available or accessible. The disease may cause significant disruption to the population and require significant public health resources.

• High: The pathogen typically causes severe disease for this population, there is a very low proportion of population immunity protecting against severe disease, and/or effective treatments are very limited or difficult to access. The disease could cause extensive disruption to normal activities and will potentially require a large amount of public health resources.

• Very high: The pathogen typically causes very severe disease for this population, there is a very low proportion of population immunity protecting against severe disease, and/or there are no effective treatments. The disease could cause prolonged and extensive disruption to normal activities and will potentially require a very high level of public health resources.

Confidence level definitions

We assess confidence based on the extent and quality of information used to make assessments, as well as how well different lines of evidence corroborate one another. We consider peer-reviewed published studies and confirmed reports from reliable government sources as high-quality evidence. Individual case reports and news articles are of lower quality.

• Low confidence: Assessment is based on information that is fragmented, poorly corroborated or upon data sources for which there are significant concerns or problems. There may be several information gaps that require numerous assumptions in order to draw conclusions for the assessment.

• Moderate confidence: Assessment is based on credibly sourced and plausible information, but the information is not of sufficient quality or corroboration to warrant a high level of confidence. The assessment acknowledges some information gaps or assumptions that underlie analysis.

• High confidence: Assessment is based on high-quality information from multiple sources, although such judgments are not a certainty. There are few information gaps and few assumptions are required to draw analytic conclusions.