Guidance

Toolkit 3: outbreak risk assessment

Published 21 January 2025

This toolkit provides more information regarding approaches to outbreak risk assessment. This includes a risk management model which may be used, adapted, or considered at a local level.

Conduct risk assessments at the beginning of an outbreak. Review them regularly. Use them to inform control strategies. Document your risk assessment approach and outcome. Different organisations use different risk assessment frameworks, therefore the framework you choose should depend on the circumstances and be agreed at the Incident Management Team (IMT).

The risk management model for communicable disease control provided below is based on a model used by UKHSA HPTs. This model considers 5 separate elements:

• severity
• confidence
• spread
• intervention
• context

It may be supplemented by additional consideration of other factors in more detail, such as health equity aspects.

Risk management model for communicable disease control

Severity

The seriousness of the incident in terms of the potential to cause harm to individuals or to the population.

Grade	Qualifier	Description	Examples
0	Very low	Seldom causing severe illness	Head lice
1	Low	Occasional serious illness, rarely with long term effects or death	Hepatitis A in a primary school
2	Moderate	Often severe illness, occasionally with long term effects or death	Shiga toxin-producing  E.coli O157, Pulmonary tuberculosis, Legionnaires’ disease
3	High	Usually severe illness, often with long term effects or death	Meningococcal disease, Diphtheria
4	Very high	Severe illness, almost invariably fatal	Rabies, Ebola virus disease

Uncertainty

The level of uncertainty that the outbreak hypothesis is correct, based on epidemiological, clinical, statistical and laboratory evidence.

Grade	Qualifier	Probability that the hypothesis is correct, according to available evidence
0	Very low	Over 85%
1	Low	50 to 84%
2	Moderate	25% to 49%
3	High	10% to 24%
4	Very high	9% or less

Spread

The likelihood of the infection spreading. This includes an assessment of the infective dose, virulence of the organism, modes and routes of transmission, the observed spread, and the susceptibility of the population (for example lack of immunity).

Grade	Qualifier	Description	Examples
0	Very low	Very low likelihood of spread with very few new cases	A single case of campylobacter
1	Low	Low likelihood of spread with few new cases	A single case of meningococcal disease, a smear negative culture positive case of Tuberculosis
2	Moderate	Moderate likelihood of spread with new cases. May develop into a limited outbreak	A small number of cases of hepatitis A occurring over a prolonged period of time in a large community, a smear positive case of Tuberculosis
3	High	High likelihood of spread with many new cases. May develop into a large outbreak	Viral gastro-enteritis in a nursing home
4	Very high	Spread is almost inevitable	Measles in a non-immune sub-population

Intervention

The feasibility and impact of intervening to alter the course of an outbreak/incident. This includes feasibility of delivering appropriate interventions, taking into consideration how simple, effective, equitable, available, affordable, acceptable, and accessible they are.

Grade	Qualifier	Description	Examples
0	Very easy	Intervention well established with clear benefits and no anticipated difficulties	Hand washing advice
1	Easy	Intervention with clear beneficial effects and few difficulties to implement	Withdrawal of contaminated food in a closed institution, Hepatitis A immunisation for a small group of vulnerable contacts of a case
2	Moderate	Intervention with some beneficial effects but some difficulties to implement	Prophylaxis for immediate family and close contacts of a meningococcal case where they are dispersed, a national food withdrawal or recall
3	Difficult	Some remedial intervention possible but either difficult to implement, relatively ineffectual, or other significant problems	Urgent mass immunisation campaign
4	Very difficult	Remedial intervention very difficult	Response to a cluster of vCJD

Context

The level of public interest and complexity regarding aspects of the broader environment in which events are occurring that may influence decisions about response.

Give consideration to:

• the level of media and public interest in an outbreak and the implications this might have on transmission, harm, and the impact of control measures
• previous local or community experience of outbreaks, and the implications this might have on management, including consideration of equity aspects of outbreak management
• lessons learned from the management of similar outbreaks
• the extent to which other similar incidents are being managed and publicised and the impact this may have on public attitudes and expectations
• the impact of existing or wider inequalities, including the distribution of vulnerability to harm and patterns of access to healthcare, or safeguarding concerns

Grade	Qualifier	Description	Examples
0	Very low	No raised level of interest	Common adverse problems are fairly well understood
1	Low	A small degree of increased interest with a low level of conflicting factors. Little public concern	Misunderstanding corrected by routine information. A few cases of diarrhoea in a nursery school
2	Moderate	A degree of unease and anxiety amongst local community.	A series of gastro-enteritis cases associated with an outdoor centre accessed by children. Tuberculosis in a school in a low incidence area
3	High	Particular sensitives identified in relation to the outbreak.	Widespread food poisoning affecting several schools
4	Very high	Significantly raised public interest and concern	Outbreak caused by novel infection disproportionately impacting children

Summary table

The below table provides a summary table of the outbreak risk assessment and a template for the recording of this assessment at each IMT.

Date	Severity	Uncertainty	Spread	Intervention	Context
Insert date of IMT	Insert risk rating	Insert risk rating	Insert risk rating	Insert risk rating	Insert risk rating