What is prevalence?
Prevalence is the percentage (or proportion) of the population that has a certain characteristic or is affected by a particular disease, such as ME/CFS.
Prevalence is calculated for a given place for either a specific time (point prevalence), or for a specific time-period (period prevalence), by considering the following:
- The number of new cases of disease (incidence).
- The number of people who recover from the disease – or no longer meet the criteria for a diagnosis.
- The number of people who relapse, or have a reoccurrence of the disease.
- The number of people who leave the specific area of interest (migration).
- The number of people who die with the disease (death may not always be due to the disease of interest but the person is no longer considered a “case”).
Epidemiologists – researchers who study disease in populations – often use the “bathtub model” to explain prevalence – this also applies to ME/CFS.
Why is prevalence important?
Prevalence is calculated by research studies that investigate disease in populations – epidemiological studies – and is useful for several reasons:
1. It provides an estimate of the proportion of people who have a disease in a population at a given time (or over a specified time period)
This indicates the number of people with a disease and suggests the proportion of the population who may require one or more of the following specifically relating to the disease of interest:
- Health care,
- Social care,
- Benefits,
- Employment support,
- Legislative protections.
2. It can also be calculated for different groups within a population.
For example, prevalence can be calculated by age, sex, for those living in different areas of residence, and by ethnic group. Prevalence can also be calculated for different disease characteristics such as severity, and duration of illness.
Looking at prevalence by characteristic can indicate groups who may be more at risk of a certain disease (or of a certain disease characteristic). It can also highlight groups who may not be accessing care and support as expected.
Where the prevalence rate of a disease is high and access to, for example, health care is low, groups who are not receiving the care they need can be identified, and interventions developed to improve access to care specifically for these groups.
3. It can be used – alongside the impact of a specific disease on an individual and wider society – when:
- Allocating health care resources such as screening tests and rehabilitation programs.
- Justifying the importance of research.
- Applying for/allocating research funding.
- To help raise awareness of the impact of a disease – such as ME/CFS – and to try to ensure it is taken seriously by others; for example, by the general public, and within health and research settings.
Regrettably, as is the case for ME/CFS, funding and resources are not always allocated in a way that reflects the prevalence and impact of a disease.
Given the importance of prevalence, in this series of articles ME Research UK will examine the following:
- 1) What the existing research says about the prevalence of ME/CFS, alongside the research’s strengths and limitations.
- 2) The limitations and difficulties in arriving at a prevalence rate for ME/CFS in the UK today.
- 3) A summary, and recommendations for the next steps.
