Recently, discussion around the adequacy and accuracy of oft-used prevalence figures of ME/CFS has increased. One reason for this is the emergence of long COVID – and the similarities in symptoms between some long COVID cases and ME/CFS. A recent review suggested that approximately 51% of people with long COVID also meet the diagnostic criteria for ME/CFS – studies included in the review used one or more of the following criteria: the Canadian Consensus Criteria, International Consensus Criteria, Institute of Medicine criteria or the DePaul Symptom Questionnaire.
Another reason for heightened interest in the prevalence of ME/CFS has been the release of a pre-print paper (an article that has not been through peer-review) by Prof. Chris Ponting and Gemma Samms at the University of Edinburgh. In this, the authors estimate the number of people in England with the diagnostic code most closely relating to ME/CFS – the code for “post-viral fatigue syndrome” – in their hospital medical records. Based on this finding, the researchers also estimate the number of people in the UK who may have ME/CFS.
This pre-print paper – and consideration of the methods used by the authors – has sparked questions relating to existing prevalence estimates for ME/CFS and how they have been calculated. Importantly, it has also raised queries regarding how well existing ME/CFS prevalence estimates might represent the proportion of the population living with the disease today – specifically in the UK, but also elsewhere in the world.
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) Why existing estimates for the prevalence of ME/CFS may no longer apply to the UK population today.
- 3) A summary, and recommendations for the next steps.
