Begonya Nafria Escalera
Begonya Nafria
Share4Rare Coordinator

Prevalence, comorbidity and disease incidence

People in the street
Measuring the frequency of a disease is essential to study its impact on society. Epidemiology is the discipline that studies this matter, and it has certainly gained visibility during the last two years due to the COVID-19 pandemic.

What is the prevalence of a disease?

In epidemiology, prevalence is the proportion of individuals in a group or population that presents a specific characteristic or event at a given time or period. There are three types of prevalence:

  • Point-prevalence: refers to the number of cases of a health event at a particular point in time. For example, if we want to study to tobacco consumption collecting data through a survey, the question that would correspond to collect this value would be “Do you currently smoke?
  • Period prevalence: the number of cases of a health event over a period of time, usually 12 months. Following the previous example, in a survey the question would be “Have smoked in the last 12 months?
  • Lifetime prevalence: the number of people that at some point in their life have experienced the health event. For example, in a survey, you would ask, “Have you ever smoked?

The prevalence of a disease quantifies the proportion of people in a population who have a disease (or any other event) at a given time and provides an estimate of the proportion of individuals in that population who have the disease at that time. It is a useful parameter because we can use it to describe a health-related phenomenon, identifying how frequently it appears on the population and generating explanatory hypotheses. It is commonly used by epidemiologists, health policymakers, insurance agencies, and in many public health settings.

In the case of rare diseases, Orphanet is the source of reference that provides data related to the prevalence of these pathologies. It has a search engine where you can look for the pathology, the affected gene, the ICD-10 code, the OMIM code or the Orphanet number.

What does comorbidity mean?

Comorbidity is the presence of one or more additional conditions co-existing with a primary condition. It is also called associated morbidity. There is no accepted method to measure this value, that is, a standard formula to calculate this value unequivocallyOne of the most used formulas is the Charlson Comorbidity Index.

Asthma comorbidities
Asthma comorbidities (Wikimedia)

Incidence of a disease

Prevalence refers to existing cases, while incidence considers new cases of a disease in a given population and in a given time period.

In the case of rare diseases, knowing their prevalence does not guarantee that we have the exact information about the number of people affected by the disease in a given country and in a specific period of time. Prevalence is an approximate indicator for a possible number of cases, but it is not reliable when it comes to predicting the number of people affected. This is because this indicator is normally calculated at a European level and is not usually updated on a regular basis. This is why it is so important to build patient registries, which are updated databases that contribute to analyse the incidence of a given disease in a particular country.

Likewise, we must consider that there are pathologies with a pan-ethnic component, with a greater presence in the case of inheritance linked to consanguinity or with a clear determination linked to epigenetics, etc. This means that both the prevalence and the incidence of a disease in different countries or regions will not be the same.