Prevalence is one of two key epidemiological terms that refers to the total existing cases of a disease, injury, or condition at a given time. It is distinct from the other key term, incidence, which refers to how many new cases of a disease, injury, or other condition occur in a specified period of time.
Deeper Dive: Incidence and prevalence
Many of the medical studies journalists cover are epidemiological, which are observational studies focusing on the health of populations. These studies tend to report on the incidence and prevalence of diseases and other conditions, so it’s important that journalists understand the difference between these two commonly confused terms in epidemiology.
In the plainest terms, “incidence” refers to new cases of a disease or injury or condition. “prevalence” refers to the total existing cases of a disease or injury or condition — whether newly occurring or ongoing from a previous diagnosis or occurrence. Although these terms can refer to any condition studied, such as gunshot wounds, short-term infectious disease or chronic conditions, this section will primarily focus on diseases for the sake of simplicity.
Whether a study, or a journalist, uses incidence or prevalence depends on what’s being communicated. For example, to have a sense of how quickly a disease is spreading through a population, incidence is more relevant because it describes new cases.
But to understand the burden of a disease, especially a chronic condition, in a population, prevalence is more relevant because it focuses on how many people are suffering, regardless of whether they were diagnosed yesterday or ten years ago.
These concepts involve more complexity, but first, here is a visual analogy to make sense of the difference: imagine a bathtub that has the faucet turned on and the drain open. The water pouring into the bathtub is the incidence — the new cases getting diagnosed. The water that is in the bathtub is the prevalence — how many currently have the condition. The water exiting the tub through the drain are the people leaving the prevalence either because they died from the condition or because they recovered from it.
Two more examples: If 500 people are diagnosed with diabetes each year, that refers to incidence, but if 15 million people are currently living with diabetes, that refers to prevalence. If 6 million people caught the flu in the first week of February, that’s the incidence, but if only 4 million people are currently suffering from symptoms of the flu on February 7, that refers to the prevalence of influenza; the other 2 million recovered or died from the flu during that week.
Incidence is typically described in one of two ways: incidence proportion or incidence rate. The incidence proportion is also called cumulative incidence, attack rate, or risk of a condition — the probability of developing it. Incidence proportion is expressed as a ratio where the numerator (top number) is the total number of new cases of a condition during a specified time interval, and the denominator (bottom number) is the population of people who are at risk for the condition.
For example, the incidence proportion of HIV in a particular country might be 25 people per 100,000 individuals per year. Similarly, the incidence of cervical cancer in the same country might be 10 per 50,000 women. Even though it’s the same population, the denominator must reflect the population that is at risk. Both males and females can get HIV, but only females can get cervical cancer, so the denominator can only include women in the second example. (The second example would probably actually be expressed as 5/100,000, but it’s important to know that the denominator still only contains women and that the HIV rate and the cervical cancer rates given here cannot be directly compared since the denominators refer to different populations within the same country.)
Even incidence proportion can be described in different ways. For example, the overall incidence proportion, or attack rate, of a listeriosis outbreak refers to the total number of individuals getting newly diagnosed with the food-borne illness out of the total population. But if the source of the outbreak is determined to be cantaloupe, the food-specific attack rate refers to the number of new cases of illness among people who ate that food. (It can get even more specific if the denominator is limited to the people who ate the cantaloupe from the farm where the outbreak originated.)
The incidence rate is less familiar to journalists even though they will come across it in studies; it refers to the number of newly diagnosed cases in the population over a set amount of time. It’s often expressed in “person-years,” which incorporates time into the denominator. In writing about this type of incidence in layperson terms, one way to express it is to do a quick division and use “cases per year” (or whatever the unit of time is, usually days or years). For example, if the rate of norovirus in Pleasantville over a 10-year period is 25,000 cases per 1 million person-years, then that actually means the population is approximately 100,000 people (100,000 people times 10 years is 1 million person-years), and 2,500 people a year got sick. (If 25,000 cases occur over that time, the annual rate is estimated by dividing by 10.) The reason researchers might express a condition in person-years instead of annual rate is that the population might change over that time and person-years is more precise and accurate for researchers. Usually, for a journalist’s purposes, that level of precision is not necessary, and the estimate of 2,500 cases per year is sufficient.)
Prevalence can also be discussed in two different ways: point prevalence and period prevalence. Just as it sounds, point prevalence refers to the number of people with a certain condition at a precise moment in time, such as a day or “right now” throughout the U.S. The numerator is the number of current cases, and the denominator is the total current population. The estimated point prevalence of HIV in the U.S. is 1.2 million people. Meanwhile, period prevalence refers to the number of current cases over a period of time, such as over a year. The point prevalence of the flu in February may be 300,000 cases, but the period prevalence of the flu over the entire year might be 9 million (which includes the 300,000 cases in February). Whether this refers to the cases in a nation, a state, a county, a city, a school or some other group depends on the study and the needs of the journalist’s story.
For more nitty gritty specifics, check out this lesson on the CDC website on Morbidity Frequency Measures.