Incidence is the rate of newly diagnosed cases of a disease. It is different from prevalence, which is the total number of cases of a disease existing in a population. If you imagine a bathtub being filled with water, incidence is the amount of water going into the tub, and prevalence is the amount of water in the tub at any given moment. Water exiting the tub through the drain are reductions in prevalence due to death or recovery from the condition.
The relationship between incidence and prevalence depends on the contagiousness of the disease and the ability to treat it and prevent further spread. There can be a high number of diagnosed cases of a disease, but low prevalence because the disease is treated quickly. With a disease with a low cure rate, but maintenance treatment permits sustained survival, then incidence contributes to a continuous growth of prevalence.
Incidence may be a measure of how well surveillance and prevention measures for a disease are working while prevalence may be an indication of the effectiveness of treatment methods.
Deeper dive
Many of the infectious disease studies journalists cover are epidemiological observational studies that include reports on the incidence and prevalence of diseases. It’s important to 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 infections or chronic conditions, this section focuses on infectious diseases.
Whether to use 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 population burden of a disease, including chronic conditions that result from an infectious disease, such as long COVID, or latent conditions, such as latent TB, prevalence is more relevant because it focuses on how many total people have the condition, whether diagnosed yesterday or a decade ago.
As an example, 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 experiencing 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. Keep in mind that the denominator (bottom number) only includes people who can actually contract the condition. If looking at incidence of cervical cancer, for example, the denominator would only include individuals who have a cervix — typically the natal female population of a population — since natal males cannot develop cervical cancer.
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 may be less familiar to journalists even though it’s reported 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.