Prevalence is the total number of cases of a disease existing in a population. It is different from incidence, which is the rate of newly diagnosed cases of a disease. If you imagine a bathtub filling 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.
Prevalence can 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.