Are new incidence added to current prevalence

In epidemiology, the relationship between new incidence and current prevalence is a bit more complex than a simple addition. Here's a breakdown:

Incidence: The number of new cases of a disease or condition that occur in a population over a specific period of time (e.g., a year).

Prevalence: The total number of cases of a disease or condition that exist in a population at a given point in time.

When new cases occur, they can affect the prevalence in two ways:

  1. Addition: New cases can increase the prevalence by adding to the existing number of cases. For example, if there are 100 people with a disease and 10 new cases occur, the prevalence would increase to 110 cases.
  2. Replacement: New cases can also replace existing cases that have resolved or been removed from the population. For instance, if someone with the disease recovers, the new case would replace the old one, keeping the overall prevalence constant.

To illustrate this, let's consider an example:

Suppose we have a population of 100 people with a disease, and the incidence rate is 5 new cases per year. Over the course of a year, 5 new cases occur, and 5 existing cases resolve (e.g., people recover or die). The net effect on prevalence would be:

In this scenario, the prevalence would remain the same, at 100 cases.

In summary, new incidence can increase or decrease the prevalence, depending on the balance between the number of new cases and the number of existing cases that resolve or are removed from the population.