15 Oct 2014

A measure of the frequency with which a disease occurs in a population over a specified time period. “Incidence rate” or “incidence” is numerically defined as the number of new cases of a disease within a time period, as a proportion of the number of people at risk for the disease. The incidence rate is typically expressed as the number of cases per person-year of observation. Only new cases are considered when computing the incidence rate, while cases that were diagnosed earlier are excluded. The “population at risk” measure is usually obtained from census data.

Several insights can be gleaned from the incidence rate. Apart from providing information on the number of new cases of a disease that have occurred within a community, it also gives a snapshot on changes in the disease’s progression within a population over time, and thus is very important metric for tracking chronic infectious diseases such as tuberculosis and malaria. The incidence rate also enables comparisons to be made of the incidence of a disease across vastly differing populations.

For example, a county in the U.S. with a population of 500,000 may have had 20 new cases of tuberculosis (TB) in 2013, for an incidence rate of 4 cases per 100,000 persons. This is higher than the incidence rate of TB for the entire United States, which recorded 9,852 new TB cases in 2013, for an incidence rate of 3 cases per 100,000 persons.

As an example of determining trends using incidence rates, consider a study on lung-cancer rates released in January 2014 by the Centers for Disease Control and Prevention (CDC). The study found that thanks to tobacco control efforts, lung cancer incidence rates from 2005 to 2009 declined by 2.6 per year among men, from 87 to 78 cases per 100,000 mean; the lung cancer incidence rate fell 1.1 per year among women, from 57 to 54 cases per 100,000 women.

Incidence should not be confused with “prevalence,” which measures the number of cases of a condition or illness in a population at a certain point in time. Thus, while incidence enables an assessment to be made of the risk of contracting a disease, prevalence shows whether the disease is widespread or not.

Covid-19 – Johns Hopkins University

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