What is Incidence?

What is incidence? 

Incidence is a term in epidemiology that measures confirmed new COVID cases per day. You might see this in our 5 key indicators for risk as “Daily New Cases.” We chose to use this term so that expert knowledge is not needed to understand the metric. To ensure incidence can be compared across geographies, we calculate it as a proportion of the population — specifically, daily new COVID cases for every 100,000 people. Adding a daily new cases or incidence, metric depicts risk more accurately, since it takes the overall number of cases into account. 

We calculate incidence as follows:

Daily New Cases        New Daily Cases (average of last 7 days)
100k Population   =   ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯
                               Population / 100,000

Why is incidence important?

Incidence measures how many new COVID cases there are per day per unit of population. It answers the question: “how many new COVID infections are in my area?” If the infection rate is the acceleration, incidence is the velocity. Infection rate reflects how quickly incidence is increasing or decreasing.

Incidence is important because it provides a more complete picture of the state of COVID in a given community. For instance, a community recovering from a major outbreak may have driven their infection growth rate (also known as R(t)) down to 0.5, but still a very high incidence of 50 daily new cases per 100 thousand population. On the other hand, a community heading towards an outbreak situation may have a very high infection growth rate (for example, 2), but low incidence (for example, 2). 

How do we grade incidence?

As with our previous four metrics — infection growth rate, test positivity rate, ICU capacity, and contact tracing — we separate case incidence into four categories: critical, high, medium, and low. To be classified as critical state or county must have over 25 daily new cases per 100,000 people. To be classified as low, a state or county must have less than one new case per 100,000 people.

Color Code Covid Risk Level Incidence
Daily new cases per 100,000 people

Critical >25

High 10<25

Medium 1<10

Low <1

What is the difference between incidence and prevalence?

Infection prevalence is another term you may have read in the news or online. It is not the same as case incidence. 

  1. Incidence refers to the number of confirmed new COVID cases within a given time, typically one day. Imagine if on day one we have three people test positive, and on day two we have two more people test positive. The “incidence” on day one would be 3, and the incidence on day two it would be 2. Incidence is reported by most states and counties, but it does not account for new infections that are not caught by testing. It also does not include the duration of each infection (how long each infected person is exhibiting symptoms and/or contagious).
  2. Prevalence refers to the actual disease prevalence (total number of active infections) at any given time. This data is what we’d really like to know, because it indicates the actual risk of encountering an infected individual in a community.  But to know the true prevalence, you would need to test the entire population (or a widespread randomized sample) every day, using a highly-accurate test. Just counting confirmed cases isn’t enough. Per our earlier example, let’s say that on day one, we have three new confirmed cases, and on day two, we have two more confirmed cases. Those numbers suggest a total of five active infections, but perhaps there were an additional five new infections each day that weren’t caught via testing. (Some cases are asymptomatic and some symptomatic people never get tested.) That puts us at 15 active infections. Plus, over the past couple weeks, there may have been an additional 5+ infections per day, and most of those people may still be sick (indicating another 70 active infections). So the actual disease prevalence (number of infected people) may be 85+.

The bottom line is that prevalence paints a more complete picture of COVID in a community, but it can only be estimated based on the current data available (incidence).

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