Infection Rate: Explained

The Infection Rate, or “R(t),” is the number of people one infected person goes on to infect in a specific area, over a specific time. The areas we look at are county and state. The period of time we look at is while a person is contagious (able to spread COVID).

In the picture to the right, R(t) is 3.

If the R(t) is 3, one person will most likely infect three other people, and those three people will each go on to infect three more people and so forth.

R(t) is one of the most important metrics Covid Act Now tracks because it tells us how fast COVID is spreading. R(t), in combination with daily new cases, tells us about how many people are spreading COVID and at what rate. R(t) also indicates risks associated with ICU headroom used.

Here is a video that helps explain infection rate:

R(t) can change based on factors like community behavior (whether there are large gatherings) and their intervention practices (whether people wear masks and maintain social distancing.) 

How Does Covid Act Now rate R(t)?

We use 0.9 as the cutoff for a green score because, at 0.9, the number of infected people significantly declines. This is because each infected person is spreading COVID to less than one other person. 

Shown in March 31st as 1.44, after which it dips and then rises again to 1.29 in June 7th

When looking at R(t), it is important to note both value and direction. In the graph above, on March 31, the R(t) value was critical, but decreasing. On June 7, the R(t) value was high and increasing.

**The dotted line on the right represents values that have yet to be finalized.

There are significant delays between when people change their behavior and when that behavior change is reflected in R(t). When people are infected, it takes time to develop symptoms, get tested, and receive results. Because of these delays, interventions like social distancing or mask orders will take at least a few days, if not weeks, to show a decrease in R(t).

Similarly, by the time you see cases rise in your county or state, COVID has likely been spreading for days or weeks. The lower test positivity is, the more accurate R(t) measurements are because the state or county is missing fewer positive cases.

Where does our data come from for this metric?

To calculate R(t), we use new positive cases and COVID death data from The New York Times.

Infection rate is just one of several important metrics to determine how well your state or county is doing in the fight against COVID. Learn more about daily new cases, test positivity, ICU headroom used, and contact tracing. For a description of assumptions and methodology, please see our references and assumptions document, along with our data sources presentation.

Check out our Youtube Channel for our educational videos.

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