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COVID-19's Effective Reproduction Number (Rt)


Author: Laurie Mellor | Thermal Proximity Systems


Rt, is a key measure of how fast the virus is growing. Basically, it’s the average number of people who become infected by someone who is infectious.

Essentially, if Rt is above one (1), the virus will spread quickly. Example if Rt is two (2), on average two (2) infected people will infect four (4) others, who will in turn infect eight (8) more people, and on and on it will go.

If Rt is below one (1), the virus will stop spreading. It is important to understand three (3) things:

1. Rt is a lagging indicator…

2. Using Rt alone without other supporting data can lead to false conclusions…

3. Rt does not account for things such as Super spreaders…


Rt is particularly useful to obtain a glimpse of how things are trending if you happen to be a pedestrian like me who is simply trying to understand how things are generally trending.

However, it is important to understand that many other factors need to be weighed to come to firm conclusions. Things such as: >> New infections recorded

>> Deaths

>> Hospital admissions

>> Hospital patients discharged


But as mentioned above - Rt is a trailing indicator and states like CA, FL, TX might look good on the index at a point in time, while all other indicators are escalating and the state has been identified as a hot spot.

It is very apparent from the information provided in the graphic, that was obtained via a tool created by epidemiologists at the Harvard T.H. Chan School of Public Health in Boston (Go to Site), that locking down the country paid off quickly and that we were indeed able to bend and flatten the curve.


Unfortunately, the picture is beginning to change as we have seen States numbers skyrocket as COVID-19 restrictions were lifted in an attempt to reopen and restart the economy.

Another tool I look at on a daily basis is rt.live that shows where all 50 States stand with regards to Rt. As of this writing,33 States have an Rt greater than 1, however, by looking only at Rt one can come to the wrong conclusions.


In the end, we here at Thermal Proximity Systems come to the same fundamental findings. We need to stay vigilant, decisive, and diligent in our use of the MATH approach which includes wearing a mask, distancing from others by at least 6 feet, performing temperature readings, and washing your hands and the surfaces around you.

Right now, this approach is the only “cure” we have. Remember - Return Safe, Stay Safe, Be Confident… We will get through this.


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