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How To Track How Fast Covid Is Growing In Your State

A good way to track how fast Covid is growing in your state is, which tracks the R-naught for each state.

R-naught is "How many people does each infected person infect?"

Say R-naught is 2, and that we start with 100 people who have covid.

That means those 100 people each infect 2 new people, so there will be 200 new cases, and those 200 new sick people each infect 2 new people, producing 400 new cases who in turn infect 800 people...  for a total of 100 + 200 + 400 + 800 =1,500 sick people after a week or so.  That's 15x growth! Insane!

That is, R-naught > 1.0 is exponential growth, which is dangerous and out-of-control.

Texas has an R-naught of 1.12 as-of July 7. Since it's above 1.0, it still grows wildly:

  • 100 sick people will infect 112 new people = 212 cases
  • Those 112 new sick people infect 125 new people = 337 cases
  • Those 125 new sick people infect 140 new people = 447 cases
  • Those 140 new people infect 157 new people = 604 cases
  • and so on

And that can all happen within 2 weeks, before the 100 initial sick people can  get better.

In this example, we pretended we started with just 100 sick people. Every state has a different number of currently-sick people, and that number of current cases in your area is important too (and you can also find that number on for your state).

But you can see from our example above that the exact number of current cases doesn't matter much if the R-naught is high - it's going to double fast!

But we can get this under control by more social distancing  - we know that because other countries have done it, and because we've done it here in the US.

For example, here's a graph of Texas's R-naught over-time (see all the states at - you can see Texas tamed its exponential growth to R-naught < 1.0, but now it's crept back up to a wild 1.12:


The great news though is the flipside - viruses are actually easy to kill if we can push R-naught below 1.0 - we get exponential decay, which is how New York got their covid under control. 

For example, if we can get R-naught down to 0.5:

  • 100 sick people will infect 50 new people = 150 cases
  • Those 50 new sick people infect 25 new people = 175 cases
  • Those 25 new sick people infect 12 new people = 187 cases
  • Those 12 new people infect 6 new people = 193 cases
  • Those 6 new people infect 3 new people = 199 cases
  • Those 3 new people infect 1 new person = 200 cases
  • That 1 new sick person gets no one sick = 200 cases

And as soon as those 200 people get better (fingers crossed!), Covid would be over.

And we know now that while only a small percent of those people might die, getting Covid can be really really miserable

So please, let's all keep up the social distancing! 

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