Track the Coronavirus’s R₀ by U.S. State

Track the Coronavirus’s R₀ by U.S. State

Remember when the coronavirus was brand new, and we were all worried about what its R0 might turn out to be? R0, pronounced “R naught,” is a measure of how many people each infected person tends to pass an infection to. To stop a pandemic, we need to bring that number down to something smaller than 1. A tool called shows each U.S. state’s current reproduction number, according to recent data.

Technically R0 refers to the “basic” reproduction number, that is, how many people will be infected in sort of a hypothetical blank-slate population. The real world number is called Re for effective reproduction number, or Rt for reproduction number at a given time.

Think of a chain letter, or a multi-level marketing scheme. If you and I each rope in two people, and they each rope in two people, pretty soon our scheme will spread worldwide, increasing exponentially. But if the average person decides, nah, I won’t pass this on, the scheme — or the pandemic — will fizzle out.

The dividing line between those scenarios is a Rt of 1. If each person infects just one other person, the pandemic isn’t accelerating but it isn’t dying out, either. An Rt of 1 means we have flattened the curve. It’s an important milestone, but not a victory.

At, we can see which U.S. states have gotten their Rt below 1, and which are still over 1. New Jersey is doing the best, so far, with an Rt around 0.8. Arizona is doing the worst, with an Rt of 1.11.

Honestly, it’s still worrying (to me) that the numbers are all so similar, and relatively high. As soon as transmission began to slow, governors began opening up states. A flat Rt of 1 isn’t really good enough.

But it does show that the lockdowns and masks and everything else have been working. Estimates of the coronavirus’s R0 put it somewhere around 2.0 to 2.5. By getting Rt down to 1.0 or less, we’ve eliminated a bunch of new infections, and slowed the pandemic’s progress quite a bit. It’s a start.

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