Effective Reproduction Number

 During a epidemic, Rt is the measure known as the effective reproduction number. It’s the number of people who become infected per infectious person at time t. The version the media only talks about is the basic reproduction  number: Ro when t = 0. But , Ro is a single measure that does not adapt with changes in behavior and restrictions.

As a pandemic evolves, increasing restrictions (or potential releasing of restrictions) changes  Rt. Knowing the current Rt is essential. When R >> 1, the pandemic will spread through a large part of the population. If Rt < 1, the pandemic will slow quickly before it has a chance to infect many people. The lower the Rt: the more manageable the situation. In general, any Rt<1 means things are under control.

The value of Rt helps us in two ways.

1. It helps us understand how effective our measures have been controlling an outbreak and 
2. it gives us vital information about whether we should increase or reduce restrictions based on our competing goals of economic prosperity and human safety.

Tracking Rt is the only way to manage through this crisis.

today, we don’t yet use Rt in this way. In fact, the only real-time measure I’ve seen has been for Hong Kong. More importantly, it is not useful to understand Rt at a national level. Instead, to manage this crisis effectively, we need a local (state, county and/or city) tracking level of Rt.

I wrote the code for model which can found here.  It calculates at a state level the Value of Rt at a state level.  This can easily be adapted for county or even city level.