The Viral Transmission Rate is not the number of cases nor is it the number of positive tests. It is a percentage based upon what is called the “Effective Contact Rate” and the number of positive tests. The short form is that when the Viral Transmission Rate is less than 1 in 100 K (which is our target goal) the spread of the virus is reducing and combined with a positivity rate of less than 5% of the population means that contact tracing can be done effectively. When the number of people infected (percentage of the population) is low enough and the Viral Transmission Rate is less than 1 in 100K any new incidents of the virus are manageable.
This rate is significant because it will be our indication of whether variants of the virus are undermining the effectiveness of the vaccine.
Disclaimer: I am a pastor with access to google and not a virologist or mathematician. Half the terms with which I have had to become familiar with this process are ones I’ve only recently learned. For more information see the links below.
Risks explained: https://globalepidemics.org/key-metrics-for-covid-suppression/
Covid Data Tracker: https://covid.cdc.gov/covid-data-tracker